Differential Expression of the Cs and Cα1 Isoforms of the Catalytic Subunit of Cyclic 3′,5′-Adenosine Monophosphate-Dependent Protein Kinase in Testicular Cells1

The amino terminus of the sperm cAMP-dependent protein kinase catalytic subunit (termed C(s)) differs from that of the Calpha1 isoform expressed in most tissues due to the use of alternative transcripts of the Calpha gene. Both Calpha1 and C(s) transcripts are present in testis; C(s) is expressed specifically in spermatogenic cells and is the only C isoform detected in mature sperm. Immunohistochemistry of mouse testis using antibodies specific for C(s) and Calpha1 now shows that Calpha1 is present in somatic testicular cells, spermatogonia, and preleptotene spermatocytes but not in cells that are in later stages of spermatogenesis. In contrast, C(s) is expressed only in midpachytene and later stage spermatocytes and in spermatids. Therefore, C(s) and Calpha1 expression do not overlap. Immunofluorescence microscopic localization of C(s) in murine and ovine sperm reveals that C(s) is located primarily in sperm tail components, including the midpiece mitochondria and the axoneme. Quantitative analysis of Western blots indicates that individual ovine sperm contain approximately 4 x 10(5) molecules of C(s), a seemingly large number for a protein that acts catalytically.