THE SCHIZOSACCHAROMYCES-POMBE CDC3+ GENE ENCODES A PROFILIN ESSENTIALFOR CYTOKINESIS

The fission yeast Schizosaccharomyces pombe divides by medial fission and, like many higher eukaryotic cells, requires the function of an F- actin contractile ring for cytokinesis. In S. pombe, a class of cdc(-) mutants defective for cytokinesis, but not for DNA replication, mitos is, or septum synthesis, have been identified. In this paper, we prese nt the characterization of one of these mutants, cdc3-124. Temperature shift experiments reveal that mutants in cdc3 are incapable of formin g an F-actin contractile ring. We have molecularly cloned cdc3 and use d the cdc3(+) genomic DNA to create a strain carrying a cdc3 null muta tion by homologous recombination in vivo. Cells bearing a cdc3-null al lele are inviable. They arrest the cell cycle at cytokinesis without f orming a contractile ring. DNA sequence analysis of the cdc3(+) gene r eveals that it encodes profilin, an actin-monomer-binding protein. In light of recent studies with profilins, we propose that Cdc3-profilin plays an essential role in cytokinesis by catalyzing the formation of the F-actin contractile ring. Consistent with this proposal are our ob servations that Cdc3-profilin localizes to the medial region of the ce ll where the F-actin contractile ring forms, and that it is essential for F-actin ring formation. Cells overproducing Cdc3-profilin become e longated, dumbbell shaped, and arrest at cytokinesis without any detec table F-actin staining. This effect of Cdc3-profilin overproduction is relieved by introduction of a multicopy plasmid carrying the actin en coding gene, act1(+). We attribute these effects to potential sequeste ration of actin monomers by profilin, when present in excess.