The (1,3)β-D-glucan synthase subunit Bgs1p is responsible for the fission yeast primary septum formation

Cytokinesis is a crucial event in the cell cycle of all living cells. In fungal cells, it requires co-ordinated contraction of an actomyosin ring and synthesis of both plasmatic membrane and a septum structure that will constitute the new cell wall end. Schizosaccharomyces pombe contains four essential putative (1,3)P-D-glucan synthase catalytic subunits, Bgs1p to Bgs4p. Here we examined the function of Bgs1p in septation by studying the lethal phenotypes of bgs1(+) shut-off and bgs1 Delta cells and demonstrated that Bgsl p is responsible and essential for linear (1,3)P-Dglucan and primary septum formation. bgs1(+) shut-off generates a more than 300-fold Bgs1p reduction, but the septa still present large amounts of disorganized linear (1,3)P-D-glucan and partial primary septa. Conversely, both structures are absent in bgs1 Delta cells, where there is no Bgs1p. The septum analysis of bgs1(+)-repressed cells indicates that linear (11,3)beta-D-glucan is necessary but not sufficient for primary septum formation. Linear (1,3)P-D-glucan is the polysaccharicle that specifically interacts with the fluorochrome Calcofluor white in fission yeast. We also show that in the absence of Bgs1p abnormal septa are formed, but the cells cannot separate and eventually die.