Protein kinase C contains two phorbol ester binding domains

A series of deletion and truncation mutants of protein kinase C (PKC) were expressed in the baculovirus-insect cell expression system in order to elucidate the ability of various domains of the enzyme to bind phorbol dibutyrate (PDBu). A PKC truncation mutant consisting of only the catalytic domain of the enzyme did not bind 3H]PDBu, whereas a PKC truncation mutant consisting of the regulatory domain (containing the tandem cysteine-rich putative zinc finger regions) bound 3H]PDBu. Deletion of the second conserved region (C2) of PKC did not abolish 3H]PDBu binding, whereas a deletion of the first conserved region (C1) of PKC, containing the two cysteine-rich sequences, completely abolished 3H]PDBu binding. Additional truncation and deletion mutants helped to localize the region necessary for 3H]PDBu binding; all PKC mutants that contained either one of the cysteine-rich zinc finger-like regions possessed phorbol ester binding activity. Scatchard analyses of these mutants indicated that each bound 3H]PDBu with equivalent affinity (21-41 nM); approximately 10-20-fold less than the native enzyme. In addition, a peptide of 146 amino acid residues from the first cysteine-rich region, as well as a peptide of only 86 amino acids residues from the second cysteine-rich region, both bound 3H]PDBu with high affinity (31 +/- 4 and 59 +/- 13 nM, respectively). These data establish that PKC contains two phorbol ester binding domains which may function in its regulation.