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We have used two complementary time-resolved spectroscopic techniques, dipolar electron–electron resonance and fluorescence resonance energy transfer to determine conformational changes in a single structural element of the myosin motor domain, the relay helix, before and after the recovery stroke. Two double-Cys mutants were labeled with optical probes or spin labels, and interprobe distances were determined. Both methods resolved two distinct structural states of myosin, corresponding to straight and bent conformations of the relay helix. The bent state was occupied only upon nucleotide addition, indicating that relay helix, like the entire myosin head, bends in the recovery stroke. However, saturation of myosin with nucleotide, producing a single biochemical state, did not produce a single structural state. Both straight and bent structural states of the relay helix were occupied when either ATP (ADP.BeF x ) or ADP.P i (ADP.AlF 4 ) analogs were bound at the active site. A greater population was found in the bent structural state when the posthydrolysis analog ADP.AlF 4 was bound. We conclude that the bending of the relay helix in the recovery stroke does not require ATP hydrolysis but is favored by it. A narrower interprobe distance distribution shows ordering of the relay helix, despite its bending, during the recovery stroke, providing further insight into the dynamics of this energy-transducing structural transition.

We have used transient time-resolved FRET (TR2-FRET) to detect structural changes within the relay helix in real time during the myosin recovery stroke. The most intriguing problem in myosin's function is the mechanism of signal transduction between the nucleotide binding site and the force-generating domain of myosin during the recovery stroke. Atomic structures of myosin provide clues, but structural measurements during transient kinetics are needed to reveal the details of this mechanism. Kinetics of nucleotide binding is reliably determined from the fluorescence of mant nucleotides or single Trp myosin mutants (W129, W131). Single Trp mutant W501 has provided insight into structural kinetics in the force-generating domain, but the complex fluorescence signals observed during the recovery stroke do not yield clear structural interpretation. To overcome this problem, we have performed FRET measurements that monitor well-defined structural changes in the relay helix, which is proposed to be a key structural element in myosin's force-generating domain. We engineered two double-Cys myosin mutants in the Cys-lite D. discoideum myosin construct (gift from J. Spudich), with one Cys placed at the C-terminus of the relay helix (K498C) and the other at one of two stable positions in the lower 50K domain (D515C or A639C). We labeled these mutants with a donor-acceptor FRET pair (IAEDANS-DABSYL), then used time-resolved FRET (TR-FRET, detects donor lifetime at nanosecond timescale) to detect interprobe distances in equilibrium states trapped by nucleotide analogs, reflecting pre- and post-powerstroke structures of myosin. We then performed these nanosecond time-resolved measurements repetitively every 0.1 ms, in the transient phase after ATP addition using stopped flow. These transient time-resolved FRET (TR2-FRET) signals showed directly that the relay helix changes from straight to bent conformation in the recovery stroke, from the post-powerstroke to the pre-powerstroke state.

The class VII myosins (M7) are expressed in a wide range of organisms. M7 mutants in mice, zebrafish and Dictyostelium exhibit phenotypes that reveal a role for M7 in adhesion in these highly divergent systems, suggesting a basic conservation of M7 function throughout evolution. M7s are characterized by the presence of two FERM domains in their C-terminal tail region, and deletion of either from the Dictyostelium M7 (DdM7) tail results in loss of function without affecting localization. A search for DdM7 binding partners has revealed that talin, an actin-binding protein that provides a key link between adhesion receptors and the actin cytoskeleton, interacts directly with DdM7. The phenotypes of the DdM7 and talin null mutants are highly similar, suggesting that these two proteins work co-operatively to maintain cell-cell and cell-surface contact and that this interaction may also be conserved throughout evolution.

We have engineered a mutant of Dictyostelium discoideum ( Dicty ) myosin II that contains the same fast-reacting “SH1” thiol as in muscle myosin, spin-labeled it, and performed electron paramagnetic resonance (EPR) to compare the structure of the force-generating region of the two myosins. Dicty myosin serves as a model system for muscle myosin because of greater ease of mutagenesis, expression, and crystallization. The catalytic domains of these myosins have nearly identical crystal structures in the apo state, but there are significant differences in ATPase kinetics, and there are no crystal structures of skeletal muscle myosin with bound nucleotides, so another structural technique is needed. Previous EPR studies, with a spin label attached to SH1 in muscle myosin, have resolved the key structural states of this region. Therefore, we have performed identical experiments on both myosins spin-labeled at equivalent sites. Spectra were identical for the two myosins in the apo and ADP-bound states. With bound ADP and phosphate analogs, ( i ) both proteins exhibit two resolved structural states (prepowerstroke, postpowerstroke) in a single biochemical state (defined by the bound nucleotide), and ( ii ) these structural states are essentially identical in the two myosins but ( iii ) are occupied to different extents as a function of the biochemical state. We conclude that ( i ) myosin structural and biochemical states do not have a one-to-one correspondence, and ( ii ) Dicty myosin can serve as a good analog for structural studies of muscle myosin only if differences in the coupling between biochemical and structural states are taken into account.

We present a structurally dynamic model for nucleotide- and actin-induced closure of the actin-binding cleft of myosin, based on site-directed spin labeling and electron paramagnetic resonance (EPR) in Dictyostelium myosin II. The actin-binding cleft is a solvent-filled cavity that extends to the nucleotide-binding pocket and has been predicted to close upon strong actin binding. Single-cysteine labeling sites were engineered to probe mobility and accessibility within the cleft. Addition of ADP and vanadate, which traps the posthydrolysis biochemical state, influenced probe mobility and accessibility slightly, whereas actin binding caused more dramatic changes in accessibility, consistent with cleft closure. We engineered five pairs of cysteine labeling sites to straddle the cleft, each pair having one label on the upper 50-kDa domain and one on the lower 50-kDa domain. Distances between spin-labeled sites were determined from the resulting spin–spin interactions, as measured by continuous wave EPR for distances of 0.7–2 nm or pulsed EPR (double electron–electron resonance) for distances of 1.7–6 nm. Because of the high distance resolution of EPR, at least two distinct structural states of the cleft were resolved. Each of the biochemical states tested (prehydrolysis, posthydrolysis, and rigor), reflects a mixture of these structural states, indicating that the coupling between biochemical and structural states is not rigid. The resulting model is much more dynamic than previously envisioned, with both open and closed conformations of the cleft interconverting, even in the rigor actomyosin complex.

Originally published in: Molecular Motors. Edited by Manfred Schliwa. Copyright © 2003 Wiley-VCH Verlag GmbH & Co. KGaA Weinheim. Print ISBN: 3-527-30594-0 The sections in this article are Introduction Functional Properties of Myosins Directionality and Processivity Protein Motifs Found in Myosins Myosin Regulation Diverse Functions for Myosins Non-muscle Contractile Structures Cell Motility and Adhesion Organelle/Cellular Component Transport Maintenance of Actin-rich Extensions Membrane Trafficking Signal Transduction Myosins in Disease Griscelli Syndrome Roles for Myosins in Hearing New Myosins and Myosin Functions on the Horizon Conclusions Acknowledgements Keywords: myosin superfamily; protein motifs; myosin regulation; cell motility; transport; actin-rich extensions; membrane trafficking; signal transduction; Griscelli syndrome

The class VII myosins (M7) are expressed in a wide range of organisms. M7 mutants in mice, zebrafish and Dictyostelium exhibit phenotypes that reveal a role for M7 in adhesion in these highly divergent systems, suggesting a basic conservation of M7 function throughout evolution. M7s are characterized by the presence of two FERM domains in their C-terminal tail region, and deletion of either from the Dictyostelium M7 (DdM7) tail results in loss of function without affecting localization. A search for DdM7 binding partners has revealed that talin, an actin-binding protein that provides a key link between adhesion receptors and the actin cytoskeleton, interacts directly with DdM7. The phenotypes of the DdM7 and talin null mutants are highly similar, suggesting that these two proteins work co-operatively to maintain cell-cell and cell-surface contact and that this interaction may also be conserved throughout evolution.

We have used site-directed spin and fluorescence labeling to test molecular models of the actin-myosin interface. Force is generated when the actin-myosin complex undergoes a transition from a disordered weak-binding state to an ordered strong-binding state. Actomyosin interface models, in which residues are classified as contributing to either weak or strong binding, have been derived by fitting individual crystallographic structures of actin and myosin into actomyosin cryo-EM maps. Our goal is to test these models using site-directed spectroscopic probes on actin and myosin. Starting with Cys-lite constructs of both yeast actin (ActC) and the Dictyostelium myosin II motor domain (S1dC), site-directed labeling (SDL) mutants were generated by mutating residues to Cys in the proposed weak and strong-binding interfaces. This report focuses on the effects of forming the strong-binding complex on four SDL mutants, two located in the proposed weak-binding interface (ActC5 and S1dC619) and two located in the proposed strong-binding interface (ActC345 and S1dC401). Neither the mutations nor labeling prevented strong actomyosin binding or actin-activation of myosin ATPase. Formation of the strong-binding complex resulted in decreased spin and fluorescence probe mobility at all sites, but both myosin-bound probes showed remarkably high mobility even after complex formation. Complex formation decreased solvent accessibility for both actin-bound probes, but increased it for the myosin-bound probes. These results are not consistent with a simple model in which there are discrete weak and strong interfaces, with only the strong interface forming under strong-binding conditions, nor are they consistent with a model in which surface residues become rigid and inaccessible upon complex formation. We conclude that all four of these residues are involved in the strong actin-myosin interface, but this interface is remarkably dynamic, especially on the surface of myosin.

Myosin VII (M7) plays a role in adhesion in both Dictyostelium and mammalian cells where it is a component of a complex of proteins that serve to link membrane receptors to the underlying actin cytoskeleton. The nature of this complex is not fully known, prompting a search for M7-binding proteins. Co-immunoprecipitation experiments reveal that Dictyostelium M7 (DdM7) interacts with talinA, an actin-binding protein with a known role in cell-substrate adhesion. No additional proteins are observed in the immunoprecipitate, indicating that the interaction is direct. The N-terminal region of the DdM7 tail that lies between the region of predicted coil and the first MyTH4 domain is found to harbor the talinA binding site. Localization experiments reveal that talinA does not serve as a membrane receptor for DdM7 and vice versa. These findings reveal that talinA is a major DdM7 binding partner and suggest that their interaction induces a conformational change in each that, in combination with membrane receptor binding, promotes the assembly of a high avidity receptor complex essential for adhesion of the cell to substrata.

Little is known about how organisms regulate the size of multicellular structures. This review condenses some of the observations about how Dictyostelium regulates the size of fruiting bodies. Very large fruiting bodies tend to fall over, and one of the ways Dictyostelium cells prevent this is by breaking up the aggregation streams when there is an excessive number of cells in the stream. Developing cells simultaneously secrete and sense counting factor (CF), a 450 kDa complex of proteins. Diffusion calculations showed that as the number of cells in a stream or group increases, the local concentration of CF will increase, allowing the cells to sense the number of cells in the stream or group. Computer simulations predicted that a high level of CF could trigger stream breakup by decreasing cell-cell adhesion and/or increasing cell motility, effectively causing the stream to dissipate and begin to fall apart. The prediction that adhesion and motility affect group size is supported by observations that decreasing adhesion by adding antibodies that bind to adhesion protein causes the formation of smaller groups, while increasing adhesion by overexpressing adhesion proteins, or decreasing motility with drugs that disrupt actin function both cause the formation of larger groups. CF both decreases adhesion and increases motility. CF increases motility in part by increasing actin polymerization and myosin phosphorylation, and decreasing myosin polymerization. New observations using a fusion of a green fluorescent protein to a protein fragment that binds polymerized actin show that in live cells CF does not affect the distribution of polymerized actin. CF increases the levels of ABP-120, an actin-bundling protein, and new observations indicate that very low levels of CF cause an increase in levels of myoB, an unconventional myosin. Our current understanding of group size regulation in Dictyostelium is thus that motility plays a key role, and that to regulate group size cells regulate the expression of at least two proteins, as well as regulating the polymerization of both actin and myosin.

Filopodia are cellular protrusions composed of bundled parallel actin filaments that initiate from the cortex and have roles in cell adhesion and signaling during directed cell migration. MyTH4-FERM myosins (including Myosin 7, 10 and 15) localize to the distal tips of filopodia, stereocilia, and microvilli. Myo10 is required for filopod growth in mammalian cells while Myo7 is essential for filopod growth in Dictyostelium. We performed live cell imaging using GFP-tagged Myo7 in null cells. Myo7 rescued filopod formation and localized to filopodia tips and actin-based pseudopods. In pseudopods, enrichment at the leading edge near the cell membrane preceded the growth of filopodia. Tip extension velocity remained constant throughout the growing phase independent of Myo7 accumulation at the tip. A construct lacking MyTH4-FERM domains but including the putative SAH domain (Myo7 Motor) failed to rescue filopodia and did not localize with cortical actin. Neither of the two FERM domains was essential for rescue of filopod formation. Deletion of the C-terminal FERM led to increased production of substrate-adhered filopodia suggesting this domain might negatively regulate Myo7 activity in vivo. We tested this by mutating conserved residues implicated in auto-inhibition of Myo10 and expressing a mutated Myo7 (KK2333/2336AA) that also showed increased production of filopodia relative to wild type Myo7. Tip extension velocity in these mutants was similar to wild type despite the increased number of filopod tips. A mutant lacking the motor domain (Myo7 Tail) was enriched near the cell membrane but failed to rescue filopod formation. Neither FERM domain was essential for localization of the tail. These results suggest that normal formation of filopodia requires membrane-localized Myo7 with precisely regulated motor activity. (This work is supported by an NSF grant to MAT. KJP is supported by NIH grant T32 AR007612.)

We have examined the function of a member of the vasodilator-stimulated phosphoprotein family of proteins (DdVASP) in Dictyostelium. Ddvasp null cells lack filopodia, whereas targeting DdVASP to the plasma membrane with a myristoyl tag results in a significant increase in filopodia. The proline-rich domain-Ena/VASP homology 2 structure is required for both actin polymerization activity and filopodia formation. Ddvasp null cells exhibit a chemotaxis defect, which appears to be due to a defect in the ability of the cells to properly adhere to the substratum and to suppress lateral pseudopod extension. We demonstrate that during chemotaxis, the anterior approximately 50% of the cell lifts from the substratum and remains elevated for up to 1 min. These defects lead to a significant decrease in chemotaxis efficiency. DdVASP localizes to the leading edge in migrating cells and to the tips of filopodia. In addition, Ddvasp null cells have a defect in particle adhesion but internalize particles normally. Our results provide new insights into the function of DdVASP in controlling the actin cytoskeleton during chemotaxis and filopodia formation.

Rapid progress has recently been made in the identification and characterization of a large number of kinesin and myosin motor proteins. Recent work has uncovered roles for these motors in processes such as vesicle trafficking, cytoskeletal organization, and chromosome movements, to name a few. A series of reviews describing some of the significant advances in our understanding of the structure and function of myosins and kinesins is presented.

Directed cell migration occurs in response to extracellular cues. Following stimulation of a cell with chemoattractant, a significant rearrangement of the actin cytoskeleton is mediated by intracellular signaling pathways and results in polarization of the cell and movement via pseudopod extension. Amoeboid myosin Is play a critical role in regulating pseudopod formation inDictyostelium, and their activity is activated by heavy chain phosphorylation. The effect of chemotactic stimulation on thein vivo phosphorylation level of aDictyostelium myosin I, myoB, was tested. The myoB heavy chain is phosphorylated in vivo on serine 322 (the myosin TEDS rule phosphorylation site) in chemotactically competent cells. The level of myoB phosphorylation increases following stimulation of starving cells with the chemoattractant cAMP. A 3-fold peak increase in the level of phosphorylation is observed at 60 s following stimulation, a time at which the Dictyostelium cell actively extends pseudopodia. These findings suggest that chemotactic stimulation results in increased myoB activity via heavy chain phosphorylation and contributes to the global extension of pseudopodia that occurs prior to polarization and directed motility.

Background: Phagocytosis, the process by which cells internalize particles, is essential for the defense of multicellular organisms against invading pathogens and is the major means by which many unicellular organisms obtain nutrients. The actin cytoskeleton plays a critical role in phagocytosis and the observation that a significant amount of force (10–20 nN) is generated during internalization, suggests that a myosin participates in the process. Although more than 15 distinct classes of myosin have been identified, their roles in phagocytosis are unknown. Results: The identification of a class VII unconventional myosin (DdMVII) in the Dictyostelium discoideum amoeba, which is a model for phagocytosis, is reported here. Mutant cells lacking DdMVII exhibited an 80% decrease in the uptake of particles whereas all other actin-based behaviors that were tested, including pinocytosis, exocytosis, cytokinesis and morphogenesis, proceeded normally. The defect in phagocytosis was neither because of altered particle binding nor inability to form actin-filled phagocytic cups. Conclusions: Molecular genetic analysis of Dictyostelium myosin VII reveals that this motor protein plays a specific and significant role in phagocytosis.

Recent results reinforce the view that actin-based and microtubule-based motility systems do not operate independently, but are used in a coordinated fashion to determine intracellular localization of cargo such as organelles.

The class I myosins play important roles in controlling many different types of actin-based cell movements.Dictyostelium cells either lacking or overexpressing amoeboid myosin Is have significant defects in cortical activities such as pseudopod extension, cell migration, and macropinocytosis. The existence of Dictyostelium null mutants with strong phenotypic defects permits complementation analysis as a means of exploring important functional features of the myosin I heavy chain. Mutant Dictyostelium cells lacking two myosin Is exhibit profound defects in growth, endocytosis, and rearrangement of F-actin. Expression of the full-length myoB heavy chain in these cells fully rescues the double mutant defects. However, mutant forms of the myoB heavy chain in which a serine at the consensus phosphorylation site has been altered to an alanine or in which the C-terminal SH3 domain has been removed fail to complement the null phenotype. The wild-type and mutant forms of the myoB heavy chain appeared to be properly localized when they were expressed in the myosin I null mutants. These results suggest that the amoeboid myosin I consensus phosphorylation site and SH3 domains do not play a role in the localization of myosin I, but are absolutely required for in vivo function.

It is an exciting time to be studying myosins and their roles in the function of cells and organisms. Past efforts aimed at finding new members of this family have now given way to a focus on identifying individual functions for each motor protein. These actin-based motors are now known to be intimately involved in the following processes: neurosensory function; vesicle trafficking; determinant partitioning; and cortical function. The following article reviews the inroads made into the functions of myosins in these processes over the past several years.

In this issue of Developmental Cell, Sokac et al. (2006) describe an intriguing new role for an actin-based motor protein in restraining actin polymerization during endocytosis in Xenopus oocytes.

The unconventional myosins are a superfamily of actin-based motors responsible for a rich array of intracellular motility events. Recent evidence suggests that these motors play important roles in cell migration, endocytosis and intracellular transport. Several genetic mutants have been identified whose abnormalities are the result of the loss of a specific myosin. This article describes how analysis of these mutants, coupled with basic studies of the intracellular localization and biochemical properties of individual myosins, is leading to a clearer understanding of the in vivo function of a number of these interesting motor proteins.

The myosins are a superfamily of protein motors. Analysis of their sequences in a wide range of organisms reveals an unexpected variety of domains, and provides insights into the nature of the earliest eukaryotes.

The simple eukaryote Dictyostelium discoideum contains at least 12 unconventional myosin genes. Here we report the characterization of one of these, myoJ, a gene initially identified through a physical mapping screen. The myoJ gene encodes a high molecular weight myosin, and analysis of the available deduced amino acid sequence reveals that it possesses six IQ motifs and sequences typical of alpha helical coiled coils in the tail region. Therefore, myoJ is predicted to exist as a dimer with up to 12 associated light chains (six per heavy chain). The 7.8 kb myoJ mRNA is expressed all throughout the life cycle of D. discoideum. The myoJ gene has been disrupted and a phenotypic analysis of the mutant cells initiated. Finally, phylogenetic analysis of the head region reveals that myoJ is most similar to two plant myosin genes, Arabidopsis MYA1 and MYA2, that have been alternatively suggested to be either members of the myosin V class or founding members of the myosin XI class.

The role of myosin Is in endosomal trafficking and the lysosomal system was investigated in a Dictyostelium discoideum myosin I double mutant myoB-/C-, that has been previously shown to exhibit defects in fluid-phase endocytosis during growth in suspension culture (Novak et al., 1995). Various properties of the endosomal pathway in the myoB-/C- double mutant as well as in the myoB- and myoC- single mutants, including intravesicular pH, and intracellular retention time and exocytosis of a fluid phase marker, were found to be indistinguishable from wild-type parental cells. The intimate connection between the contractile vacuole complex and the endocytic pathway in Dictyostelium, and the localization of a myosin I to the contractile vacuole in Acanthamoeba, led us to also examine the structure and function of this organelle in the three myosin I mutants. No alteration in contractile vacuole structure or function was observed in the myoB-, myoC- or myoB-/C- cell lines. The transport, processing, and localization of a lysosomal enzyme, alpha-mannosidase, were also unaltered in all three mutants. However, the myoB- and myoB-/C- cell lines, but not the myoC- cell line, were found to oversecrete the lysosomal enzymes alpha-mannosidase and acid phosphatase, during growth and starvation. None of the mutants oversecreted proteins following the constitutive secretory pathway. Two additional myosin I mutants, myoA- and myoA-/B-, were also found to oversecrete the lysosomally localized enzymes alpha-mannosidase and acid phosphatase. Taken together, these results suggest that these myosins do not play a role in the intracellular movement of vesicles, but that they may participate in controlling events that occur at the actin-rich cortical region of the cell. While no direct evidence has been found for the association of myosin Is with lysosomes, we predict that the integrity of the lysosomal system is tied to the fidelity of the actin cortex, and changes in cortical organization could influence lysosomal-related membrane events such as internalization or transit of vesicles to the cell surface.

A central part of the machinery of cell division is the spindle. The creation and operation of this structure seem to require a component of the cell's infrastructure not previously associated with it.

The protozoan myosin Is are widely expressed actin-based motors, yet their in vivo roles remain poorly understood. Molecular genetic studies have been carried out to determine their in vivo function in the simple eukaryote Dictyostelium, an organism that contains a family of four myosin Is. Here we report the characterization of myoC, a gene that encodes a fifth member of this family. Analysis of the deduced amino acid sequence reveals that the myoC gene encodes a myosin that is homologous to the well-described Acanthamoeba myosin Is as well as to Dictyostelium myoB and -D. The expression pattern of the myoC mRNA is similar to that of myoB and myoD, with a peak of expression at times of maximal cell migration, around 6 hours development. Deletion of the myoB gene has been previously shown to result in mutant cells that are defective in pseudopod extension and phagocytosis. However, no obvious differences in cell growth, development, phagocytosis or motility were detected in cells in which the myoC gene had been disrupted by homologous recombination. F-actin localization and ultrastructural organization also appeared unperturbed in myoC- cells. This apparent ‘lack’ of phenotype in a myosin I single knockout cannot be simply explained by redundancy of function. Our results rather suggest that the present means of assessing myosin I function in vivo are insufficient to identify the unique roles of these actin-based motors.

We have used pulsed EPR spectroscopy (DEER) to determine the structural effects of point mutations in the force generation region of myosin (I499A, F506A, F692A, D.discoideum sequence). As it was previously reported, these myosin mutants maintain basal ATPase activity, but completely lose their motor function. It was proposed that F506A, located in the relay loop, disrupts communication between the nucleotide binding site and the force generation region in myosin, and F692A and I499A, located on the interface with the converter domain, affect coordination of the converter domain relative to the myosin head. In this study we assayed the structure of the relay helix in these mutants and in wild-type myosin. The mutations were introduced into the A639C:K498C myosin construct, both Cys were labeled with maleimide spin probes, and interprobe distance was measured. Previous studies showed that probes at these sites detect the nucleotide-induced bending of the relay helix. Observed spin echo decays were interpreted in terms of one or two Gaussian distance distributions, corresponding to one or two myosin structural states. Nucleotide analogs were used to trap myosin in several biochemical states. In the wild type, two structural states, corresponding to pre- and post-recovery stroke, are present in a single biochemical state. The mutations affect the equilibrium between these two structural states and increase the conformational disorder, indicating that these mutants destabilize the structure of the force-generation region, particularly in the post-recovery state. These results provide a structural explanation for the functional perturbation, introduced to myosin.

The number and variety of myosins that have been identified has increased greatly over the past few years, and is still growing. Myosins have been classified into at least six distinct classes. Research during the last year has concentrated on identifying the roles of various myosins.

We suggest that the vertebrate myosin-I field adopt a common nomenclature system based on the names adopted by the Human Genome Organization (HUGO). At present, the myosin-I nomenclature is very confusing; not only are several systems in use, but several different genes have been given the same name. Despite their faults, we believe that the names adopted by the HUGO nomenclature group for genome annotation are the best compromise, and we recommend universal adoption.

We have used transient time-resolved FRET (TR2FRET) to monitor the conformation of the relay helix in a myosin II functional mutant during the recovery stroke in real time. Myosin was perturbed with the F506A mutation (Dictyostelium discoideum sequence), located within the relay loop in the force-generating region. F506 is a highly conserved residue in myosin II and is a hypertrophic cardiomyopathy mutation site. Previous studies [Tsiavaliaris, EMBO Rep, 2002, 3(11), 1099] showed a significant effect of the F506A mutation on myosin function. Actin affinity in the presence of ATP was increased, and the mutant did not move actin filaments in in vitro motility assays. A small decrease in intrinsic fluorescence was observed upon addition of excess ATP, but ATP binding and hydrolysis were not affected by the mutation. It was proposed that the F506A disrupts the communication between the active site and the lever arm. We engineered a double-Cys myosin mutant (A639C:K498C) in the Cys-less background with the F506A functional mutation, and labeled the mutant with optical probes. We used TR-FRET to determine the interprobe distance, and TR2FRET measurements after rapid mixing with ATP revealed changes in the relay helix conformation during the recovery stroke in real time. The mutation induced significant disorder of the relay helix in the force-generating region, but myosin still produces a recovery stroke, changing the relay helix conformation from straight to bent. We conclude that (a) the relay helix is disordered in myosin functional mutant F506A, which demonstrates the importance of the relay loop - relay helix interaction in the relay helix stabilization, and (b) the relay helix is the major structural element in the force-generating region of myosin, responsible for communication from the active site to the converter domain and the lever arm.

We have used dipolar electron-electron resonance (DEER) and time-resolved fluorescence resonance energy transfer (TR-FRET) to investigate the role of the myosin relay helix in coupling between the active site and the force-generating region of myosin II. Two double-Cys Dictyostelium myosin constructs have been engineered, and the structure of the relay helix was monitored by measuring interprobe distances in MSL/MSL or IAEDANS/Dabcyl-labeled myosin. Experiments were performed on WT myosin and on F506A, a functional mutant that has close to normal enzymatic activity but completely lacks motor functions (e.g. unable to move actin filaments or support cell development). We found that the WT myosin relay helix adopts two distinct states (straight and bent), with the bent conformation populated when ATP and ADP.Pi analogs are bound at the active site. In contrast, binding of nucleotide analogs had very little effect on relay helix conformation in the F506A mutant. In addition, the width of the distance distribution was significantly larger in the F506A compared to WT myosin, indicating loss of structural organization. Our results demonstrate that the relay helix plays a key role in coupling of myosin ATPase and motor activities. Loss of functionality observed in F506A myosin can be explained by the disruption of the relay helix-relay loop interactions that normally stabilize well-defined conformations of the myosin force-generating region allowing it to switch between distinct structural states.

We have used three complementary spectroscopic techniques, pulsed EPR (DEER), time-resolved fluorescence resonance energy transfer (TR-FRET) and transient time-resolved FRET [(TR)2FRET], to elucidate the mechanisms of energy transduction in myosin - an important motility protein involved in a variety of processes including muscle contraction. We engineered two double-Cys myosin mutants that were labeled with optical probes or spin labels, and determined structural changes in a single structural element of the myosin motor domain, the relay helix, as affected by nucleotide binding and hydrolysis. In all cases, time-domain detection permitted the reliable determination of the interprobe distance distribution, quantitating both order and disorder, and resolving coexisting structural states. While DEER offered superior distance resolution, (TR)2FRET permitted detection of transient structural changes after rapid mixing (stopped-flow) with ATP. All methods demonstrated two structural states of myosin during the recovery stroke, corresponding to straight and bent conformations of the relay helix, in parallel with straight and bent conformations of the entire myosin head. A narrower interprobe distance distribution in the post-recovery state shows ordering of the relay helix structure during the recovery stroke. These experiments reveal changes in both structure and dynamics of the relay helix and identify it as a key player in the interdomain coupling mechanism. This methodology is applicable to any enzyme in which double Cys mutants can be engineered in a key region of energy transduction, permitting resolution of structural states and dynamics in real time during the transient phase of a biochemical reaction.

MOLECULAR MOTORS . MyOSINS . MyosinIl '" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Myosin I .... . . .. . . ... . . . ... . ... . ....... . . ... . KINESIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DYNEINS . Axonemal Dynein . . . . . . . . . . . . . . . . . .. . . . . . . . ... . . . . . . . . . . . . . . .. . . . . Cytoplasmic Dynein . . . . . . . . .. . . . .. . . . ... . . .. . . . ... . ..... . .... . ... . DISCOVERY OF NEW FAMILY MEMBERS . ';!fnO::i�� : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Dyneins .. . . . .... . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . MUTANT PHENOTYPES AND MOTOR PROTEIN INTERACTIONS . ';!fnO::i�� : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Dyneins . . . . . . ... . . . . . . . . ... . ... . ..... . . . . . . . . . . . . . . . . . . ... . . . . . GENETIC EVIDENCE FOR MOTOR PROTEIN INTERACTIONS . FUTURE PROSPECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roles in the Cell. . . . . . . . . . . . . .. .... . . . .. . . . Mechanisms of Motor Protein Action . . . . . . . . . . . Regulation of Motor Protein Activity . . . ... . . .. . . . . . . . . .. . . .... . . . . . . . . . CONCLUDING REMARKS . AFTERWORD .

We have trapped Dictyostelium myosin II in a weak actin-binding conformation by chemically crosslinking two engineered Cys across the actin-binding cleft using a bifunctional spin label (BSL). With sites in the lower and upper 50 kDa domains, the crosslink restricts the conformation of the actin-binding cleft. The crosslinking reaction was monitored by electron paramagnetic resonance (EPR) based on the spin label immobilization that occurs upon reaction of both Cys. The EPR spectrum of crosslinked myosin is sensitive to structural changes induced by both nucleotide- and actin-binding. Functional assays demonstrate that crosslinking partially impairs actin binding and actin-activation but has negligible effects on basal ATPase activity. We propose that crosslinked myosin is trapped in a weak actin-binding structure in which phosphate release is inhibited by the presence of actin. This conformation presumably binds actin weakly but cannot transition to the “closed” cleft structure that is populated with strong actin-binding (Klein et al, 2008, PNAS 105:12867-72). The weak actin-binding structure has proven difficult to characterize because of its transient nature; BSL-crosslinked myosin provides a stable model system for analysis of structural dynamics. We are using EPR to analyze the orientation of BSL-crosslinked myosin attached to actin in skinned muscle fibers, and we are using nucleotide probes to investigate the coupling between the actin-binding cleft and the nucleotide-binding pocket. This work is complementary to a study in which BSL was used to crosslink SH1 (C707) and SH2 (C697) in the force-generating domain of myosin, producing a stable complex that bound weakly to actin with slow orientational disorder (Thompson et al., 2008, Biophys. J., in press). This work was supported by grants from NIH (AR32961, AR07612) and the Minnesota Supercomputing Institute.

We have used DEER (double electron-electron resonance) and TR-FRET (time-resolved fluorescence resonance energy transfer) to study conformational changes within the myosin II relay helix. Major structural changes during the myosin II ATPase cycle take place in the force-generating domain. Crystal structures show that the converter domain, the relay helix, and SH1 helix have different conformations in the proposed pre- and post- powerstroke structural states of myosin. This dramatic structural change is detected in solution studies by intrinsic fluorescence [1, 2] and EPR of a spin label attached to SH1 [3]. In the present study, we focus on the relay helix as a crucial structural element involved in coupling between the force-generating domain and the nucleotide-binding pocket. Cysteine mutations were introduced into a Cys-lite construct of Dictyostelium discoideum (Dicty) myosin in the lower 50k domain (either D515C or A639C) and the C-terminal end of the relay helix (K498C). These constructs were selectively modified with either MSL/MSL or IAEDANS/DABCYL pairs, and the distance between probes was measured in different myosin conformations trapped with nucleotides or nucleotide analogs Two conformations of the relay helix (with distinct probe-to-probe distances, presumably corresponding to the “straight” and “bent“ states of the relay helix) were resolved. Observed distances were in good agreement with existing crystal structures, but at least two distinct structural states were present in certain biochemical states (e.g, with bound ADP.BeF3, ADP.Vi, ADP.AlF4). The mole fraction of the “bent” conformation was higher with post-hydrolysis analogs (ADP.Vi, ADP.AlF4) bound at the active site. Our results reveal structural rearrangements within a single subdomain of myosin and provide insights into the coupling between ATP binding and changes in the force-generating region.

Publisher Summary Dictyostelium discoideum has a number of features that make it an attractive system for cell biological studies. The ability of Dictyostelium cells to perform active ameboid crawling and chemotaxis has made it a popular system for cell motility and signal transduction studies. The lack of a cell wall and ease of cultivation make Dictyostelium an excellent organism for biochemical approaches, allowing large quantities of material to be obtained and lysed without difficulty. This chapter describes molecular genetic tools that are used for transformation, construction of null cell lines, and expression of the cloned myosin gene fragments. Brief coverage is given to other tools and methods common in the field, but the emphasis is on those approaches that are currently being used. Straightforward gene disruption protocols have been developed and used to construct myosin null lines of Dictyostelium . These techniques have also been successfully applied to the study of other genes in Dictyostelium , such as the α-actinin gene.

There are changes afoot in the way that US granting agencies, most notably the NIH, evaluate proposals. Change can be a good thing, and it is probably high time that the NIH reassessed how grants are evaluated. Frankly, I was becoming blase about the entire granting process. It seemed that once I became inured to the low probability of funding, competition from the mega-labs, and the shift towards preferring science that can be called ‘medically relevant’, all of the fun had been taken out of writing grants and reading the subsequent scathing evaluations. Now, the evaluation of science will include a more precise and specific ranking of several individual aspects of a proposal, one of which is the (real or imagined) significance or impact of the proposed research. Ah … life is worth living again!How can one argue with the notion that significant science should be funded and insignificant science should not? Although explicit ranking of the impact of the proposed research seems to be a great idea, surely reviewers have been overtly or covertly considering significance when rating grants all along. So why has this criterion now come out of the closet and how will it be applied evenly to all grant applications?One major reason for the change in the method of evaluation is that a mechanism for provoking reviewers into giving out a wider range of rankings seems to be needed, since so many grants score high and so few can be funded. If the evaluation can be broken down into smaller quanta there is a chance that the overall scores may have greater variability. But how can we put a number on such an inherently unpredictable aspect of science as its future significance? Yes, some projects have obvious impact, but one of the attractions of this business is the fact that there are times when biology is determined to undermine your nice, tidy, and oh-so-logical model in a new, fascinating and wholly unpredicted way. Apparently routine questions are forever coming up with unexpected answers. Will the new system reduce our ability to check and recheck the truth of our assumptions?Moving to the personal level, how can I assess the likely impact of my own research? How will I know that my work will affect the concepts or methods that drive my field, and how can I be sure that I am working on an important problem? I increasingly suspect that the fact that I think my science is interesting and important is irrelevant to the misguided people out there who don’t share my views. Is there a generally acceptable way to define high-impact science?One simple-minded criterion is to gauge significance using the publication record: the number of papers published, and where they are published. This criterion is convenient and readily quantifiable, with the added bonus that the editors and reviewers of the ‘correct’ journals do a good deal of the dirty work by deciding what areas of science are significant and which papers are likely to have an impact on the field. Perhaps in the future journals will be able to buy the right from the NIH to affix the label “The place where high-impact scientists publish!” to the front cover. But are we really happy with the idea that these faceless editors and reviewers who already have so much power will now have more? Are they really the best people to decide the overall scientific direction of the country? Perhaps.It's hard to guess which journal the paper is going to end up in before the work is even done, however. So can we define high-impact science by the area of research? Maybe the NIH should provide a list of high-impact topics, to alleviate confusion for those of us who might be unwittingly considering working in medium or low impact areas. In assembling this list, the NIH may want to consider whether high-impact science is work that even the man on the street has heard of, having an appeal not solely confined to the rarified circles of the scientist. By this criterion, research to improve the quality of the beers produced by the major brewers in the US would probably be the ultimate in high-impact biological science.However we decide to define high-impact science, there is one problem that nobody seems to have recognized yet. We need to provide therapy for people who are addicted to science of lesser significance. I envision a high-quality, extremely confidential clinic (the Watson & Crick Clinic for Significant Science) modeled along the lines of the Betty Ford treatment center for substance abusers. The anonymous patients would be offered a series of seminars alerting them to the warning signs of medium- or low-impact science, and would be taught to avoid self-destructive tendencies such as an affection for un-trendy research. This approach alone would greatly improve the perceived quality of science in many labs almost overnight.Of course it would be a good idea to shift the way that people develop projects in such a way as to make it more likely that truly significant work will be done. But will this change in NIH reviewing guidelines help or hinder this goal? I suspect the latter.

There is growing evidence for a coupling of actin assembly and myosin motor activity in cells. However, mechanisms for recruitment of actin nucleators and motors on specific membrane compartments remain unclear. Here we report how Spir actin nucleators and myosin V motors coordinate their specific membrane recruitment. The myosin V globular tail domain (MyoV-GTD) interacts directly with an evolutionarily conserved Spir sequence motif. We determined crystal structures of MyoVa-GTD bound either to the Spir-2 motif or to Rab11 and show that a Spir-2:MyoVa:Rab11 complex can form. The ternary complex architecture explains how Rab11 vesicles support coordinated F-actin nucleation and myosin force generation for vesicle transport and tethering. New insights are also provided into how myosin activation can be coupled with the generation of actin tracks. Since MyoV binds several Rab GTPases, synchronized nucleator and motor targeting could provide a common mechanism to control force generation and motility in different cellular processes. DOI: http://dx.doi.org/10.7554/eLife.17523.001

Myosin Is are associated with specific membranes, however, the mechanism for regulating their intracellular localization is unclear. As a first step towards understanding this mechanism, membrane rebinding assays using Dictyostelium myoB were performed. Crude, cytosolic myoB bound to intact, but not to NaOH-treated plasma membranes. In contrast, partially purified myoB binds to both intact and NaOH-treated plasma membranes. Chemical cross-linking of cytosolic myoB yielded several products, whereas none were found with the partially purified myoB. These results suggest a model where proteins regulating the specific binding of myoB to the plasma membrane may exist both in the cytosol and on the plasma membrane.

Background: The initial stages of phagocytosis and cell motility resemble each other. The extension of a pseudopod at the leading edge of a migratory cell and the formation of a phagocytic cup are actin dependent, and each rely on the plasma membrane adhering to a surface during dynamic extension. Results: A myosin VII null mutant exhibited a drastic loss of adhesion to particles, consistent with the extent of an observed decrease in particle uptake. Additionally, cell–cell adhesion and the adhesion of the leading edge to the substratum during cell migration were defective in the myosin VII null cells. GFP-myosin VII rescued the phagocytosis defect of the null mutant and was distributed in the cytosol and recruited to the cortical cytoskeleton, where it appeared to be enriched at the tips of filopods. It was also localized to phagocytic cups, but only during the initial stages of particle engulfment. During migration, GFP-myosin VII is found at the leading edge of the cell. Conclusions: Myosin VII plays an important role in mediating the initial binding of cells to substrata, a novel role for an unconventional myosin.

The recent observation that class VI myosins move in a direction opposite to all other known myosins, taken together with analyses of mutant flies and mice, suggests that, instead of moving vesicles out towards the cell periphery, myosin VI more likely brings materials or membranes into the cell.

SummaryNew work shows that the motor protein myosin VI, acting through vinculin, plays a key role in the maturation of cadherin-based adherens junctions in epithelial cells.

The reactive thiol of the myosin head, SH-1, can be selectively labelled in glycerinated rabbit muscle fibres. This residue has been used as an attachment site for either fluorescent or spectroscopic probes which report on head movements and orientations in various functional states of muscle. We have specifically modified SH-1in vitro, using purified rabbit myosin and conditions similar to those employed in the labelling of muscle fibres (low ionic strength [40mM NaCl] at 4°C), with stoichiometric amounts of either [14C]-iodoacetamide, 5-(2((iodoacetyl)amino)ethyl) aminonaphthalene-1-sulphonic acid (IAEDANS), or 4-(2-iodoacetamido-2,2,6,6-tetramethyl piperidinooxyl (IASL). The specificity of modification was determined by measuring the well-defined alterations in the high salt ATPase activities of myosin and by localizing both IAAm and IAEDANS to the 20-kDa C-terminal subfragment 1 (S1) which contains SH-1. The low ionic strength actin-activated Mg2+-ATPase of SH-1-modified rabbit myosin was measured in the presence of the thin filament regulatory, complex, troponin-tropomyosin. A significant increase in this activity in the absence of calcium, concomitant with a decrease in activity in the presence of calcium, was observed as the extent of SH-1 modification was incrementally increased from zero to one mole of label bound per mole of SH-1. The elevated myosin Mg2+-ATPase, which results from SH-1 modification, does not account for the increased actin-activated Mg2+-ATPase in resting conditions (i.e. in the absence of calcium). Thein vitro actin-activated Mg2+-ATPase activities become equal in both active and resting conditions when one mole of SH-1 is modified per mole of myosin head. These results demonstrate that SH-1 is located in a region of the myosin head which plays a part in the calcium-sensitive regulation of the actin-activated Mg2+-ATPase by troponin-tropomyosin. These studies also indicate that SH-1-labelled preparations may not be suitable for the analysis of myosin head motion and/or orientation in the resting state.

The eukaryotic slime mold Dictyostelium discoideum has a single conventional myosin heavy chain gene (mhcA). The elimination of the mhcA gene was achieved by homologous recombination. Two gene replacement plasmids were constructed, each carrying the G418 resistance gene as a selective marker and flanked by either 0.7 kb of 5' coding sequence and 0.9 kb of 3' coding sequence or 1.5 kb of 5' flanking sequence and 1.1 kb of 3' flanking sequence. Myosin null mutants (mhcA- cells) were obtained after transformation with either of these plasmids. The mhcA- cells are genetically stable and are capable of a variety of motile processes. Our results provide genetic proof that in Dictyostelium the conventional myosin gene is required for growth in suspension, normal cell division and sporogenesis, and illustrate how gene targeting can be used as a tool in Dictyostelium.