The endo-β-1,3-glucanase eng1p is required for dissolution of the primary septum during cell separation inSchizosaccharomyces pombe

Abstract

Schizosaccharomyces pombe cells divide by medial fission throughout contraction of an actomyosin ring and deposition of a multilayered division septum that must be cleaved to release the two daughter cells. Although many studies have focused on the actomoysin ring and septum assembly,little information is available concerning the mechanism of cell separation. Here we describe the characterization of eng1+, a new gene that encodes a protein with detectable endo-β-1,3-glucanase activity and whose deletion is not lethal to the cells but does interfere in their separation. Electron microscopic observation of mutant cells indicated that this defect is mainly due to the failure of the cells to degrade the primary septum, a structure rich in β-1,3-glucans, that separates the two sisters cells. Expression of eng1+ varies during the cell cycle,maximum expression being observed before septation, and the protein localizes to a ring-like structure that surrounds the septum region during cell separation. This suggests that it could also be involved in the cleavage of the cylinder of the cell wall that covers the division septum. The expression of eng1+ during vegetative growth is regulated by a C2H2 zinc-finger protein (encoded by the SPAC6G10.12c ORF), which shows significant sequence similarity to the Saccharomyces cerevisiae ScAce2p,especially in the zinc-finger region. Mutants lacking this transcriptional regulator (which we have named ace2+) show a severe cell separation defect, hyphal growth being observed. Thus, ace2p may regulate the expression of the eng1+ gene together with that of other genes whose products are also involved in cell separation.