Sbg1 Is a Novel Regulator for the Localization of the β-Glucan Synthase Bgs1 in Fission Yeast

Abstract

Glucan synthases synthesize glucans, complex polysaccharides that are the major components in fungal cell walls and division septa. Studying regulation of glucan synthases is important as they are essential for fungal cell survival and thus popular targets for anti-fungal drugs. Linear 1,3-β-glucan is the main component of primary septum and is synthesized by the conserved β-glucan synthase Bgs1 in fission yeast cytokinesis. It is known that Rho1 GTPase regulates Bgs1 catalytic activity and the F-BAR protein Cdc15 plays a role in Bgs1 delivery to the plasma membrane. Here we characterize a novel protein Sbg1 that is present in a complex with Bgs1 and regulates its protein levels and localization. Sbg1 is essential for contractile-ring constriction and septum formation during cytokinesis. Sbg1 and Bgs1 physically interact and are interdependent for localization to the plasma membrane. Bgs1 is less stable and/or mis-targeted to vacuoles in sbg1 mutants. Moreover, Sbg1 plays an earlier and more important role in Bgs1 trafficking and localization than Cdc15. Together, our data reveal a new mode of regulation for the essential β-glucan synthase Bgs1 by the novel protein Sbg1.

Two S. pombe septation phases differ in ingression rate, septum structure, and response to F-actin loss

“…In addition, the fact that many septa imaged through different techniques of EM were observed to exhibit sharp edges in previously published studies (at least 18 previous studies and 39 images) confirms that the three types of sharp septum edge described here are not mere artifacts caused by fixation and dehydration for TEM imaging (Johnson et al, 1973;Kanbe et al, 1989Kanbe et al, , 1993Mateos and Domínguez, 1991;Arai et al, 1998;Barnett and Robinow, 2002;Nakano et al, 2003;Donoso et al, 2005;Osumi et al, 2006;Cortés et al, 2012;Jourdain et al, 2012;Osumi, 2012;Pollard et al, 2012;Muñoz et al, 2013;Oberti et al, 2015;Davidson et al, 2016;Sipiczki, 2016;Wang et al, 2016).…”

Inhibition of cell membrane ingression at the division site by cell wall in fission yeast

“…Next, we considered the possibility that the extracellular glycan matrix inhibited ring contraction and membrane ingression in cps1-191 mutants. The Cps1 is a transmembrane protein that (along with other integral membrane proteins, such as Ags1, Bgs3, and Bgs4) links actomyosin rings underneath the cell membrane to the extracellular glycan matrix (Muñoz et al, 2013;Cortés et al, 2012;Davidson et al, 2016;Sethi et al, 2016;Arasada and Pollard, 2015;Cortés et al, 2005). It was possible that in the absence of division septum synthesis (and thereby cell wall remodeling), the actomyosin rings are permanently fixed to the inactive cps1-191 gene-product anchored at the division site.…”

“…The septum assembly machinery consists of glucan synthases such as β-glucan synthase Bgs1/Cps1 and α-glucan synthase Ags1/Mok1. Cps1 synthesizes the β-glucan matrix at the division site and couples the extracellular glycan matrix to the actomyosin ring via intermediate protein complexes (Sethi et al, 2016;Davidson et al, 2016). The deposition of extracellular glycan matrix coordinates with actomyosin ring contraction and stabilizes the contracting actomyosin ring at the division site (Muñoz et al, 2013).…”

Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe

“…Rga7 has been shown to specifically promote Bgs4 recruitment to the division site (Arasada and Pollard, ). The integral membrane protein Sbg1 has also been shown to promote Bgs1 localization at the septum (Davidson et al , ; Sethi et al , ). Bgs1 and Sbg1 localization are dependent on each other and Sbg1 associates with both Bgs1 as well as the actomyosin ring components thus linking the two (Sethi et al , ).…”

“…In cdc15 mutants, where Bgs1 recruitment to the division site is impaired, the actomyosin ring is not properly anchored to the membrane and has been shown to slide (Roberts-Galbraith et al, 2009;Arasada and Pollard, 2014). The integral membrane protein Sbg1 links the actomyosin ring with Bgs1 (Davidson et al, 2016;Sethi et al, 2016). It is unclear if Bgs1 enzyme activity is needed for anchoring the ring.…”