date two, of the fifteen unconventional myosins have been discovered to
be minus end directed motors; Myosin VI
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!
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).
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
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
et al, 1998).
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.
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,
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.
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.