Sequence and developmental expression of a mRNA encoding a putative protein of rat sperm outer dense fibers

We have isolated a cDNA from rat testis homologous to the Drosophila melanogaster gene mst(3)gl-9 by screening a rat testis cDNA library with mst(3)gl-9 and by direct PCR amplification of upstream sequences out of the cDNA library. Homologous genes are also expressed in testes of different mammalian species. In rat testis, two different transcripts are found. Evidences are presented which suggest that these two transcripts are alternative splicing products. As proved by Northern blot analysis of testis RNA prepared from rats of different ages and by in situ hybridization to rat testis tissue sections, the mRNAs are first transcribed in early spermatids. The longest open reading frame of the cDNA encodes a polypeptide of 244 amino acids which contains 16.4% cysteine, 9.9% proline, and 5.7% glycine and closely resembles the sizes and amino acid compositions of two major polypeptides isolated from outer dense fibers of rat spermatozoa. The COOH-terminal end consists mostly of the tripeptide motif Cys-Gly-Pro, the main motif in D. melanogaster mst(3)gl-9. It is suggested that the isolated rat cDNA encodes a polypeptide which is a protein component of the outer dense fibers of spermatozoa.