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Summary Centrioles are cylindrical assemblies whose peripheral microtubule array displays a 9-fold rotational symmetry that is established by the scaffolding protein SAS6. Centriole symmetry can be broken by centriole-associated structures, such as the striated fibers in Chlamydomonas that are important for ciliary function. The conserved protein CCDC61/VFL3 is involved in this process, but its exact role is unclear. Here, we show that CCDC61 is a paralog of SAS6. Crystal structures of CCDC61 demonstrate that it contains two homodimerization interfaces that are similar to those found in SAS6, but result in the formation of linear filaments rather than rings. Furthermore, we show that CCDC61 binds microtubules and that residues involved in CCDC61 microtubule binding are important for ciliary function in Chlamydomonas. Together, our findings suggest that CCDC61 and SAS6 functionally diverged from a common ancestor while retaining the ability to scaffold the assembly of basal body-associated structures or centrioles, respectively., Graphical Abstract, Highlights • CCDC61 is a paralog of SAS6, XRCC4, XLF, and PAXX • CCDC61 can form protofilaments with a 3-fold screw axis in vitro • CCDC61 binds to microtubules mainly via its coiled-coil domain • Microtubule binding of CCDC61 is important for its function in Chlamydomonas, Members of the XRCC4 superfamily of proteins have scaffolding functions in DNA repair or centriole formation. Ochi et al. describe the centrosomal protein CCDC61 as a member of this superfamily. CCDC61 oligomerizes and also binds to microtubules which plays a role in establishing accessory structures of centrioles in Chlamydomonas.

Insertional mutagenesis, in which a piece of exogenous DNA is integrated randomly into the genomic DNA of the recipient cell, is a useful method to generate new mutants with phenotypes of interest. The unicellular green alga Chlamydomonas reinhardtii is an outstanding model for studying many biological processes. We developed a new computational algorithm, MAPINS (mapping insertions), to efficiently identify insertion sites created by the integration of an APHVIII (aminoglycoside 3'-phosphotransferase VIII) cassette that confers paromomycin resistance. Using whole-genome sequencing data, this method eliminates the need for genomic DNA manipulation and retains all the sequencing information provided by paired-end sequencing. We experimentally verified 38 insertion sites out of 41 sites (93%) identified by MAPINS from 20 paromomycin-resistant strains. Using meiotic analysis of 18 of these strains, we identified insertion sites that completely cosegregate with paromomycin resistance. In six of the seven strains with a mutant phenotype, we demonstrated complete cosegregation of the mutant phenotype and the verified insertion site. In addition, we provide direct evidence of complex rearrangements of genomic DNA in five strains, three of which involve the APHVIII insertion site. We suggest that strains obtained by insertional mutagenesis are more complicated than expected from previous analyses in Chlamydomonas To map the locations of some complex insertions, we designed 49 molecular markers based on differences identified via whole-genome sequencing between wild-type strains CC-124 and CC-125. Overall, MAPINS provides a low-cost, efficient method to characterize insertional mutants in Chlamydomonas.

We determined how the ciliary motor I1 dynein is transported. A specialized adapter, IDA3, facilitates I1 dynein attachment to the ciliary transporter called intraflagellar transport (IFT). Loading of IDA3 and I1 dynein on IFT is regulated by ciliary length., Axonemal dyneins, including inner dynein arm I1, assemble in the cytoplasm prior to transport into cilia by intraflagellar transport (IFT). How I1 dynein interacts with IFT is not understood. We take advantage of the Chlamydomonas reinhardtii ida3 mutant, which assembles the inner arm I1 dynein complex in the cytoplasm but fails to transport I1 into the cilium, resulting in I1 dynein-deficient axonemes with abnormal motility. The IDA3 gene encodes an ∼115-kDa coiled-coil protein that primarily enters the cilium during ciliary growth but is not an axonemal protein. During growth, IDA3, along with I1 dynein, is transported by anterograde IFT to the tip of the cilium. At the tip, IDA3 uncouples from IFT and diffuses within the cilium. IFT transport of IDA3 decreases as cilia lengthen and subsides once full length is achieved. IDA3 is the first example of an essential and selective IFT adapter that is regulated by ciliary length.

In some special hydraulic position control systems, the actuator is required to operate at a high speed in a short time. In order to reduce the installed power of the hydraulic pump station, the accumulator can be used as the auxiliary power source to provide sufficient flow for the actuator in a short time. However, due to factors such as accumulator oil supply pressure variation, hydraulic system nonlinearity, and disturbance of load and frictional resistance, position control of the system is more difficult. Aiming at this problem, this paper designs a nonlinear active disturbance rejection control (NLADRC) strategy based on the mathematical model of electro-hydraulic position control system with energy accumulator as power source, this controller estimates the system internal and external disturbances by nonlinear extended state observer (NLESO) in real time and the disturbance is compensated actively by nonlinear error feedback controller (NLSEF). Finally, the control method is verified through the joint simulation of MATLAB and AMESim. The simulation results show that compared with the PID controller, the NLADRC strategy effectively suppresses the internal and external disturbance of the system, reduces the adverse effect caused by the variable pressure oil supply of the accumulator, improves the position control accuracy of the system, improves the system static and dynamic control effect and verifies the effectiveness and superiority of the NLADRC strategy.

Mutations in transition zone genes change the composition of the ciliary proteome. We isolated new mutations in RPGRIP1L (denotated as RPG1 in algae) that affect the localization of the transition zone protein NPHP4 in the model organism Chlamydomonas reinhardtii. NPHP4 localization is not affected in multiple new intraflagellar transport (IFT) mutants. We compared the proteome of cilia from wild-type and mutants that affect the transition zone (RPGRIP1L) or IFT (IFT172 and DHC1b) by mass spectrometry. The rpg1-1 mutant cilia show the most dramatic increase in cytoplasmic proteins. These nonciliary proteins function in translation, membrane remodeling, ATP production and as chaperonins. These proteins are excluded in isolated cilia from fla11-1 (IFT172) and fla24-1 (DHC1b). Our data support the idea that RPGRIP1L, but not IFT proteins, acts as part of the gate for cytoplasmic proteins. The rpg1-1 cilia lack only a few proteins, which suggests that RPGRIP1L only has a minor role of in the retention of ciliary proteins. The fla11-1 mutant shows the greatest loss/reduction of proteins, and one-third of these proteins have a transmembrane domain. Hence, IFT172 may play a role in the retention of proteins., Summary: RPGRIP1L, a key protein of the transition zone that is needed for NPHP4 localization, helps to exclude cytoplasmic proteins from the cilia in Chlamydomonas reinhardtii.

Figure 1: Protein sequence alignment of IFT81; Figure 2: The IFT81 transcripts and the IFT81 protein at 32˚C are similar to those at 21˚C; Figure 3: Protein sequence alignment of DGR14.; Figure4: Nucleotide sequences and splicing details of IFT121 in various ift121 mutants.; Figure 5: Protein sequence alignment of IFT121.

Asymmetric placement of the photosensory eyespot organelle in Chlamydomonas is patterned by mother-daughter differences between the two basal bodies, which template the anterior flagella. Each basal body is associated with two bundled microtubule rootlets, one with two microtubules and one with four, forming a cruciate pattern. In wild-type cells, the single eyespot is positioned at the equator in close proximity to the plus end of the daughter rootlet comprising four microtubules, the D4. Here we identify mutations in two linked loci, MLT1 and MLT2, which cause multiple eyespots. Antiserum raised against MLT1 localized the protein along the D4 rootlet microtubules, from the basal bodies to the eyespot. MLT1 associates immediately with the new D4 as it extends during cell division, before microtubule acetylation. MLT1 is a low-complexity protein of over 300,000 Daltons. The expression or stability of MLT1 is dependent on MLT2, predicted to encode a second large, low-complexity protein. MLT1 was not restricted to the D4 rootlet in cells with the vfl2-220 mutation in the gene encoding the basal body-associated protein centrin. The cumulative data highlight the role of mother-daughter basal body differences in establishing asymmetry in associated rootlets, and suggest that eyespot components are directed to the correct location by MLT1 on the D4 microtubules. © 2015 Wiley Periodicals, Inc.

To address the mechanisms of ciliary radial spoke assembly, we took advantage of the Chlamydomonas pf27 mutant. The radial spokes that assemble in pf27 are localized to the proximal quarter of the axoneme, but otherwise are fully assembled into 20S radial spoke complexes competent to bind spokeless axonemes in vitro. Thus, pf27 is not defective in radial spoke assembly or docking to the axoneme. Rather, our results suggest that pf27 is defective in the transport of spoke complexes. During ciliary regeneration in pf27, radial spoke assembly occurs asynchronously from other axonemal components. In contrast, during ciliary regeneration in wild-type Chlamydomonas, radial spokes and other axonemal components assemble concurrently as the axoneme grows. Complementation in temporary dikaryons between wild-type and pf27 reveals rescue of radial spoke assembly that begins at the distal tip, allowing further assembly to proceed from tip to base of the axoneme. Notably, rescued assembly of radial spokes occurred independently of the established proximal radial spokes in pf27 axonemes in dikaryons. These results reveal that 20S radial spokes can assemble proximally in the pf27 cilium but as the cilium lengthens, spoke assembly requires transport. We postulate that PF27 encodes an adaptor or modifier protein required for radial spoke – IFT interaction.

Cilia are microtubule based organelles that project from cells. Cilia are found on almost every cell type of the human body and numerous diseases, collectively termed ciliopathies, are associated with defects in cilia, including respiratory infections, male infertility, situs inversus, polycystic kidney disease, retinal degeneration, and Bardet-Biedl Syndrome. Here we show that Illumina-based whole-genome transcriptome analysis in the biflagellate green alga Chlamydomonas reinhardtii identifies 1850 genes up-regulated during ciliogenesis, 4392 genes down-regulated, and 4548 genes with no change in expression during ciliogenesis. We examined four genes up-regulated and not previously known to be involved with cilia (ZMYND10, NXN, GLOD4, SPATA4) by knockdown of the human orthologs in human retinal pigment epithelial cells (hTERT-RPE1) cells to ask whether they are involved in cilia-related processes that include cilia assembly, cilia length control, basal body/centriole numbers, and the distance between basal bodies/centrioles. All of the genes have cilia-related phenotypes and, surprisingly, our data show that knockdown of GLOD4 and SPATA4 also affects the cell cycle. These results demonstrate that whole-genome transcriptome analysis during ciliogenesis is a powerful tool to gain insight into the molecular mechanism by which centrosomes and cilia are assembled.

Whole-genome sequencing (WGS) provides a new platform for the identification of mutations that produce a mutant phenotype. We used Illumina sequencing to identify the mutational profile of three Chlamydomonas reinhardtii mutant strains. The three strains have more than 38,000 changes from the reference genome. NG6 is aflagellate and maps to 269 kb with only one nonsynonymous change; the V12E mutation falls in the FLA8 gene. Evidence that NG6 is a fla8 allele comes from swimming revertants that are either true or pseudorevertants. NG30 is aflagellate and maps to 458 kb that has six nonsynonomous changes. Evidence that NG30 has a causative nonsense allele in IFT80 comes from rescue of the nonswimming phenotype with a fragment bearing only this gene. This gene has been implicated in Jeune asphyxiating thoracic dystrophy. Electron microscopy of ift80-1 (NG30) shows a novel basal body phenotype. A bar or cap is observed over the distal end of the transition zone, which may be an intermediate in preparing the basal body for flagellar assembly. In the acetate-requiring mutant ac17, we failed to find a nonsynonymous change in the 676 kb mapped region, which is incompletely assembled. In these strains, 43% of the changes occur on two of the 17 chromosomes. The excess on chromosome 6 surrounds the mating-type locus, which has numerous rearrangements and suppressed recombination, and the changes extend beyond the mating-type locus. Unexpectedly, chromosome 16 shows an unexplained excess of single nucleotide polymorphisms and indels. Overall, WGS in combination with limited mapping allows fast and accurate identification of point mutations in Chlamydomonas.

SummaryDevelopmental mechanisms that yield multicellular diversity are proving to be well conserved within lineages, generating interest in their origins in unicellular ancestors. We report that molecular regulation of the haploid-diploid transition in Chlamydomonas, a unicellular green soil alga, shares common ancestry with differentiation pathways in land plants. Two homeoproteins, Gsp1 and Gsm1, contributed by gametes of plus and minus mating types respectively, physically interact and translocate from the cytosol to the nucleus upon gametic fusion, initiating zygote development. Their ectopic expression activates zygote development in vegetative cells and, in a diploid background, the resulting zygotes undergo a normal meiosis. Gsm1/Gsp1 dyads share sequence homology with and are functionally related to KNOX/BELL dyads regulating stem-cell (meristem) specification in land plants. We propose that combinatorial homeoprotein-based transcriptional control, a core feature of the fungal/animal radiation, may have originated in a sexual context and enabled the evolution of land-plant body plans.

The sex-determination system of the unicellular green alga, Chlamydomonas reinhardtii, is governed by genes in the mating-type (MT) locus and entails additional genes located in autosomes. Gene expression is initiated by nitrogen starvation, and cells differentiate into plus or minus gametes within 6h. Reviewed is our current understanding of gametic differentiation and fertilization, initiation of zygote development, and the uniparental inheritance of organelle genomes.

CCDC39 and CCDC40 were first identified as causative mutations in primary ciliary dyskinesia patients; cilia from patients show disorganized microtubules, and they are missing both N-DRC and inner dynein arms proteins. In Chlamydomonas, we used immunoblots and microtubule sliding assays to show that mutants in CCDC40 (PF7) and CCDC39 (PF8) fail to assemble N-DRC, several inner dynein arms, tektin, and CCDC39. Enrichment screens for suppression of pf7; pf8 cells led to the isolation of five independent extragenic suppressors defined by four different mutations in a NIMA-related kinase, CNK11. These alleles partially rescue the flagellar length defect, but not the motility defect. The suppressor does not restore the missing N-DRC and inner dynein arm proteins. In addition, the cnk11 mutations partially suppress the short flagella phenotype of N-DRC and axonemal dynein mutants, but do not suppress the motility defects. The tpg1 mutation in TTLL9, a tubulin polyglutamylase, partially suppresses the length phenotype in the same axonemal dynein mutants. In contrast to cnk11, tpg1 does not suppress the short flagella phenotype of pf7. The polyglutamylated tubulin in the proximal region that remains in the tpg1 mutant is reduced further in the pf7; tpg1 double mutant by immunofluorescence. CCDC40, which is needed for docking multiple other axonemal complexes, is needed for tubulin polyglutamylation in the proximal end of the flagella. The CCDC39 and CCDC40 proteins are likely to be involved in recruiting another tubulin glutamylase(s) to the flagella. Another difference between cnk11-1 and tpg1 mutants is that cnk11-1 cells show a faster turnover rate of tubulin at the flagellar tip than in wild-type flagella and tpg1 flagella show a slower rate. The double mutant shows a turnover rate similar to tpg1, which suggests the faster turnover rate in cnk11-1 flagella requires polyglutamylation. Thus, we hypothesize that many short flagella mutants in Chlamydomonas have increased instability of axonemal microtubules. Both CNK11 and tubulin polyglutamylation play roles in regulating the stability of axonemal microtubules., Author Summary Cilia are specialized projections found on the surface of eukaryotic cells. They play crucial sensory functions, as well as motile functions needed for clearing airways or propelling cells. Ciliary motility is perturbed in the inherited disease, Primary Ciliary Dyskinesia (PCD). Two coiled coil domain-containing (CCDC39 and CCDC40) proteins are needed for the assembly of multiple key structures/complexes that are required for generating ciliary motility. Using the unicellular green alga, Chlamydomonas, we have identified a kinase (CNK11) that when mutated is able to partially rescue the short flagella phenotype of the ccdc39 and ccdc40 mutants as well as mutants lacking axonemal dyneins or the N-DRC complex. In addition, CCDC40 is required for tubulin polyglutamylation at the proximal end of flagella. We suggest that substructures like dynein arms and the N-DRC, which are needed for motility, play a second role in stabilizing the axonemal microtubules and are needed for proper length control. The polyglutamylase, TTLL9, and the kinase, CNK11, play roles in stabilizing the axonemal microtubules based on their ability to partially rescue the short flagella phenotypes of multiple mutants.

Flagellar assembly requires intraflagellar transport of components from the cell body to the flagellar tip for assembly. The understanding of flagellar assembly has been aided by the ease of biochemistry and the availability of mutants in the unicellular green alga, Chlamydomonas reinhardtii. In this chapter, we discuss means to identify genes involved in these processes using forward and reverse genetics. In particular, the ease and low cost of whole genome sequencing (WGS) will help to make gene identification easier and promote the understanding of this important process.

Polycystic kidney disease (PKD) is one of the most common genetic diseases in the world and is characterized by chronic renal cystic growth and kidney failure in children and adults. The kidney as well as the liver and pancreas undergo a buildup of fluid-filled cysts. The renal cysts arise from the

Asymmetric placement of the photosensory eyespot organelle in Chlamydomonas is patterned by mother-daughter differences between the two basal bodies, which template the anterior flagella. Each basal body is associated with two bundled microtubule rootlets, one with two microtubules and one with four, forming a cruciate pattern. In wild type cells, the single eyespot is positioned at the equator in close proximity to the plus end of the daughter rootlet comprising four microtubules, the D4. Here we identify mutations in two linked loci, MLT1 and MLT2, which cause multiple eyespots. Antiserum raised against MLT1 localized the protein along the D4 rootlet microtubules, from the basal bodies to the eyespot. MLT1 associates immediately with the new D4 as it extends during cell division, before microtubule acetylation. MLT1 is a low-complexity protein of over 300,000 daltons. The expression or stability of MLT1 is dependent on MLT2, predicted to encode a second large, low-complexity protein. MLT1 was not restricted to the D4 rootlet in cells with the vfl2-220 mutation in the gene encoding the basal body-associated protein centrin. The cumulative data highlight the role of mother-daughter basal body differences in establishing asymmetry in associated rootlets, and suggest that eyespot components are directed to the correct location by MLT1 on the D4 microtubules.

Genetic disorders that affect the development or function of flagella and their associated signalizing pathways are collectively known as ciliopathies. Intraflagellar transport (IFT) is the process that is responsible for the regulation of flagellar growth and signaling. In this study, we perform a single-molecule fluorescence imaging investigation of IFT in a Chlamydomonas mutant (IMP3) that exhibits unexplained phenotypes of slower flagellar regeneration and anterograde IFT speed. Using TIRF microscopy and two-color imaging, we show that the loading and unloading of flagellar membrane signaling proteins is compromised in the IMP3 mutant. We first use BBS4-GFP to quantify IFT speed changes in Chlamydomonas, we then use two-color imaging of IFT20-mCherry and PKD2-GFP to verify that these speed changes correspond to the loading and unloading of cargo membrane signaling proteins onto and from IFT trains, respectively. Preliminary results suggest that the IMP3 mutant is less efficient at the loading of transmembrane cargo proteins onto IFT trains, thus consequently generating a signaling defect.

In the unicellular algae Chlamydomonas reinhardtii, the plus and minus mating types are controlled by a complex locus, MT, where the dominant MID gene in the MT− locus has been shown to be necessary for expression of minus-specific gamete-specific genes in response to nitrogen depletion. We report studies on MID expression patterns during gametogenesis and on a second gene unique to the MT− locus, MTD1. Vegetative cells express basal levels of MID. An early activation of MID transcription after nitrogen removal, and its sequence similarity to plant RWP-RK proteins involved in nitrogen-responsive processes, suggest that Mid conformation/activity may be nitrogen sensitive. A second stage of MID upregulation correlates with the acquisition of mating ability in minus gametes. Knockdown of MTD1 by RNAi in minus strains results in a failure to differentiate into gametes of either mating type after nitrogen deprivation. We propose that intermediate Mid levels are sufficient to activate MTD1 transcription and to repress plus gamete-specific genes and that MTD1 expression in turn allows the threshold-level MID expression needed to turn on minus gamete-specific genes. We further propose that an MTD1-equivalent system, utilizing at least one gene product encoded in the MT+ locus, is operant during plus gametogenesis.

In the past decade, cilia/flagella have come to be known as essential sensory organelles for cells. They perform their sensory duties via signaling pathways comprised of transmembrane signaling proteins (TSP) and intraflagellar transport (IFT) machineries that are responsible for translocating TSPs within the flagellum. While IFT average speed, IFT train frequency, and IFT train size have already been studied, little is known about IFT dynamics at the flagellar tip region where anterograde IFT trains deposit their cargo and rearrange for retrograde transport to the cell body. To address this issue, we use single-molecule fluorescence imaging methods to study the motion of GFP-tagged BBS4, an IFT-associated protein that is part of the BBSome complex, at the flagellar tip in Chlamydomonas reinhardtii. These investigations have yielded the following results: (i) In the tip region, BBSomes remain attached to their cargo TSPs and diffuse along the flagellar membrane with a diffusion coefficient of 1130 nm2/s. This result contrasts with the current understanding that the BBSome either remains bound to IFT train on the microtubule or diffuses in the flagellar lumen. (ii) On average, BBSomes remain at the flagellar tip for 2.6 seconds before undergoing retrograde IFT. (iii) Our BBS4-GFP image-size investigation has helped further uncover the nature of the BBSome's oligomerization on IFT trains both during active transport along the flagellum and during the turn-around period at the flagellar tip.

Whole genome sequencing is a powerful tool in the discovery of single nucleotide polymorphisms (SNPs) and small insertions/deletions (indels) among mutant strains, which simplifies forward genetics approaches. However, identification of the causative mutation among a large number of non-causative SNPs in a mutant strain remains a big challenge. In the unicellular biflagellate green alga Chlamydomonas reinhardtii, we generated a SNP/indel library that contains over 2 million polymorphisms from four wild-type strains, one highly polymorphic strain that is frequently used in meiotic mapping, ten mutant strains that have flagellar assembly or motility defects, and one mutant strain, imp3, which has a mating defect. A comparison of polymorphisms in the imp3 strain and the other 15 strains allowed us to identify a deletion of the last three amino acids, Y313F314L315, in a protein phosphatase 2A catalytic subunit (PP2A3) in the imp3 strain. Introduction of a wild-type HA-tagged PP2A3 rescues the mutant phenotype, but mutant HA-PP2A3 at Y313 or L315 fail to rescue. Our immunoprecipitation results indicate that the Y313, L315, or YFLΔ mutations do not affect the binding of PP2A3 to the scaffold subunit, PP2A-2r. In contrast, the Y313, L315, or YFLΔ mutations affect both the stability and the localization of PP2A3. The PP2A3 protein is less abundant in these mutants and fails to accumulate in the basal body area as observed in transformants with either wild-type HA-PP2A3 or a HA-PP2A3 with a V310T change. The accumulation of HA-PP2A3 in the basal body region disappears in mated dikaryons, which suggests that the localization of PP2A3 may be essential to the mating process. Overall, our results demonstrate that the terminal YFL tail of PP2A3 is important in the regulation on Chlamydomonas mating., Author Summary Whole genome sequencing is a powerful tool to detect changes in genomic DNA. However, how to identify a causative mutation from over 20,000 changes remains a big challenge. For the unicellular green alga Chlamydomonas, we built a library that consists of over 2 million changes from 16 strains. A comparison of changes found in a mutant strain with a mating defect, imp3, to 15 other strains, leads to the identification of a three amino acid deletion in the catalytic subunit of a protein phosphatase 2A (PP2A3). The mating defect of imp3 is rescued by an HA-tagged PP2A3 gene. Introduction of the HA-tagged PP2A3 gene with various mutations in these three amino acids reveals that they play a key role in stabilizing and ensuring the proper localization of PP2A3. The ubiquitous enzyme PP2A is involved in diverse cellular processes. Our discovery that PP2A3 is involved in the Chlamydomonas mating signaling pathway, which also contains the polycystin2 homolog (PKD2), makes Chlamydomonas mating an excellent model to study ciliary/flagellar signaling. Since both PP2A and PKD2 play important roles in human health, further investigation of the relationship between these two proteins in Chlamydomonas will facilitate better understanding of their functions.

Background The building of a cilium or flagellum requires molecular motors and associated proteins that allow the relocation of proteins from the cell body to the distal end and the return of proteins to the cell body in a process termed intraflagellar transport (IFT). IFT trains are carried out by kinesin and back to the cell body by dynein. Methods We used whole genome sequencing to identify the causative mutations for two temperature-sensitive flagellar assembly mutants in Chlamydomonas and validated the changes using reversion analysis. We examined the effect of these mutations on the localization of IFT81, an IFT complex B protein, the cytoplasmic dynein heavy chain (DHC1b), and the dynein light intermediate chain (D1bLIC). Results The strains, fla18 and fla24, have mutations in kinesin-2 and cytoplasmic dynein, respectively. The fla18 mutation alters the same glutamic acid (E24G) mutated in the fla10-14 allele (E24K). The fla18 strain loses flagella at 32°C more rapidly than the E24K allele but less rapidly than the fla10-1 allele. The fla18 mutant loses its flagella by detachment rather than by shortening. The fla24 mutation falls in cytoplasmic dynein and changes a completely conserved amino acid (L3243P) in an alpha helix in the AAA5 domain. The fla24 mutant loses its flagella by shortening within 6 hours at 32°C. DHC1b protein is reduced by 18-fold and D1bLIC is reduced by 16-fold at 21°C compared to wild-type cells. We identified two pseudorevertants (L3243S and L3243R), which remain flagellated at 32°C. Although fla24 cells assemble full-length flagella at 21°C, IFT81 protein localization is dramatically altered. Instead of localizing at the basal body and along the flagella, IFT81 is concentrated at the proximal end of the flagella. The pseudorevertants show wild-type IFT81 localization at 21°C, but proximal end localization of IFT81 at 32°C. Conclusions The change in the AAA5 domain of the cytoplasmic dynein in fla24 may block the recycling of IFT trains after retrograde transport. It is clear that different alleles in the flagellar motors reveal different functions and roles. Multiple alleles will be important for understanding structure-function relationships.

Kinesins are microtubule-based motors that deliver cargo to their destinations in a highly regulated manner. Although numerous regulators of cargo delivery have been identified in recent years, the regulation mechanism for kinesin through the cargo delivery and recycling process is not known. By performing single molecule fluorescence imaging measurements in Chlamydomonas flagella, which are 200 nm in diameter and 10 microns in length with 9 sets of doublet microtubules, we tracked intraflagellar transport (IFT) trains, BBSome cargoes, and kinesin-2 motors through the entire cargo delivery process and determined their reorganization and recycling dynamics. Upon arrival at the microtubule plus end at the flagellar tip, (1) IFT trains and BBSome cargoes remain intact and dissociate away from the kinesins and microtubules together, diffuse, and reorganize along the flagellar membrane over the course of 2.3 seconds before commencing retrograde travel. (2) Kinesin motors remain bound to and diffuse along microtubules for 1.3 seconds before dissociating into the flagellar lumen for recycling.