Class V myosins are processive actin-based motors that have multiple functions in the cell ranging from mRNA transport, cell polarity, and membrane trafficking.
IntroductionThe myosin family of actin-based molecular mo-tors consists of 15 known classes that are structurallydistinct based on comparisons of the primary struc-ture of the motor domains of the known myosinheavy chain genes 1³. Information regarding thefunction and/or biochemical properties of most ofthese myosin classes is sparse relative to the well-characterized class II and class I myosins; neverthe-less, the range of proposed functions for these myo-sins is already remarkably broad 1. There are anumber of recent reviews that provide an overviewof the rapidly growing myosin gene family 1, 4⁸. Among the best characterized and functionally di-verse of the recently discovered myosin classes arethe class V myosins, the focus of this review (forother reviews see 9¹1). Myosin-V was initially characterized as an unusualcalmodulin binding protein from brain with a num-ber of myosin-like biochemical properties 12¹4. Subsequently, myosin-V heavy chain genes werecloned from mouse, yeast and chicken, thus de¢ningthe ¢fth class of actin-based motors 15¹9. Studiesof the mouse and yeast class V myosins provided the¢rst insights regarding the cellular function of myo-sin-V. Phenotypes of the mutant dilute mouse andthe temperature-sensitive yeast mutant, myo2-66 ledto the hypothesis that class V myosins may functionin cell polarity and membrane tra⁄cking 15, 16. Moreover, mutations in the human ortholog of thedilute heavy chain gene cause Griscelli syndrome, arare recessive disease characterized by pigmentarydilution and in most, but not all cases immunode¢-ciency 20²2. Neurological disorders have also beenreported in Griscelli syndrome patients 22, 23. Sincethese ¢rst studies, a great deal has been learnedabout the biochemistry, biophysics and cellular func-tion of the class V myosins. This review will discussthe emerging evidence that myosin-V is a processiveactin-based motor that has multiple functions in thecell ranging from mRNA transport, cell polarity andmembrane tra⁄cking. There are currently nine complete myosin-V heavychain sequences known (Fig. 1). Analysis of the twocompleted eukaryotic genomes of Saccharomyces ce-revisiae and Caenorhabditis elegans reveals that yeasthave two class V myosins, while C. elegans has asingle class V myosin heavy chain gene. In verte-brates, there are at least three distinct subclasses ofmyosin-V. The three most closely related heavy chainsequences, that of chicken brain myosin-V, the
Reck‐Peterson et al. (2000) reported a review. Class V myosins was evaluated. Class V myosins are processive actin-based motors that have multiple functions in the cell ranging from mRNA transport, cell polarity, and membrane trafficking.