THE PSB GLYCOPROTEIN COMPLEX IS SECRETED AS A PREASSEMBLED PRECURSOR OF THE SPORE COAT IN DICTYOSTELIUM-DISCOIDEUM

The PsB glycoprotein in Dictyostelium discoideum is one of a diverse g roup of developmentally regulated, prespore-cell-specific proteins, th at contain a common O-linked oligosaccharide. This post-translational modification is dependent on the wild-type modB allele. The PsB protei n exists as part of a multiprotein complex of six different proteins, which have different post-translational modifications and are held tog ether by both covalent and noncovalent interactions (Watson et al. (19 93). J. Biol. Chem. 268, 22634-22641). In this study we have used micr oscopic and biochemical analyses to examine the cellular localization and function of the PsB complex during development. We found that the PsB complex first accumulates in prespore vesicles in slug cells and i s secreted later during culmination and becomes localized to both the extracellular matrix of the apical spore mass of mature fruiting bodie s and to the inner layer of the spore coat. The PsB associated with th e spore coat is covalently bound by disulfide bridges. The PsB protein always exists in a multiprotein complex, but the composition of the P sB complex changes during secretion and spore maturation. Some of the PsB complex proteins have been identified as spore coat proteins. Thes e data demonstrate that some of the proteins that form the spore coat exist as a preassembled precursor complex. The PsB complex is secreted in a developmentally regulated manner during the process of spore dif ferentiation, at which time proteins of the complex, as well as additi onal spore coat proteins, become covalently associated in at least two forms of extracellular matrix: the interspore matrix and the spore co at. These and other studies show that proteins with modB dependent O-l inked oligosaccharides are involved in a wide variety of processes und erlying morphogenesis in this organism. These developmental processes are the direct result of cellular mechanisms regulating protein target ing, assembly and secretion, and the assembly of specific extracellula r matrices.