The Meiosis-Specific Sid2p-related Protein Slk1p Regulates Forespore Membrane Assembly in Fission Yeast

Abstract

Cytokinesis in all organisms involves the creation of membranous barriers that demarcate individual daughter cells. In fission yeast, a signaling module termed the septation initiation network (SIN) plays an essential role in the assembly of new membranes and cell wall during cytokinesis. In this study, we have characterized Slk1p, a protein-kinase related to the SIN component Sid2p. Slk1p is expressed specifically during meiosis and localizes to the spindle pole bodies (SPBs) during meiosis I and II in a SIN-dependent manner. Slk1p also localizes to the forespore membrane during sporulation. Cells lacking Slk1p display defects associated with sporulation, leading frequently to the formation of asci with smaller and/or fewer spores. The ability of slk1⌬ cells to sporulate, albeit inefficiently, is fully abolished upon compromise of function of Sid2p, suggesting that Slk1p and Sid2p play overlapping roles in sporulation. Interestingly, increased expression of the syntaxin Psy1p rescues the sporulation defect of sid2-250 slk1⌬. Thus, it is likely that Slk1p and Sid2p play a role in forespore membrane assembly by facilitating recruitment of components of the secretory apparatus, such as Psy1p, to allow membrane expansion. These studies thereby provide a novel link between the SIN and vesicle trafficking during cytokinesis. INTRODUCTIONCytokinesis in all organisms requires the formation of new membranes between segregated chromosomes, leading eventually to the formation of independent daughter cells (Guertin et al., 2002;Wu et al., 2003;Glotzer, 2001Glotzer, , 2005. In animal cells, assembly of new membranes occurs in coordination with constriction of the actin and myosin containing contractile ring and proceeds in a centripetal direction Gromley et al., 2005;Matheson et al., 2005;Baluska et al., 2006). In plant cells, new membrane growth occurs in a centrifugal manner, and this centrifugal expansion is mediated by targeted vesicle delivery to the spindle midzone (Jurgens, 2005;Baluska et al., 2006). Fungi appear to use two distinct mechanisms depending on the mode of cell cycle regulation. During vegetative growth, as in the case of animal cells, new membrane assembly occurs concomitant with constriction of an actomyosin ring Wu and Pollard, 2005;Wu et al., 2006). In contrast, during meiosis and sporulation, new membrane assembly is initiated around the spindle pole bodies, in an apparently actomyosin ring-independent manner (Shimoda, 2004;Neiman, 2005). Although different cell types use different modes of cytokinesis, elements important for membrane targeting and vesicle fusion are highly conserved. How these conserved molecules participating in membrane biogenesis respond to different signals remains largely unanswered.In recent years, the fission yeast Schizosaccharomyces pombe has become particularly attractive for the study of cytokinesis, due to the ease with which genetic and cell biological analysis are applied in this organism (Gould and Simanis, 1997;McCollum and Gould, 2001;Wu and Pollard, 2005;Wu et al....

Dma1-dependent degradation of Septation Initiation Network proteins during meiosis inSchizosaccharomyces pombe

“…S1A). The failure of cdc14-118 to form spores is consistent with the requirement for its partner Sid1p in spore formation (Krapp et al, 2006), and the results for spg1, cdc11 and sid4 confirm those of earlier studies (Krapp et al, 2006;Yan et al, 2008). The ability of sid2-250 to form spores has been documented and explained previously (see Introduction), but it was surprising to find that mob1-R4 and cdc7-24 formed spores, because these mutant proteins are inactive in vegetative cells at 32˚C (Goyal and Simanis, 2012).…”

“…Heat-sensitive and null mutants have implicated the SIN and its regulators in spore formation (Krapp et al, 2006;Ohtaka et al, 2008;Pérez-Hidalgo et al, 2008;Yan et al, 2008). We extended this analysis to other components of the SIN.…”

“…Loss of SIN signalling produces multinucleate cells, whereas constant activation of the SIN results in multiseptated cells (reviewed by Goyal et al, 2011;. The SIN is also essential during meiosis (Krapp et al, 2006;Ohtaka et al, 2008;Pérez-Hidalgo et al, 2008;Yan et al, 2008) -SIN mutants complete both meiotic divisions, but fail to form spores. SIN signalling is modulated by the nucleotide status of the GTPase Spg1p (Schmidt et al, 1997;Sohrmann et al, 1998), which is regulated by the balance of spontaneous nucleotide exchange, and a GTPase-activating protein (GAP), Cdc16p Minet et al, 1979), with which Spg1p interacts through a scaffold, Byr4p (Furge et al, 1999;Furge et al, 1998;Song et al, 1996).…”

“…During meiosis II, Cdc16p disappears from the SPBs, to be replaced by Cdc7p and Sid1p-Cdc14p. A Sid2p-related protein kinase called Mug27 (also known as Ppk35 or Slk1) is expressed specifically during meiosis (Ohtaka et al, 2008;Pérez-Hidalgo et al, 2008;Yan et al, 2008). Mug27p localises to the SPB during nuclear division and associates with the FSM during spore formation before being degraded at the end of meiosis II.…”

“…Increased expression of the syntaxin Psy1p rescues the sporulation defect of mug27-D sid2-250 (Yan et al, 2008), suggesting that Mug27p and Sid2p cooperate to promote the recruitment of components of the secretory apparatus to allow FSM expansion. Previous studies have shown that the association of all the SIN proteins with the SPB during meiosis depends upon the scaffold proteins Sid4p and Cdc11p (Krapp et al, 2006;Ohtaka et al, 2008;Yan et al, 2008). However, the hierarchy of localisation of the other SIN proteins in meiosis has not been investigated in detail.…”