The unique catalytic subunit of sperm cAMP-dependent protein kinase is the product of an alternative Cα mRNA expressed specifically in spermatogenic cells

cAMP-dependent protein kinase has a central role in the control of mammalian sperm capacitation and motility. Previous protein biochemical studies indicated that the only cAMP-dependent protein kinase catalytic subunit (C) in ovine sperm is an unusual isoform, termed C-s, whose amino terminus differs from those of published C isoforms of other species. Isolation and sequencing of cDNA clones encoding ovine C-s and C alpha 1 (the predominant somatic isoform) now reveal that C-s is the product of an alternative transcript of the C alpha gene. C-s cDNA clones from murine and human testes also were isolated and sequenced, indicating that C-s is of ancient origin and widespread in mammals. In the mouse, C-s transcripts were detected only in testis and not in any other tissue examined, including ciliated tissues and ovaries. Finally, immunohistochemistry of the testis shows that C-s first appears in pachytene spermatocytes. This is the first demonstration of a cell type-specific expression fur any C isoform. The conservation of C-s throughout mammalian evolution suggests that the unique structure of C-s is important in the subunit's localization or function within the sperm.