Identification of Acidic Amino Acid Residues in the Protein Kinase Cα V5 Domain That Contribute to Its Insensitivity to Diacylglycerol

The protein kinase C (PKC) isoforms are maintained in an inactive and closed conformation by intramolecular interactions. Upon activation these are disrupted by activators, binding proteins and cellular membrane. We have seen that autophosphorylation of two sites in the C-terminal V5 domain is crucial to keep PKCα insensitive to the activator diacylglycerol, which presumably is caused by a masking of the diacylglycerol-binding C1a domain. Here we demonstrate that the diacylglycerol sensitivity of the PKCβ isoforms also is suppressed by autophosphorylation of the V5 sites. To analyze conformational differences, a fusion protein ECFP-PKCα-EYFP was expressed in cells and the FRET signal was analyzed. The analogous mutant with autophosphorylation sites exchanged for alanine gave rise to a substantially lower FRET signal than wild-type PKCα indicating a conformational difference elicited by the mutations. Expression of the isolated PKCα V5 domain led to increased diacylglycerol sensitivity of PKCα. We identified acidic residues in the V5 domain that, when mutated to alanines or lysines, rendered PKCα sensitive to diacylglycerol. Furthermore, mutation to glutamate of four lysines in a lysine-rich cluster in the C2 domain gave a similar effect. Simultaneous reversal of the charges of the acidic residues in the V5 and the lysines in the C2 domain gave rise to a PKCα that was insensitive to diacylglycerol. We propose that these structures participate in an intramolecular interaction that maintains PKCα in a closed conformation. The disruption of this interaction leads to an unmasking of the C1a domain and thereby increased diacylglycerol sensitivity of PKCα.