ASYMMETRIC SEGREGATION ON SPINDLE POLES OF THE SCHIZOSACCHAROMYCES-POMBE SEPTUM-INDUCING PROTEIN-KINASE CDC7P

Schizosaccharomyces pombe divides by means of a centrally placed divis ion septum. The initiation of septation must be tightly coordinated wi th events in mitosis, as premature formation of the septum san lethall y cut the undivided nucleus. The Spg1p GTPase and the Cdc7p kinase, wi th which it interacts, play a central role in signaling the initiation of septum formation. Loss-of-function mutations in either gene preven t septation, whereas inappropriate activation of Spg1p ran induce sept um formation from G(1) or G(2) interphase cells. Increased expression of either gene leads to multiple rounds of septation without cell clea vage, 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 mitosi s and cytokinesis. We demonstrate that Spg1p localizes to the spindle pole body in interphase and to both spindle poles during mitosis. In c ontrast, Cdc7p shows no discrete localization during interphase, but e arly in mitosis it associates with both spindle pole bodies and, as th e spindle extends, is seen on only one pole of the spindle during anap hase B. Spg1p activity if required for localization of Cdc7p in vivo b ut not for its kinase activity in vitro. Staining with an antiserum th at recognizes preferentially GDP-Spg1p indicates that activated GTP-Sp g1p predominates during mitosis when Cdc7p, is associated with the spi ndle pole body. Furthermore, staining with this antibody shows that as ymmetric distribution of Cdc7p map be mediated by inactivation of Spg1 p on one spindle pole. Deregulated septation in mutant cells correlate s with segregation of Cdc7p to both spindle poles.