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Myosin-motors are conserved from yeast to human and transport a great variety of cargoes. Most plus-end directed myosins. which constitute the vast majority of all myosin motors, form stable dimers and interact constitutively with their cargo complexes. To date, little is known about regulatory mechanisms for cargo-complex assembly. In this study, we show that the type V myosin Myo4p binds to its cargo via two distinct binding regions, the C-terminal tail and a coiled-coil domain-containing fragment. Furthermore, we find that Myo4p is strictly monomeric at physiologic concentrations. Because type V myosins are thought to require dimerization for processive movement, a mechanism must be in place to ensure that oligomeric Myo4p is incorporated into cargo- translocation complexes. Indeed, we find that artificial dimerization of the Myo4p C-terminal tail promotes stabilization of myosin- cargo complexes, suggesting that full-length Myo4p dimerizes in the cocomplex as well. We also combined the Myo4p C-terminal tail with the coiled-coil region, lever arm, and motor domain from a different myosin to form constitutively dimeric motor proteins. This heterologous motor successfully translocates its cargo in vivo, suggesting that wild-type Myo4p may also function as a dimer during cargo-complex transport. [ABSTRACT FROM AUTHOR]

In eukaryotes, developmental processes and cell differentiation, as well as basic cellular functions require the propagation of information in an asymmetric manner. Localization of mRNA is a key mechanism to establish asymmetric cell fate. The first part of this review provides an overview of our current knowledge of motor protein-dependent mRNA transport in eukaryotes. The second part provides a more detailed description of the most comprehensively studied mRNA translocation complex to date: the ASH1 messenger ribonucleoprotein particle (mRNP) from Saccharomyces cerevisiae. During budding of yeast, the ASH1 mRNP transports cell fate determinants exclusively into the daughter cell. The core factors of the ASH1 mRNP have been identified, their interactions have been studied in detail, and the three-dimensional structure of its mRNA-binding protein, She2p, has been determined. Because no other mRNP has been studied in such detail, the ASH1 mRNP could serve as a model for asymmetric segregation of cell fate determinants in higher eukaryotes. [ABSTRACT FROM AUTHOR]

Cytokinesis in Saccharomyces cerevisiae involves coordination between actomyosin ring contraction and septum formation and/or targeted membrane deposition. We show that Mlc1p, a light chain for Myo2p (type V myosin) and lqg1p (IQGAP), is the essential light chain for Myo1p, the only type II myosin in S. cerevisiae. However, disruption or reduction of MIc1p-Myo1p inter- action by deleting the Mlc1p binding site on Myo1p or by a point mutation in MLC1, mlc 1-93, did not cause any obvious defect in cytokinesis. In contrast, a different point mutation, mlcl-11, displayed defects in cytokinesis and in interactions with Myo2p and lqg1p. These data suggest that the major function of the Mlc1p-Myo1p interaction is not to regulate Myo1p activity but that Mlc1p may interact with Myo1p, lqg1p, and Myo2p to coordinate actin ring formation and targeted membrane deposition during cytokinesis. We also identify Mlc2p as the regulatory light chain for Myo1p and demonstrate its role in Myo1p ring disassembly, a function likely conserved among eukaryotes. [ABSTRACT FROM AUTHOR]

Myo4p is a nonessential type V myosin required for the bud tip localization of ASH1and IST2 mRNA. These mRNAs associate with Myo4p via the She2p and She3p proteins. She3p is an adaptor protein that links Myo4p to its cargo. She2p binds to ASH1 and 1ST2 mRNA, while She3p binds to both She2p and Myo4p. Here we show that Myo4p and She3p, but not She2p, are required for the inheritance of cortical ER in the budding yeast Saccharomyces cerevisiae. Consistent with this observation, we find that cortical ER inheritance is independent of mRNA transport. Cortical ER is a dynamic network that forms cytoplasmic tubular connections to the nuclear envelope. ER tubules failed to grow when actin polymerization was blocked with the drug latrunculin A (Lat-A). Additionally, a reduction in the number of cytoplasmic ER tubules was observed in Lat-A--treated and myo44 cells. Our results suggest that Myo4p and She3p facilitate the growth and orientation of ER tubules. [ABSTRACT FROM AUTHOR]

mRNA localization and localized translation is a common mechanism that contributes to cell polarity and cellular asymmetry. In metazoan, mRNA transport participates in embryonic axis determination and neuronal plasticity. Since the mRNA localization process and its molecular machinery are rather complex in higher eukaryotes, the unicellular yeast Saccharomyces cerevisiae has become an attractive model to study mRNA localization. Although the focus has so far been on the mechanism of ASH1 mRNA transport, it has become evident that mRNA localization also assists in protein sorting to organelles, as well as in polarity establishment and maintenance. A diversity of different pathways has been identified that targets mRNA to their destination site, ranging from motor protein-dependent trafficking of translationally silenced mRNAs to co-translational targeting, in which mRNAs hitch-hike to organelles on ribosomes during nascent polypeptide chain elongation. The presence of these diverse pathways in yeast allows a systemic analysis of the contribution of mRNA localization to the physiology of a cell.