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.
Highlights
Cell walls protect a cell from environmental stresses, withstand high internal turgor pressure, and maintain cell shape
Germinated spores in (A, F, and H) were imaged 12–24 h after tetrad dissection onto YE5S plates. (A) Time course of a sbg1Δ cell lysed at 180 min. (B) A sbg1Δ cell germinated on YE5S + sorbitol and imaged after 3 d of growth. (C-E) Micrograph (C) and quantifications (D and E) of sbg1 depletion phenotype of strain 81nmt1-mECitrine-sbg1 grown in YE5S + thiamine for 60 h. (D) Cell width at the widest part of the cell perpendicular to cell long axis
In this study we find that Sbg1, a previously uncharacterized protein with a SKN1 domain from the Skn1/Kre6 family, plays a prominent role in regulating the β-glucan synthase Bgs1
Summary
Cell walls protect a cell from environmental stresses, withstand high internal turgor pressure, and maintain cell shape. In many fungal species including the fission yeast Schizosacchromyces pombe, the cell wall and the septum mainly consist of polysaccharide glucans [7,8]. The F-BAR protein Cdc scaffolds several proteins involved in cytokinesis at the division site and maintains the contractile ring stability during anaphase. Rho-GAP Rga plays a role in β-1,3-glucan synthase Bgs delivery from Golgi to the cleavage site during cytokinesis [29,30]. In A. nidulans, the α-COP protein SodVICp plays a role in cell wall synthesis and protein secretion by affecting β-glucan synthase activity [32]. We investigated the roles of the essential protein Sbg in cytokinesis and revealed a novel mode of regulation for the β-glucan synthase Bgs. We find that Sbg has a more important role than the F-BAR protein Cdc in Bgs trafficking and localization
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