Received for publication June 28, 2007.
Revised October 8, 2007.
Accepted for publication November 27, 2007.

An Intracellular, Allosteric Site for a Specific Class of Antagonists of the CC Chemokine G Protein-Coupled Receptors, CCR4 and CCR5

Abstract

A novel mechanism for antagonism of the human chemokine receptors CCR4 and CCR5 has been discovered with a series of small-molecule compounds that appears to interact with an allosteric, intracellular site on the receptor. The existence of this site is supported by a series of observations: 1) intracellular access of these antagonists is required for their activity, 2) specific, saturable binding of a radiolabeled antagonist requires the presence of the receptor and 3) through engineering receptor chimeras by reciprocal transfer of C-terminal domains between CCR4 and CCR5, compound binding and the selective structural/activity relationship for antagonism of these receptors appears to be associated with the integrity of that intracellular region. Published antagonists from other chemical series do not appear to bind to the novel site, and their interaction with either CCR4 or CCR5 is not affected by alteration of the C-terminal domain. The precise location of the proposed binding site remains to be determined, but the known close association of the C-terminal domain, including Helix 8, as a proposed intracellular region that interacts with transduction proteins (e.g., G Proteins and -arrestin) suggests that this could be a generic allosteric site for chemokine receptors and perhaps more broadly for Class A G Protein-Coupled Receptors. The existence of such a site that can be targeted for drug discovery has implications for screening assays for receptor antagonists, which would need, therefore, to consider compound properties for access to this intracellular site.

Key words: Chemotactic peptides, Func. analysis receptor/ion channel mutants, Mutagenesis/Chimeric approaches, Receptor binding studies