Deletion of the regulatory domain of protein kinase C α exposes regions in the hinge and catalytic domains that mediate nuclear targeting

Members of the protein kinase C (PKC) family are characterized by an NH₂-terminal regulatory domain containing binding sites for calcium, phosphatidylserine, and diacylglycerol (or tumor-promoting phorbol esters), a small central hinge region and a COOH-terminal catalytic domain. We have constructed fusion proteins in which the regulatory domain of PKCa was removed and replaced by a 19-amino acid leader sequence containing a myristoylation consensus or by the same sequence in which the amino-terminal glycine was changed to alanine to prevent myristoylation. The goal was to generate constitutively active mutants of PKC that were either membrane bound, due to their myristoylation, or cytoplasmic. Western blotting of fractions from COS cells transfected with plasmids encoding wild-type and mutant proteins revealed that PKCα resided entirely in a Triton X-100 soluble (TS) fraction, whereas both the myristoylated and nonmyristoylated mutants were associated primarily with the nuclear envelope fraction. A similar mutant that lacked the 19 amino acid leader sequence was also found almost entirely in the nuclear envelope, as was a truncation mutant containing only the regulatory domain, hinge region, and a small portion of the catalytic domain. However, an additional truncation mutant consisting of only the regulatory domain plus the first one-third of the hinge region was almost entirely in the TS fraction. A nonmyristoylated fusion protein containing only the catalytic domain was also found in the nuclear envelope. Immunostaining of cells transfected with these constructs revealed that both the myristoylated and nonmyristoylated mutants were localized in nuclei, whereas wild-type PKCa was primarily cytoplasmic and perinuclear. Phorbol dibutyrate treatment of PKCa-transfected cells resulted in increased perinuclear and nuclear staining. The results are consistent with a model in which activation of PKC, by phorbol esters or by deletion of the regulatory domain, exposes regions in the hinge and catalytic domains that interact with a PKC "receptor" present in the nuclear envelope, and may explain the ability of wild-type PKC to be translocated to the nucleus under certain conditions.