Sequence and characterization of the sperm protein Sp17 from the baboon

In this study, cDNAs encoding the sperm protein Sp17 from the baboon (Papio papio) have been cloned and sequenced. Three clones, differing in the lengths of their 3' untranslated regions, were identified, which were encoded by mRNA transcripts of 0.8-1.35 kb. The open reading frame encodes 163 amino acids with a predicted molecular mass of 18.8 kDa. The baboon Sp17 protein sequence is 97% identical to human Sp17 but differs significantly by the addition of 12 amino acids at the C-terminal, providing an additional potential protein kinase C phosphorylation site. Northern blot analysis demonstrated that the baboon Sp17 mRNA was specific to the baboon testes and was not detected in the ovary, placenta, or any of the other somatic tissues tested. Western blot analysis using anti-Sp17 antibodies demonstrated that the native baboon sperm Sp17 protein consists of a doublet with an apparent M(r) of 26.5 and 27.2 kDa. Immunocytochemical staining of baboon testis with anti-Sp17 antibodies demonstrated Sp17 in spermatocytes, spermatids, and spermatozoa within the seminiferous epithelium. No specific staining was observed on spermatogonia, Sertoli cells, Leydig cells, or other somatic cell types.