A new member of the third class in the protein kinase C family, PKC lambda, expressed dominantly in an undifferentiated mouse embryonal carcinoma cell line and also in many tissues and cells.

Protein kinase C (PKC)-related cDNA clones isolated from cDNA libraries of mouse P19 embryonal carcinoma cells and mouse brain encoded a 67-kDa protein, PKC lambda. PKC lambda shows the highest amino acid sequence identity with PKC zeta (72%), the third class of the PKC family. Northern blot analysis showed that the mRNA for PKC lambda is expressed in a wide variety of cells and tissues, including P19 and NIH 3T3 cells, as well as brain, kidney, testis, and ovary. In undifferentiated P19 cells, the mRNA for PKC lambda is the most abundant among all the PKC family members. The differentiation of P19 cells results in an increase in PKC alpha and epsilon, and a decrease in PKC lambda. Antiserum raised against a peptide of PKC lambda identified a 74-kDa protein in P19 cell extracts as well as in extracts from COS cells transfected with the PKC lambda expression plasmid. Autophosphorylation of the PKC lambda that immunoprecipitated with the specific antiserum was observed, indicating that PKC lambda possesses protein kinase activity. A phorbol ester binding assay using intact COS cells expressing PKC lambda failed to detect binding activity specific to PKC lambda at phorbol dibutylate concentrations up to 300 nM, suggesting that PKC lambda does not possess phorbol ester binding activity. These results, in conjunction with the results obtained in parallel experiments with PKC zeta and other PKC members, suggest a biochemical similarity between PKC lambda and zeta and their clear difference from other PKC members.