Role of SP65 in assembly of the Dictyostelium discoideum spore coat

Like the cyst walls of other protists, the spore coat of Dictyostelium discoideum is formed de novo to protect the enclosed dormant cell from stress. Spore coat assembly is initiated by exocytosis of protein and polysaccharide precursors at the cell surface, followed by the infusion of nascent cellulose fibrils, resulting in an asymmetrical trilaminar sandwich with cellulose filling the middle layer. A molecular complex consisting of cellulose and two proteins, SP85 and SP65, is associated with the inner and middle layers and is required for proper organization of distinct proteins in the outer layer. Here we show that, unlike SP85 and other protein precursors, which are stored in prespore vesicles, SP65 is, like cellulose, synthesized just in time. By tagging the SP65 locus with green fluorescent protein, we find that SP65 is delivered to the cell surface via largely distinct vesicles, suggesting that separate delivery of components of the cellulose-SP85-SP65 complex regulates its formation at the cell surface. In support of previous in vivo studies, recombinant SP65 and SP85 are shown to interact directly. In addition, truncation of SP65 causes a defect of the outer layer permeability barrier as seen previously for SP85 mutants. These observations suggest that assembly of the cellulose-SP85-SP65 triad at the cell surface is biosynthetically regulated both temporally and spatially and that the complex contributes an essential function to outer layer architecture and function.