PKC theta II, a new isoform of protein kinase C specifically expressed in the seminiferous tubules of mouse testis

Protein kinase C (PKC) theta, a Ca2+-independent isoform of PKC, has been k nown to be expressed in skeletal muscle and T cells. In the present study, we isolated and characterized a smaller transcript expressed in the mouse t estis, the cDNA of which is referred hereafter as PKC theta II and the orig inal PKC theta as PKC thetaI. The cDNA clone of PKC theta II 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 PKC thetaI sequence of 44 4 amino acids. Genomic DNA analysis revealed that transcription of PKC thet a II is initiated from the PKC theta II-specific exon, which is located bet ween exons 7 and 8 of the PKC theta gene, indicating that alternative splic ing is the mechanism by which PKC theta II is generated. PKC theta II is ex pressed exclusively in the testis in an age-dependent manner with sexual ma turation. In situ hybridization and reverse transcription-polymerase chain reaction of microdissected tissues clearly demonstrated that PKC theta II i s expressed in the seminiferous tubules of the mouse testis. Consistent wit h its molecular structure lacking the C1 regulatory domain, PKC theta II is constitutively active as determined by an in vitro kinase assay, being ind ependent of PKC activators, e.g. phosphatidylserine and phorbol ester. PKC theta II may play a crucial role in spermatogenesis or some related functio n of the testis.