The last 10 amino acid residues beyond the hydrophobic motif are critical for the catalytic competence and function of protein kinase Cα

The segment C-terminal to the hydrophobic motif at the V5 domain of protein kinase C (PKC) is the least conserved both in length and in amino acid identity among all PKC isozymes. By generating serial truncation mutants followed by biochemical and functional analyses, we show here that the very C terminus of PKC alpha is critical in conferring the full catalytic competence to the kinase and for transducing signals in cells. Deletion of one C-terminal amino acid residue caused the loss of similar to 60% of the catalytic activity of the mutant PKC alpha, whereas deletion of 10 C-terminal amino acid residues abrogated the catalytic activity of PKC alpha in immune complex kinase assays. The PKC alpha C-terminal truncation mutants were found to lose their ability to activate mitogen-activated protein kinase, to rescue apoptosis induced by the inhibition of endogenous PKC in COS cells, and to augment melatonin-stimulated neurite outgrowth. Furthermore, molecular dynamics simulations revealed that the deletion of 1 or 10 C-terminal residues results in the deformation of the V5 domain and the ATP-binding pocket, respectively. Finally, PKC alpha immunoprecipitated using an antibody against its C terminus had only marginal catalytic activity compared with that of the PKC alpha immunoprecipitated by an antibody against its N terminus. Therefore, the very C-terminal tail of PKC alpha is a novel determinant of the catalytic activity of PKC and a promising target for selective modulation of PKC alpha function. Molecules that bind preferentially to the very C terminus of distinct PKC isozymes and suppress their catalytic activity may constitute a new class of selective inhibitors of PKC.