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Background Information: Fragile X mental retardation protein (FMRP) is a selective RNA binding protein that functions as a translational inhibitor. It also plays a role in directing the transport of a subset of mRNAs to their site of translation and several recent reports have implicated microtubule motor proteins in the transport of FMRP-messenger ribonucleoprotein (mRNP) granules in neurons. Earlier work reported the association of the actin-based motor protein myosin Va with FMRP granules., Results: Here, we follow up on this finding and confirm that myosin Va does in fact associate with FMRP and is required for its correct intracellular localisation. FMRP is concentrated in the perinuclear region of myosin Va-null mouse melanoma cells which contrasts starkly with the evenly distributed punctate pattern observed in wild-type cells. Similarly, overexpression of a dominant-negative mutant of myosin Va results in the accumulation of FMRP in large aggregate-like structures. FRAP experiments demonstrate that FMRP is largely immobile in the absence of myosin Va., Conclusions: Combining these data, we propose a model in which myosin Va and kinesin play key roles in the assembly and subsequent transport of FMRP granules along microtubules to the periphery of the cell. Myosin Va captures the complex onto peripheral actin structures and mediates the local delivery of the FMRP granule to the site of mRNA translation., (© 2013 Société Française des Microscopies and Société de Biologie Cellulaire de France. Published by John Wiley & Sons Ltd.)

Myosin Va is a widely expressed actin-based motor protein that binds members of the Rab GTPase family (3A, 8A, 10, 11A, 27A) and is implicated in many intracellular trafficking processes. To our knowledge, myosin Va has not been tested in a systematic screen for interactions with the entire Rab GTPase family. To that end, we report a yeast two-hybrid screen of all human Rabs for myosin Va-binding ability and reveal 10 novel interactions (3B, 3C, 3D, 6A, 6A', 6B, 11B, 14, 25, 39B), which include interactions with three new Rab subfamilies (Rab6, Rab14, Rab39B). Of interest, myosin Va interacts with only a subset of the Rabs associated with the endocytic recycling and post-Golgi secretory systems. We demonstrate that myosin Va has three distinct Rab-binding domains on disparate regions of the motor (central stalk, an alternatively spliced exon, and the globular tail). Although the total pool of myosin Va is shared by several Rabs, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes. We also present evidence that myosin Va is necessary for maintaining a peripheral distribution of Rab11- and Rab14-positive endosomes.

Mammals express three class V myosins. Myosin Va is widely expressed, but enriched in the brain, testes and melanocytes, myosin Vb is expressed ubiquitously, and myosin Vc is believed to be epithelium-specific. Myosin Va is the best characterized of the three and plays a key role in the transport of cargo to the plasma membrane. Its cargo includes cell-surface receptors, pigment and organelles such as the endoplasmic reticulum. It is also emerging that RNA and RNA-BPs (RNA-binding proteins) make up another class of myosin Va cargo. It has long been established that the yeast class V myosin, Myo4p, transports mRNAs along actin cables into the growing bud, and now several groups have reported a similar role for class V myosins in higher eukaryotes. Myosin Va has also been implicated in the assembly and maintenance of P-bodies (processing bodies), cytoplasmic foci that are involved in mRNA storage and degradation. The present review examines the evidence that myosin Va plays a role in the transport and turnover of mRNA.

In the present study we demonstrate an association between mammalian myosin Va and cytoplasmic P bodies, microscopic ribonucleoprotein granules that contain components of the 5'-3' mRNA degradation machinery. Myosin Va colocalizes with several P body markers and its RNAi-mediated knockdown results in the disassembly of P bodies. Overexpression of a dominant-negative mutant of myosin Va reduced the motility of P bodies in living cells. Co-immunoprecipitation experiments demonstrate that myosin Va physically associates with eIF4E, an mRNA binding protein that localizes to P bodies. In contrast, we find that myosin Va does not play a role in stress granule formation. Stress granules are ribonucleoprotein structures that are involved in translational silencing and are spatially, functionally, and compositionally linked to P bodies. Myosin Va is found adjacent to stress granules in stressed cells but displays minimal localization within stress granules, and myosin Va knockdown has no effect on stress granule assembly or disassembly. Combined with recently published reports demonstrating a role for Drosophila and mammalian class V myosins in mRNA transport and the involvement of the yeast myosin V orthologue Myo2p in P body assembly, our results provide further evidence that the class V myosins serve an important role in the transport and turnover of mRNA.

It is becoming increasingly clear that the mammalian class V myosins are involved in a wide range of cellular processes such as receptor trafficking, mRNA transport, myelination in oligodendrocytes and cell division. Using paralog-specific antibodies, we observed significant nuclear localisation for both myosin Va and myosin Vb. Myosin Vb was present in nucleoli where it co-localises with RNA polymerase I, and newly synthesised ribosomal RNA (rRNA), indicating that it may play a role in transcription. Indeed, its nucleolar pattern was altered upon treatment with RNA polymerase I inhibitors. In contrast, myosin Va is largely excluded from nucleoli and is unaffected by these inhibitors. Myosin Vb was also found to physically associate with RNA polymerase I and actin in co-immunoprecipitation experiments. We propose that myosin Vb serves a role in rRNA transcription., ((c) 2009 Wiley-Liss, Inc.)