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* Imperial Cancer Research Fund, London WC2A 3PX, United Kingdom; and As in many other eukaryotic cells, cell division in fission yeast depends on the assembly of an actin
ring that circumscribes the middle of the cell. Schizosaccharomyces pombe cdc12 is an essential gene necessary for actin ring assembly and septum formation. Here we show that cdc12p is a member of a family of
proteins including Drosophila diaphanous, Saccharomyces cerevisiae BNI1, and S. pombe fus1, which are involved in cytokinesis or other actin-mediated processes.
Using indirect immunofluorescence, we show that
cdc12p is located in the cell division ring and not in
other actin structures. When overexpressed, cdc12p is
located at a medial spot in interphase that anticipates
the future ring site. cdc12p localization is altered in actin ring mutants. cdc8 (tropomyosin homologue), cdc3 (profilin homologue), and cdc15 mutants exhibit no
specific cdc12p staining during mitosis. cdc4 mutant
cells exhibit a medial cortical cdc12p spot in place of a
ring. mid1 mutant cells generally exhibit a cdc12p spot
with a single cdc12p strand extending in a random direction. Based on these patterns, we present a model in
which ring assembly originates from a single point on
the cortex and in which a molecular pathway for the
functions of cytokinesis proteins is suggested. Finally,
we found that cdc12 and cdc3 mutants show a syntheticlethal genetic interaction, and a proline-rich domain of
cdc12p binds directly to profilin cdc3p in vitro, suggesting that one function of cdc12p in ring assembly is to
bind profilin.
Department of Molecular and Cell Biology,
University of California, Berkeley, California 94720-3202
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