Myosin VI

Myosin VI

Page updated 26/5/02

Myosin VI was first identified in Drosophila (Kellerman & Miller, 1992). Together with myosin IX, it is the member that breaks the rule that all myosins are "plus" end directed motor proteins (Wells et al, 1999) (See Minus-end motors). Myosin VI is implicated in animal deafness, being pivotally involved in the maintenance of microvilli.

Figure 1. General structure of myosin VIs. This myosin has two insertions in the head domain (cyan, oblong), a single IQ calmodulin binding domain (blue), a self-associating (see right) coiled-coiled domain (purple cylinder) and a C-terminal domain (grey).

Expression
Myosin VI are found in vertebrates (Avraham et al, 1995;1997), nematodes (Baker & Titus, 1997), arthropods (Kellerman & Millar, 1992), but not in plants, yeast, nor has it been discovered in Dictyostelium as yet (Buss et al, 2001). In organisms where it exists, the expression of myosin VI is restricted to a few tissues in vertebrates and even in these tissues the amount of myosin VI tends to be low.

Organism	Accession number	Reference
Caenorhabditis elegans (nematode)		Baker & Titus, 1997
Drosophila melanogaster (fruit fly)	NP_524478	Kellerman & Millar, 1992
Gallus gallus (chicken)	AJ278608; CAB96536	Buss et al, 1998
Homo sapiens (human)	U90236; Q9UM54	Avraham et al, 1997
Morone saxatilis (Striped bass) VIa VIb	AAD52005 AAD52006	Breckler et al, 2000
Mus musculus (mouse)	U49739; Q64331;AAB00194	Avraham et al, 1995
Sus scrofa (pig)	A54818; CAA84559	Hasson et al,
Strongylocentrotus purpuratus (sea urchin)	AF248485	Sirotkin et al, 2000
Table 1 Known Myosin VI cDNAs

Mechanism of Myosin VI
Upon discovering that myosin VI is a pointed end motor (Wells et al, 1999), the authors proposed that this was due to a unique insert of 53 amino-acid in the neck region. This was suggested to comprise the converter/lever arm domain which converted structural changes within the head following ADP release in the opposite direction due to the changed geometry imposed by the 53 amino-acids (Wells et al, 1999; Schliwa 1999). This seemed a very satisfying structure function story, however, almost disappointingly, this was found not to be the case. Experiments in which the head domain of myosin VI (minus the converter) was exchanged with the head of myosin V, showed that the polarity is determined by the head region itself (Homma et al, 2001).

The functions of Myosin VI
Cell locomotion
Being a pointed end directed myosin the functions that myosin VI is put to in the cell are expected to be distinct from the other myosin classes. A number of situations that arise in cells that may require a minus end myosin are tabulated in the page Minus-end myosins. A number of these may involve myosin VI. Myosin VI is located at the leading edge of moving cells (Buss et al, 1998) and in the periphery of neuronal growth cones (Suter et al, 2000). If this population was somehow fixed to cell adhesions, then interaction with F-actin, predominantly presenting their barbed ends to the direction of travel, then the action of myosin VI would tend to push the F-actin array outward. Thus myosin VI may perform a function in lamellae protrusion (Rodriguez & Cheney, 2000).

Figure 2. A diagram of a lamellae extending from right to left. The membrane (blue), is attached to the glass (black line) by adhesion complexes (red blocks). Myosin VI are attached to adhesions and act on the microfilaments pulling the pointed ends towards them and as a result, pushing the barbed ends left ward, the way the cell is protruding.

A role for Myosin VI in endocytosis
Actin filaments tend to be attached to the plasma-membrane by their barbed ends at the cell cortex, so a myosin that moves to the pointed end of actin filaments while remaining attached to membranes would immediately seem ideal for an involvement in endocytosis (Buss et al, 2001a). Although myosin VI has been reported to localise to vesicles (especially clathrin coated vesicles) and the plasma-membrane, and myosin VI immuno-precipitates with both AP-2 and clathrin (Buss et al, 2001b). There is some support for a role in endocytosis from observations in myosin VI mutants that cause deafness in animals where endocytosis is often inefficient (see Hearing and hair cells).

Figure 3. Myosin VI is proposed to drag clathrin coated vesicles into the cell by its association with actin filaments combined with its ability to move toward the pointed ends.

Myosin VI and microvillus structure
Myosin VI are associated with deafness (Melchionda et al, 2001), the protein is localised at the cuticular plate (Hasson et al, 1997). See Hearing & Hair cells. Its function in the hair cell seems clear enough, as in the absence of the functional protein, the membrane is not pulled down between the individual microvilli (figure 4). Myosin VI is also involved in maintenance of the intetinal (Heintzelman et al, 1994) and renal brush border (Biesmesderfer et al, 2002), where it presumably performs the same function.

Figure 4. The role of maintenance of microvillar structures

Binding partner

Function

Reference

CLIP-170

A microtubule binding protein

Lantz & Miller, 1998

DOC-2/DAB2

RAS signalling protein

Inoue et al, 2002; Morris et al, 202

AP-2/Clathrin

Components of the coated pit

Buss et al, 2001

GLUT1CBP

Binds to the glucose transporter GLUT1

Bunn et al, 1999

Table 2 Myosin VI binding proteins

Regulation of Myosin VI.
Myosin VI is phosphorylated by p21 the Rac-activate Ser/Thr kinase (PAK) (Buss et al, 1998), and it is phosphorylated at Thr⁴⁰⁶ (in the motor domain ) by PAK3 (Yoshimura et al, 2001). Another clear regulatory mechanism is the calmodulin light-chain. This phosphorylation of myosin VI at Thr⁴⁰⁶ significantly activated its motility, while calcium inhibited it (Yoshimura et al, 2001).

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References:-

Avraham, K. B., Hasson, T., Steel, K. P., Kingsley, D. M., Russell, L. B., Mooseker, M. S., Copeland, N. G. & Jenkins, N. A. (1995) The mouse Snell's waltzer deafness gene encodes an unconventional myosin required for structural integrity of inner ear hair cells. Nat.Genet. 11, 369-375.

Avraham, K. B., Hasson, T., Sobe, T., Balsara, B., Testa, J. R., Skvorak, A. B., Morton, C. C., Copeland, N. G. & Jenkins, N. A. (1997) Characterization of unconventional myosin MYO6, the human homologue of the gene responsible for deafness in Snell's waltzer mice. Human Mol.Genetics. 6, 1255-1231.

Baker, J.P. & Titus, M.A. (1997) A family of unconventional myosins from the nematode Caenorhabditis elegans. J.Mol.Biol. 272, 523-535.

Biemesderfer, D., Mentone, S. A., Mooseker, M. S. & Hasson, T. (2002) Expression of myosin VI within the early endocytic pathway in adult and developing proximal tubules. Am.J.Physiol.Renal Physiol. 282, F785-F794.

Breckler, J., Au, K., Cheng, J., Hasson, T. & Burnside, B. (2000) Novel myosin VI isoform is abundantly expressed in retina. Exp.Eye Res. 70, 121-134.

Bunn, R. C., Jensen, M. A. & Reed, B. C. (1999) Protein interactions with the glucose transporter binding protein GLUT1CBP that provide a link between GLUT1 and the cytoskeleton. Mol.Biol.Cell. 10, 819-832.

Buss, F., Kendrick-Jones, J., Lionne, C., Knight, A. E., Cote, G. P. & Luzio, J. P. (1998) The localization of myosin VI at the Golgi complex and leading edge of fibroblasts and its phosphorylation and recruitment into membrane ruffles of A431 cells after growth factor stimulation. J.Cell Biol. 143, 1535-1545.

Buss, F., Luzio, J. P. & Kendrick-Jones, J. (2001a) Myosin VI, a new force in clathrin mediated endocytosis. FEBS letters. 508, 295-299.

Buss, F., Arden, S. D., Lindsay, M., Luzio, J. P. & Kendrick-Jones, J. (2001b) Myosin VI isoform localized to clathrin-coated vesicles with a role in clathrin-mediated endocytosis. EMBO J. 20, 3676-3684.

Hasson, T. & Mooseker, M. S. (1994) Porcine myosin-VI: characterization of a new mammalian unconventional myosin. J.Cell Biol. 127, 425-440.

Heintzelman, M. B., Hasson, T. & Mooseker, M. S. (1994) Multiple unconventional myosin domains of the intestinal brush border cytoskeleton. J.Cell.Sci. 107, 3535-3543.

Kellerman, K. A. & Millar, K. G. (1992) An unconventional myosin heavy chain gene from Drosophila melanogaster. J.Cell Biol. 119, 823-834.

Lantz, V. A. & Miller, K. G. (1998) A class VI unconventional myosin is associated with a homologue of a microtubule-binding protein, cytoplasmic linker protein-170, in neurons and at the posterior pole of Drosophila embryos. J.Cell Biol. 140, 897-910.

Melchionda, S., Ahituv, N., Bisceglia, L., Sobe, T., Glaser, F., Rabionet, R., Arbones, M. L., Notarangelo, A., Di Iorio, E., Carella, M., Zelante, L., Estivill, X., Avraham, K. B. & Gasparini, P. (2001) MYO6, the human homologue of the gene responsible for deafness in Snell's Waltzer mice, is mutated in autosomal dominant nonsyndromic hearing loss. Am.J.Hum.Genet. 69, 635-640.

Morris, S. M., Arden, S. D., Roberts, R. C., Kendrick-Jones, J., Cooper, J. A., Luzio, J. P. & Buss, F. (2002) Myosin VI binds to and localises with Dab2, potentially linking receptor-mediated endocytosis and the actin cytoskeleton. Traffic. 3, 331-341.

Rodriquez, O. C. & Cheney, R. E. (2000) A new direction for myosin. Trends Cell Biol. 10, 307-311.

Schliwa, M. (1999) Myosin steps backwards. Nature. 401, 431-432.

Self, T., Sobe, T., Copeland, N. G., Jenkins, N. A., Avraham, K. B. & Steel, K. P. (1999) Role of myosin VI in the differentiation of cochlear hair cells. Dev.Biol. 214, 331-341.

Uchida, G., Chinzei, T. & Matsuura, H. (1999) Reverse motion of organelles with myosin molecules along bundles of the actin filaments in a Characean internodal cell. Biochem.Biophys.Res.Comm. 257, 223-227.

Wells, A. L., Lin, A. W., Chen, L.-Q., Safer, D., Cain, S. M., Hasson, T., Carragher, B. O., Milligan, R. A. & Sweeney, H. L. (1999) Myosin VI is an actin-based motor that moves backwards. Nature. 401, 505-508.

Yoshimura, M., Homma, K., Saito, J., Inoue, A., Ikebe, R. & Ikebe, M. (2001) Dual regulation of mammalian myosin VI motor function. J.Biol.Chem. 276, 39600-39607.