Molecular models of N-benzyladriamycin-14-valerate (AD 198) in complex with the phorbol ester-binding C1b domain of protein kinase C-delta.

N-Benzyladriamycin-14-valerate (AD 198) is a semisynthetic anthracycline with experimental antitumor activity superior to that of doxorubicin (DOX). AD 198, unlike DOX, only weakly binds DNA, is a poor inhibitor of topoisomerase II, and circumvents anthracycline-resistance mechanisms, suggesting a unique mechanism of action for this novel analogue. The phorbol ester receptors, protein kinase C (PKC) and beta2-chimaerin, were recently identified as selective targets for AD 198 in vitro. In vitro, AD 198 competes with [3H]PDBu for binding to a peptide containing the isolated C1b domain of PKC-delta (deltaC1b domain). In the present study molecular modeling is used to investigate the interaction of AD 198 with the deltaC1b domain. Three models are identified wherein AD 198 binds into the groove formed between amino acid residues 6-13 and 21-27 of the deltaC1b domain in a manner similar to that reported for phorbol-13-acetate and other ligands of the C1 domain. Two of the identified models are consistent with previous experimental data demonstrating the importance of the 14-valerate side chain of AD 198 in binding to the C1 domain as well as current data demonstrating that translocation of PKC-alpha to the membrane requires the 14-valerate substituent. In this regard, the carbonyl of the 14-valerate participates in hydrogen bonding to the deltaC1b while the acyl chain is positioned for stabilization of the membrane-bound protein-ligand complex in a manner analogous to the acyl chains of the phorbol esters. These studies provide a structural basis for the interaction of AD 198 with the deltaC1b domain and a starting point for the rational design of potential new drugs targeting PKC and other proteins with C1 domains.