Potent inhibitors of LXXLL-based protein-protein interactions

Protein-protein interactions between estrogen receptors, ER alpha and ER beta, and their coactivators (CoAs) ore an attractive target for drug intervention. This interaction is mediated by a small penta-peptide motif (LXXLL), termed the NR box. Based on this motif a variety of cyclic and linear peptides were synthesized in order to gain a better understanding of the association of CoA proteins with the ER isoforms. Utilizing a time-resolved florescence-based coactivator interaction assay, we determined the abilities of these peptides to inhibit this interaction. Using molecular modeling and CD spectroscopy, we have examined the structural basis of their bioactivities with both hormone receptor isoforms. Either homocysteine or penicillamine was utilized as a substitute for cysteine in the disulfide-bridged peptides, while tertiary leucine and neopentyl glycine were used as the surrogates for the NR box leucines. The most potent disufide-bridged peptide (K,70 pm, with ER alpha) incorporates neopentyl glycine in the NR box, while the most active peptide in this series with ER beta (K-i = 350 pm) incorporates tertiary leucine. Surprisingly, several linear peptides containing a single cysteine residue showed activities with low nanomolar K-i values. Collectively, our results suggest a synthetic approach for designing potent and selective peptidomimetics for ERa and ER beta interactions with CoA proteins effecting estrogen action.