Synthesis and characterization of model peptides incorporating the phorbol ester-binding domain of protein kinase C

Protein kinase C (PKC), a key enzyme family involved in cellular signal transduction, is the main target of the potent phorbol ester-type tumor promoters. In order to elucidate at the molecular level the role of PKC in tumor promotion and signal transduction, the first and the second cysteine-rich domains (CRD's) of rat brain PKC gamma and mouse skin PKC eta (gamma-CRD1, gamma-CRD2, eta-CRD1, and eta-CRD2) have been prepared by solid phase synthesis. These peptides except for eta-CRD1 properly folded upon zinc treatment to produce PKC regulatory domain surrogates that bind [H-3]phorbol 12,13-dibutyrate (PDBu) with high affinity in the presence of phosphatidylserine. The binding affinity of eta-CRD2 was especially high (K-d=0.91 nM), comparable to that reported for native PKC eta (K-d=0.87 nM). The similar binding behavior of these peptides and native PKC suggests that these peptides serve as useful molecular probes for elucidation of the structural requirements for the recognition of phorbol ester-type tumor promoters. We have recently used these peptides for binding assay of new conformationally restricted analogues of teleocidins and for affinity labeling by photolabile phorbol esters.