PKCθII, a New Isoform of Protein Kinase C Specifically Expressed in the Seminiferous Tubules of Mouse Testis

Protein kinase C (PKC) θ, a Ca 2+ -independent isoform of PKC, has been known to be expressed in skeletal muscle and T cells. In the present study, we isolated and characterized a smaller transcript expressed in the mouse testis, the cDNA of which is referred hereafter as PKCθII and the original PKCθ as PKCθI. The cDNA clone of PKCθII has 2184 base pairs and 464 amino acids in the possible open reading frame, consisting of the 5′ unique sequence of 20 amino acids and the PKCθI sequence of 444 amino acids. Genomic DNA analysis revealed that transcription of PKCθII is initiated from the PKCθII-specific exon, which is located between exons 7 and 8 of the PKCθ gene, indicating that alternative splicing is the mechanism by which PKCθII is generated. PKCθII is expressed exclusively in the testis in an age-dependent manner with sexual maturation. In situ hybridization and reverse transcription-polymerase chain reaction of microdissected tissues clearly demonstrated that PKCθII is expressed in the seminiferous tubules of the mouse testis. Consistent with its molecular structure lacking the C1 regulatory domain, PKCθII is constitutively active as determined by an in vitro kinase assay, being independent of PKC activators, e.g. phosphatidylserine and phorbol ester. PKCθII may play a crucial role in spermatogenesis or some related function of the testis.