CDC12P, A PROTEIN REQUIRED FOR CYTOKINESIS IN FISSION YEAST, IS A COMPONENT OF THE CELL-DIVISION RING AND INTERACTS WITH PROFILIN

As in many other eukaryotic cells, cell division in fission yeast depe nds on the assembly of an actin ring that circumscribes the middle of the cell. Schizosaccharomyces pombe cdc12 is an essential gene necessa ry for actin ring assembly and septum formation. Here we show that cdc 12p is a member of a family of proteins including Drosophila diaphanou s, Saccharomyces cerevisiae BNI1, and S. pombe fus1, which are involve d in cytokinesis or other actin-mediated processes. Using indirect imm unofluorescence, we show that cdc12p is located in the cell division r ing and not in other actin structures. When overexpressed, cdc12p is l ocated at a medial spot in interphase that anticipates the future ring site. cdc12p localization is altered in actin ring mutants. cdc8 (tro pomyosin homologue), cdc3 (profilin homologue), and cdc15 mutants exhi bit no specific cdc12p staining during mitosis. cdc4 mutant cells exhi bit a medial cortical cdc12p spot in place of a ring, midi mutant cell s generally exhibit a cdc12p spot with a single cdc12p strand extendin g in a random direction. Based on these patterns, we present a model i n which ring assembly originates from a single point on the cortex and in which a molecular pathway for the functions of cytokinesis protein s is suggested, Finally, we found that cdc12 and cdc3 mutants show a s ynthetic-lethal genetic interaction, and a proline-rich domain of cdc1 2p binds directly to profilin cdc3p in vitro, suggesting that one func tion of cdc12p in ring assembly is to bind profilin.