Messenger Ribonucleic Acid Encoding an Apparent Isoform of Phosphorylase Kinase Catalytic Subunit is Abundant in the Adult Testis

The complete amino acid sequence for a novel member of the protein kinase family was deduced from the nucleotide sequence of a cloned human cDNA. This putative protein kinase, given the preliminary designation "PSK-C3," is similar in primary structure to phosphorylase kinase catalytic subunit (PhK-gamma) isolated from rabbit skeletal muscle. The level of similarity does not appear sufficient, however, to suggest that PSK-C3 represents the human homolog of skeletal muscle PhK-gamma. Rather, it seems likely that PSK-C3 is a novel PhK-gamma isoform. From a cross-species Northern hybridization experiment using adult rat tissue RNA, a transcript homologous to PSK-C3 was found to be abundant in the testis but could not be detected in any of 12 other tissues tested, including skeletal muscle, liver, and ovary. Increasing levels of PSK-C3 mRNA in the testis correlate with postnatal testicular development, suggesting possible hormonal regulation of gene transcription. Energy released by glycogeneolysis in the testis may help fuel the process of spermatogenesis.