Two type V myosins with non-overlapping functions in the fission yeast Schizosaccharomyces pombe: Myo52 is concerned with growth polarity and cytokinesis, Myo51 is a component of the cytokinetic actin ring

The fission yeast genome project has identified five myosin genes: one type I myosin, myo1(+), two type II myosins, myo2(+) and myp(2+), and two type V myosins, myo51(+) and myo52(+). Cells deleted for myo51(+) show normal mo rphology and growth rates whereas deletion of myo52(+) results in a partial loss of cell polarity, slow growth and cytokinetic defects. Combining both deletions in a single strain is phenotypically non-additive, myo52 Delta b eing epistatic to myo51 Delta Overproduction of Myo51 gives rise to elongat ed cells which fail to form functional septa whereas overproduction of Myo5 2 results in branched cells with aberrant septa that fail to cleave. Myo52 localises to the poles of growing cells but during cell division it relocal ises to the cell equator as a bar that is bisected by the cytokinetic septu m, Myo51 shows no obvious localisation during interphase but at cytokinesis it is associated with the contractile cytokinetic actin ring (CAR), Both m yosins are dependent upon an intact actin cytoskeleton for localisation. My o52 partially colocalises with the alpha -glucan synthase Mok1 at the cell tips and to a lesser extent at the septum, Mok1 is delocalised and upregula ted in myo52 Delta and myo52 Delta cell walls are resistant to digestion by the cell wall degrading enzyme zymolyase, Thus myo52+ appears to be involv ed in the local delivery or positioning of vesicles containing cell wall pr ecursors at the cell tips and has a role in the maturation or cleavage of t he septum. Myo51 has a non-essential role in cytokinesis as a component of the cytokinetic actin ring.