THE SPG1P GTPASE IS AN ESSENTIAL, DOSAGE-DEPENDENT INDUCER OF SEPTUM FORMATION IN SCHIZOSACCHAROMYCES-POMBE

The spg1 gene (septum-promoting GTPase) was cloned as a multicopy supp ressor of a dominant-negative mutant of the Cdc7p kinase. It encodes a small GTPase of the Ras superfamily. spg1 is an essential gene. Null or heat-sensitive alleles do not make a division septum, but growth, S -phase, and mitosis continue in the absence of cell division, producin g elongated, multinucleate cells. Increased expression of Spg1p induce s septum formation in G(2), S-phase, and pre-Start G(1)-arrested cells . This requires the activity of Cdc7p kinase, but not p34(cdc2). Incre ased expression of Cdc7p bypasses the requirement for Spg1p. Spg1p and Cdc7p can be coimmunoprecipitated from cell extracts, and interact in the two-hybrid system. These data indicate that Spg1p is a key elemen t in controlling the onset of septum formation in Schizosaccharomyces pombe, and that it acts through the Cdc7p kinase.