SP75 is encoded by the DP87 gene and belongs to a family of modular Dictyostelium discoideum outer layer spore coat proteins.

Highly purified spore coats of Dictyostelium discoideum each contained about 5 x 10(6) protein molecules as determined by amino acid composition analysis. By two-dimensional gel electrophoresis the coats were found to contain nine major-abundance and numerous minor protein species, most of which were highly enriched relative to the adjacent interspore matrix. Protein was nearly quantitatively eluted by denaturants and 2-mercaptoethanol, showing that it was not irreversibly cross-linked. Because a reducing agent is required together with denaturants to elute most proteins if their free thiol groups have been prealkylated, it was concluded that the D. discoideum spore coat proteins are disulfide cross-linked into the matrix. One major coat protein, SP75, was partially sequenced and found to be encoded by the previously identified DP87 gene; this finding was supported by additional physical, genetic, biochemical and microscopic evidence. The five major proteins for which genes have been cloned were associated with the outer layer of the coat. In coats missing one or more of four of these proteins as a result of gene disruption, there were physical changes but, with one exception, the other major coat proteins appeared to be incorporated normally. Sequence analysis showed that these five outer layer coat proteins are homologous and consist of alternating sequence motifs related to epithelial mucin repeats, basic proline repeats found in salivary acidic proline-rich proteins, the NH2-terminal subdomain of epidermal growth factor modules and other cysteine repeats. Based on these and other observations, outer layer coat proteins are predicted to organize indeterminately to form a cell surface microenvironment supportive of cellulose morphogenesis during spore coat formation.