The cellulose-binding activity of the PsB multiprotein complex is requiredfor proper assembly of the spore coat and spore viability in Dictyosteliumdiscoideum

The terminal event of spore differentiation in the cellular slime mould Dic tyostelium discoideum is the assembly of the spore coat, which surrounds th e dormant amoeba and allows the organism to survive during extended periods of environmental stress. The spore coat is a polarized extracellular matri x composed of glycoproteins and cellulose. The process of spore coat format ion begins by the regulated secretion of spore coat proteins from the presp ore vesicles (PSVs). Four of the major spore coat proteins (SP96, PsB/SP85, SP70 and SP60) exist as a preassembled multiprotein complex within the PSV s. This complete complex has an endogenous cellulose-binding activity. Muta nt strains lacking either the SP96 or SP70 proteins produce partial complex es that do not have cellulose-binding activity, while mutants lacking SP60 produce a partial complex that retains this activity. Using a combination o f immunofluorescence microscopy and biochemical methods we now show that th e lack of cellulose-binding activity in the SP96 and SP70 mutants results i n abnormally assembled spore coats and spores with greatly reduced viabilit y. In contrast, the SP60 mutant, in which the PsB complex retains its cellu lose-binding activity, produces spores with apparently unaltered structure and viability. Thus, it is the loss of the cellulose-binding activity of th e PsB complex, rather than the mere loss of individual spore coat proteins, that results in compromised spore coat structure. These results support th e idea that the cellulose-binding activity associated with the complete PsB complex plays an active role in the assembly of the spore coat.