Homoeostasis between the GTPase Spg1p and its GAP in the regulation of cytokinesis in S. pombe

Abstract

Cytokinesis in Schizosaccharomyces pombe begins at mitotic entry, when the site of division is defined by formation of the contractile acto-myosin ring (CAR) at the cell cortex. Contraction of the CAR and formation of the division septum are triggered at the end of mitosis by septation initiation network (SIN) proteins associated with the spindle pole body (SPB). SIN signalling requires activation of the GTPase Spg1p, which is regulated by the bipartite GTPase-activating protein (GAP) Byr4p-Cdc16p. We show that, for Spg1p to associate with the SPB, it must be bound to its GAP or to its mitotic effector, the protein kinase Cdc7p. Analysis of the GAP proteins reveals that the steady-state level of Byr4p reflects that of Spg1p. Furthermore, if the interaction of Byr4p with Spg1p is compromised, the level of Byr4p decreases dramatically. The adaptation of the level of Byr4p to that of Spg1p requires the presence of Cdc16p and is mediated by proteasome-dependent destruction. It requires neither association with the SPB nor an active SIN. We propose a mechanism that limits the amount of the Byr4p-Cdc16p GAP to the amount required to inhibit Spg1p signalling.Supplementary material available online at

SIN-Inhibitory Phosphatase Complex Promotes Cdc11p Dephosphorylation and Propagates SIN Asymmetry in Fission Yeast

“…The presence of uninucleate cells with a septum and binucleate cells with more than one septum suggested that Csc1p might participate in inhibiting the SIN. Previous studies have shown that the inactivation of SIN inhibitors leads to septation even in interphase [21, 24, 27]. Consistent with Csc1p participating as an inhibitor of the SIN, 4–6% of S phase arrested csc1 Δ cells contained a division septum (Figure 2B).…”

The fission yeast septation initiation network (SIN) kinase, Sid2, is required for SIN asymmetry and regulates the SIN scaffold, Cdc11

“…However, Cdc7 becomes asymmetrically localized to SPBs in cdc11 phosphomutants, indicating that additional Sid2 targets must exist that contribute to the positive-feedback loop. Candidates for additional Sid2 substrates include Cdc7 itself; the Spg1 GAP component Byr4, which is a heavily phosphorylated protein (Song et al , 1996; Krapp et al , 2008; Johnson and Gould, 2011); the Polo kinase Plo1, which is essential for SIN signaling (Ohkura et al , 1995; Bahler et al , 1998a; Mulvihill et al , 1999); and one or more phosphatase complexes that influence the establishment of SIN asymmetry and SIN signaling (Le Goff et al , 2001; Krapp et al , 2003; Lahoz et al , 2010; Singh et al , 2011). Further study will be required to define the complete repertoire of Sid2 substrates influencing the establishment of asymmetric SIN signaling.…”

Analysis ofS. pombeSIN protein SPB-association reveals two genetically separable states of the SIN

“…The first-cycle block to septation in spg1-B8, and the fact that Spg1-B8p-GFP is not associated with the SPB (A.K. and V.S., unpublished data; Krapp et al, 2008), argue that SIN signalling is not functional. Nonetheless, we cannot formally exclude that spg1-B8 is a hypomorphic allele and that the presence of the Cdc7p of the SPB in the first mitosis reflects residual Spg1-B8p function.…”