FKS3 encodes a presumed subunit of 1,3-beta-D-glucan synthase, similar to Fks1p and Gsc2p. 1,3-beta-D-glucan synthase (GS) is an enzymatic complex in the plasma membrane responsible for the synthesis of 1,3-beta-D-glucan, a polysaccharide that is the main structural component of the cell wall (reviewed in 4). The GS complex consists of a catalytic subunit and a regulatory subunit, both of which are essential for the complex activity (5). The regulatory subunit is a GTP-binding protein encoded by RHO1, which acts as an activator responsive to cell morphogenesis signals (6, 7, 8). FKS1 and GSC2 are believed to encode the catalytic subunit with the activity of UDP-glucose:1,3-beta-D-glucan 3-beta-glucose transferase, which catalyzes the transfer of a glucose moiety from UDP-glucose to the glucan chain (9). The two genes encode closely related integral membrane proteins that appear to act as alternate subunits with overlapping functions. FKS1 is primarily expressed during vegetative growth, whereas GSC2 is induced under starvation, during sporulation, and in response to mating pheromones (1). Null mutation of either gene is not lethal, although the fks1 null mutant shows a decrease of glucan and an increase of the chitin and mannoprotein levels in the cell wall (10) and the gsc2 null mutant shows defects in spore wall formation (3). The double null mutant fks1 gsc2 is not viable, indicating that the GS function is essential (1). The third gene, FKS3, has been identified based on sequence similarity to FKS1 and GSC2 and shown to be involved in spore wall assembly by mutant analysis, but its exact function remains unclear (3).

Last updated: 2007-06-22