Asymmetric segregation on spindle poles of theSchizosaccharomyces pombeseptum-inducing protein kinase Cdc7p

Abstract

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 G 1 or G 2 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.

Septum formation in Aspergillus nidulans

“…In S. pombe, the SIN/MEN is regulated by a Ras superfamily GTPase, Spg1, which, in its active GTP-bound state, controls the localization of the Cdc7 protein kinase [26]. Similarly, in S. cerevisiae, the Cdc7 orthologue Cdc15 is activated by the GTPase Tem1 [27].…”

Yeast polo-like kinases: functionally conserved multitask mitotic regulators

“…At present, it is not known whether Plo1 phosphorylates and regulates Byr4 function as observed with budding yeast homologs. However, similar to the Tem1-Cdc15 interaction in budding yeast (Ro et al, 2002), the GTP-bound Spg1 recruits and binds directly with Cdc7 (Cdc15 homolog) at the SPB after the dissociation of Byr4/Cdc16 from the SPB (Sohrmann et al, 1998;Li et al, 2000). Sid1-Cdc14 is then recruited to the SPB that contains activated Spg1 and Cdc7 (Guertin et al, 2000).…”

“…Genetic analysis places Plo1 upstream of the Spg1 GTPase (Tem1 homolog) (Mulvihill et al, 1999;Tanaka et al, 2001). Spg1 localizes to the SPB throughout the cell cycle (Sohrmann et al, 1998), and the two-component GAP composed of Byr4 and Cdc16 (Bfa1 and Bub2 homologs, respectively) (Furge et al, 1998) inhibits Spg1 until the initiation of mitosis. At present, it is not known whether Plo1 phosphorylates and regulates Byr4 function as observed with budding yeast homologs.…”

cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation inSchizosaccharomyces pombe

“…There are, however, no putative modification sites for this enzyme in the Cut11p sequence. The polo kinase plo1 ϩ (Ohkura et al, 1995;Lane and Nigg, 1996) and the cdc7 ϩ kinase (Fankhauser and Simanis, 1994;Sohrmann et al, 1998) have also been localized to the SPB or affect SPB behavior. It will be interesting to explore any interactions between these enzymes and cut11 ϩ in the future.…”