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Cellular situations that require a minus end myosin motor.

This page updated 25/5/02

To date two, of the fifteen unconventional myosins have been discovered to be minus end directed motors; Myosin VI (Wells et al, 1999) and Myosin IX (Inoue et al, 2002). The presence of these minus end motors makes sense of a number of perplexing observations over the years!  

1). As cells round up in preparation for cytokinesis, the cell is held in place by slender "guy ropes" containing actin uni-directional microfilaments with their barbed ends at the adhesion, however nodules can be seen travelling down and up the  retraction fibres (Cramer & Mitchison, 1997).

2).  Another observation that by be explained by a minus end motor is that a sub-population of organelles in the algae Chara moves in the "wrong direction" (Uchida et al, 1999) (However, so far no Myosin VI or Myosin IXs have been described in plants!!).

3).  The locust (and presumably many insects) have an ingenious method to acclimatise to different light conditions that involves organelles being transported in order to change the refractive index of the cell allowing more or less light to be taken into the photoreceptors (Sturmer et al, 1995).  One of these motions seems to require a minus-end directed myosin motor (Sturmer et al, 1998).

References:-

Cramer, L. P. & Mitchison, T. J. (1997) Investigation of the mechanism of retraction of the cell margin and rearward flow of nodules during mitotic cell rounding, Mol.Biol.Cell. 8, 109-119.

Inoue, A., Saito, J., Ikebe, R. & Ikebe, M. (2002) Myosin IXb is a single-headed minus-end-directed processive motor., Nature Cell Biol. 4, 302-306.

Stürmer, K., Baumann, O. & Walz, B. (1995) Actin-dependent light-induced translocation of mitochondria and ER cisternae in the photoreceptor cells of the locust Schistocerca gregaria. J.Cell Sci. 108, 2273-2283.

Stürmer, K. & Baumann, O. (1998) Immunolocalization of a putative unconventional myosin on the surface of motile mitochondria in locust photoreceptors. Cell Tissue Res. 292, 219-227.

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.

 
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