Molecular basis for protein kinase C isozyme-selective binding: The synthesis, folding, and phorbol ester binding of the cysteine-rich domains of all protein kinase C isozymes

The protein kinase C (PKC) family of enzymes plays a crucial role in cellular signal transduction and tumor promotion. Conventional and novel PKC isozymes consist of a catalytic domain for protein phosphorylation and a regulatory domain which binds the endogenous messenger diacyl glycerol or exogenous agents such as phorbol esters. The N-terminal regulatory region of these isozymes contains two cysteine-rich domains (C1A and C1B, also known as CRD1 and CRD2), both of which are candidates for the phorbol ester-binding site. To determine the phorbol ester-binding sites of these isozymes and to elucidate the structural requirements for isozyme selective PKC modulation, the C1 peptides, consisting of ca. 50 amino acids of all conventional and novel PKCs, along with those of atypical PKCs have been synthesized by a solid-phase Fmoc strategy. Exceptionally high overall yields (10-20%) were achieved in the syntheses of most of the C1 peptides on a Pioneer Peptide Synthesizer (PerSeptive Biosystems) through the use of HATU as a coupling reagent. These peptides were successfully folded by zinc treatment, as monitored by CD spectroscopy. Importantly, only the C1Bs of all conventional and novel PKCs, except for PKC gamma, bound [H-3]phorbol-12,13-dibutyrate (PDBu) with high affinities, comparable to those of the native isozymes. Of special significance, both C1 peptides of PKC gamma, (i.e., gamma-C1A and gamma-C1B) exhibited high-affinity binding, providing the structural basis for a novel approach to PKC gamma-selective modulators, compounds of potential significance for the treatment of neuropathic pain. The effects of metal cations other than zinc on the binding of these isozymes were also investigated. Only the PKC gamma surrogates (gamma-C1A and gamma-C1B), when treated with cadmium, exhibited no binding, while other similarly treated conventional and novel PKC surrogates strongly bound PDBu, as did the zinc-folded peptides. These results suggest that cadmium ion could serve as a new and effective tool for controlling the activation of PKC gamma.