A NEW MEMBER OF THE PROTEIN-KINASE-C FAMILY, NPKC-THETA, PREDOMINANTLY EXPRESSED IN SKELETAL-MUSCLE

A new protein kinase C (PKC)-related cDNA with unique tissue distribution has been isolated and characterized. This cDNA encodes a protein, nPKC-theta, which consists of 707 amino acid residues and showed the highest sequence similarity to nPKC-delta (67.0% in total). nPKC-theta has a zinc-finger-like cysteine-rich sequence (C1 region) and a protein kinase domain sequence (C3 region), both of which are common in all PKC family members. However, nPKC-theta lacks a putative Ca2+ binding region (C2 region) that is seen only in the conventional PKC subfamily (cPKC-alpha, -beta-I, beta-II, and -gamma) but not in the novel PKC subfamily (nPKC-delta, -epsilon, -zeta, and -eta). Northern (RNA) blot analyses revealed that the mRNA for nPKC-theta is expressed predominantly in skeletal muscle. Furthermore, nPKC-theta mRNA is the most abundantly expressed PKC isoform in skeletal muscle among the nine PKC family members. nPKC-theta expressed in COS1 cells serves as a phorbol ester receptor. By the use of an antipeptide antibody specific to the D2-D3 region of the nPKC-theta sequence, nPKC-theta was recognized as a 79-kDa protein upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis in mouse skeletal muscle extract and also in an extract from COS1 cells transfected with an nPKC-theta cDNA expression plasmid. Autophosphorylation of immunoprecipitated nPKC-theta was observed; it was enhanced by phosphatidylserine and 12-O-tetradecanoylphorbol-13-acetate but attenuated by the addition of Ca2+. These results clearly demonstrate that nPKC-theta should be considered a member of the PKC family of proteins that play crucial roles in the signal transduction pathway.