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* Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Hongo, Tokyo 113; and We cloned the myo2 gene of Schizosaccharomyces pombe, which encodes a type II myosin heavy
chain, by virtue of its ability to promote diploidization
in fission yeast cells. The myo2 gene encodes 1,526 amino acids in a single open reading frame. Myo2p shows homology to the head domains and the coiledcoil tail of the conventional type II myosin heavy chain
and carries putative binding sites for ATP and actin. It
also carries the IQ motif, which is a presumed binding
site for the myosin light chain. However, Myo2p apparently carries only one IQ motif, while its counterparts in other species have two. There are nine proline residues, which should break
Division of Cell Proliferation, National Institute for Basic Biology, Okazaki, Aichi 444, Japan
-helix, in the COOH-terminal coiled-coil region of Myo2p. Thus, Myo2p is rather
unusual as a type II myosin heavy chain. Disruption of
myo2 inhibited cell proliferation. myo2
cells showed
normal punctate distribution of interphase actin, but
they produced irregular actin rings and septa and were
impaired in cell separation. Overproduction of Myo2p
was also lethal, apparently blocking actin relocation.
Nuclear division proceeded without actin ring formation and cytokinesis in cells overexpressing Myo2p, giving rise to multinucleated cells with dumbbell morphology. Analysis using tagged Myo2p revealed that Myo2p
colocalizes with actin in the contractile ring, suggesting
that Myo2p is a component of the ring and responsible
for its contraction. Furthermore, genetic evidence suggested that the acto-myosin system may interact with
the Ras pathway, which regulates mating and the maintenance of cell morphology in S. pombe.
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