A PRECURSOR PRODUCT RELATIONSHIP IN MOLLUSCAN SPERM PROTEINS FROM ENSIS MINOR

A cDNA library prepared from mRNA extracted from immature male gonads of the bivalve mollusc Ensis minor (razor shell) was probed with a 133 -bp reverse-transcriptase PCR product corresponding to a segment of th e sperm protein EM6 [Giancotti, V., Russo, E., Gasparini, M., Serrano, D., Del Piero, D., Thorne, A. W., Cary, P. D. & Crane-Robinson, C. (1 993) Eur. J. Biochem. 136, 509-516]. A single 1.5-kb clone was found t o encode both sperm proteins EM1 and EM6. Mass spectrometry was used t o define the C-terminus of EM1, and since the N-terminus of EM6 is kno wn from Edman degradation, this showed that the pentapeptide NTNNS mus t be lost on proteolytic processing. Both EM1 and EM6 contain highly r epeated amino acid sequences, suggestive of extended structures. EM1 c ontains seven tandem repeats of the dipeptide S(K/R), followed by six potential cdc2 phosphorylation sites and seven repeats of the octapept ide KRSASKKR, with occasional WR substitutions. EM6 contains a globula r domain preceded by 17 almost identical uninterupted tandem repeats o f the motif KKRSXSRKRSAS, where X is charged. Its C-terminus contains 15 shea basic clusters. Assignment of EM1 and EM6 to the established c ategories of molluscan sperm proteins [PLI, PLII, PLIII, PLIV; Ausio, J. (1992) Mol. Cell. Biochem 115, 163-172] is discussed.