Synthesis and characterization of the second cysteine-rich region of mouse skin PKCGh

Establishment of a binding assay for protein kinase C isozymes using synthetic C1 peptides and development of new medicinal leads with protein kinase C isozyme and C1 domain selectivity

Conventional and novel protein kinase C (PKC) isozymes contain two cysteine-rich C1 domains (C1A and C1B), both of which are candidate phorbol-12, 13-dibutyrate (PDBu)-binding sites. We synthesized C1 peptides of 50-70 residues corresponding to all PKC isozyme C1 domains using an Fmoc solid-phase strategy. These C1 peptides were successfully folded by zinc treatment, as monitored by electrospray ionization time-of-flight mass spectrometry. We measured the K(d)'s of [3H]PDBu for all PKC C1 peptides. Most of the C1 peptides, except for delta-C1A and theta-C1A, showed strong PDBu binding affinities with K(d)'s in the nanomolar range (0.45-7.4 nM) comparable with the respective whole PKC isozymes. The resultant C1 peptide library can be used to screen for new ligands with PKC isozyme and C1 domain selectivity. Non-tumor-promoting 1-oleoyl-2-acetyl-sn-glycerol and bryostatin 1 showed relatively strong binding to all CIA peptides of novel PKCs (delta, epsilon, and eta). In contrast, the tumor promoters (-)-indolactam-V, ingenol-3-benzoate, and PDBu bound selectively to all C1B peptides of novel PKCs. The preference of tumor promoters for the domain might be related to tumorigenesis since recent investigations proposed the involvement of novel PKCs in tumor promotion in vivo using transgenic or knockout mice. Moreover, we recently have found that a new lactone analogue of benzolactams (6) shows significant selectivity in PKCeta-C1B binding.

A new class of simplified phorbol ester analogues: synthesis and binding to PKC and eta PKC-C1B (eta PKC-CRD2)

[formula: see text] A unique class of simplified phorbol ester analogues is described for the first time. A highly efficient retro-annelation sequence was developed in order to remove the five-membered ring from the phorbol diterpene core, allowing access to BCD ring analogues of the phorbol esters. The binding of these analogues to protein kinase C (PKC) and the truncated peptide eta PKC-C1B (eta PKC-CRD2) is also reported.

Solid-phase synthesis, mass spectrometric analysis of the zinc-folding, and phorbol ester-binding studies of the 116-mer peptide containing the tandem cysteine-rich C1 domains of protein kinase C gamma

Tumor-promoting phorbol esters activate protein kinase C (PKC) isozymes by binding to the zinc-finger like cysteine-rich domains in the N-terminal regulatory region. Our recent studies have revealed that only PKCgamma has two high affinity phorbol ester-binding domains, providing a structural blueprint for the rational design of PKCgamma-selective modulators for the treatment of neuropathic pain. To extend this approach, the 116-mer peptide containing the double cysteine-rich motifs of PKCgamma (gamma-C1A-C1B) has been synthesized for the first time using an Fmoc-solid phase strategy with a stepwise chain elongation. This peptide was purified by the reversed phase HPLC to give satisfactory mass data (MALDI-TOF-MS and ESI-TOF-MS). The peptide was successfully folded by zinc treatment and the folded peptide was analyzed intact under neutral conditions by ESI-TOF-MS. The multiple charge mass envelopes shifted to those of the lower mass charge state by addition of 4 molar equiv. ZnCl2, suggesting that gamma-C1A-C1B preserves some higher order structure by the zinc folding. Moreover, the mass spectrum of the zinc-folded peptide in the presence of EDTA clearly showed that gamma-C1A-C1B coordinates exactly four atoms of zinc. This zinc stoichiometry is identical to that of native PKCgamma. Scatchard analysis of the zinc-folded peptide revealed two binding sites of distinctly different affinities (Kd=6.0 +/- 1.5 and 47.0 +/- 6.6 nM) comparable to those reported by Quest and Bell for the GST fusion protein of gamma-C1A-C1B prepared by DNA recombination. These results indicate that gamma-C1A-C1B serves as an effective surrogate for native PKCgamma for the study of the structural characteristics of the binding recognition event and the design, discovery, and development of new PKCgamma-selective modulators.

Comparison of chemical characteristics of the first and the second cysteine-rich domains of protein kinase C gamma

Protein kinase C (PKC) is a key enzyme family involved in cellular signal transduction. The binding of endogenous diacyl glycerol (DAG) to the cysteine-rich domain (CRD) of PKC is associated with normal cell signaling and function. In contrast, the binding of exogenous phorbol esters to the CRD of PKC is considered to be a key initiating event in tumor promotion. Conventional PKC isozymes (PKC alpha, beta I, beta II, and gamma) contain two CRDs, both of which are candidates for the phorbol ester binding site. In order to elucidate the binding requirements of phorbol esters and to obtain information on the phorbol ester binding site in native PKC gamma, several key chemical characteristics of the first and the second CRDs consisting of ca. 50 amino acids of rat PKC gamma (gamma-CRD1 and gamma-CRD2) were examined. In the presence of Zn2+ and phosphatidylserine (PS), both CRDs gave similar Kd values (65.3 nM for gamma-CRD1, 44.1 nM for gamma-CRD2) in phorbol 12,13-dibutyrate (PDBu) binding assays. In comparison, the binding affinity of PDBu for native rat PKC gamma was found to be 6.8 nM. Zn2+ was shown to play an important role in the folding and PDBu binding of both CRDs. A Zn(2+)-induced conformational change was observed for the first time by CD spectroscopic analysis of the complexed and uncomplexed CRDs. Relative to the pronounced Zn2+ effect, most divalent first row transition metal ions along with Ca2+, Mg2+, and Al3+ were ineffective in folding either CRD. Notably, however, Co2+ exhibited a gamma-CRD1-selective effect, suggesting that metal ions, not unlike extensively used organic probes, might also become effective tools for controlling isozyme selective activation of PKC. Moreover, group Ib (Cu2+ and Ag+) and group IIb element ions other than Zn2+ (Cd2+ and Hg2+) were found to abolish PDBu binding of both CRDs. Importantly, these inhibitory effects of Cu2+, Ag+, and Cd2+, and Hg2+ were also observed with native PKC gamma. These results indicate that recent reports on the modulation of conventional PKC by heavy metal ions could be explained by their coordination to the CRDs. While the similar affinities of gamma-CRD1 and gamma-CRD2 for PDBu suggest that either site qualifies as the PDBu binding site, new molecular probes of these CRD3 have now been identified that provide information on the preferred site. These novel ligands (5a and 5b) were synthesized by aza-Claisen rearrangement of (-)-N13-desmethyl-N13-allylindolactam-G (4). These compounds did not significantly affect the specific PDBu binding of gamma-CRD1 but did inhibit that of gamma-CRD2 with similar potency to (-)-indolactam-V. Moreover, these new probes did not significantly inhibit the PDBu binding of native PKC gamma. (-)-Indolactam-V itself bound almost equally to gamma-CRD1, gamma-CRD2, and native PKC gamma. These results suggest that the major PDBu binding site in native PKC gamma is the first CRD, not the second CRD, unlike the novel PKCs.