Asymmetric segregation on spindle poles of the Schizosaccharomyces pombe septum-inducing protein kinase Cdc7p.

Schizosaccharomyces pombe divides by means of a centrally placed division septum. The initiation of septation must be tightly coordinated with events in mitosis, as premature formation of the septum can lethally cut the undivided nucleus. The Spg1p GTPase and the Cdc7p kinase, with which it interacts, play a central role in signaling the initiation of septum formation. Loss-of-function mutations in either gene prevent septation, whereas inappropriate activation of Spg1p can induce septum formation from G1 or G2 interphase cells. Increased expression of either gene leads to multiple rounds of septation without cell cleavage, emphasizing the need for precise cell cycle regulation of their activity. To understand the mechanisms underlying this regulation, we have investigated whether these key initiators of septum formation are controlled by changes in their activity and/or location during mitosis and cytokinesis. We demonstrate that Spg1p localizes to the spindle pole body in interphase and to both spindle poles during mitosis. In contrast, Cdc7p shows no discrete localization during interphase, but early in mitosis it associates with both spindle pole bodies and, as the spindle extends, is seen on only one pole of the spindle during anaphase B. Spg1p activity is required for localization of Cdc7p in vivo but not for its kinase activity in vitro. Staining with an antiserum that recognizes preferentially GDP-Spg1p indicates that activated GTP-Spg1p predominates during mitosis when Cdc7p is associated with the spindle pole body. Furthermore, staining with this antibody shows that asymmetric distribution of Cdc7p may be mediated by inactivation of Spg1p on one spindle pole. Deregulated septation in mutant cells correlates with segregation of Cdc7p to both spindle poles.