The G-protein coupled receptor CCR5 binds three distinct B-chemokines on the solvent exposed segments of the extracellular surface. This same region of CCr5 also interacts with certain macrophage-tropic strains of HIV-1. To investigate structural features of CCR5 that contribute toward HIV-1 coreceptor activity, the amino-terminal and extracellular loop segments of CCR5 were linked to form contiguous polypeptides or grafted onto a seven transmembrane helix bacteriorhodopsin (bR) scaffold either singly, or in combination, to produce a series of CCR5/bR chimeras. The CCR5 extracellular surface polypeptides and CCR5/bR chimeras were expressed in E. coli or COS-1 cells and tested for cell surface expression, all trans retinal binding, and/or CD4 dependent env mediated coreceptor activity. The results show that some the CCR5/bR chimeras are transported to the cell surface and form a bR-like chromophore will all-trans retinal. Further, env mediated cell fusion assays identified structural features in the amino-terminal region of the CCR5/bR chimeras that are sufficient for interaction with JRFL env. Mutations at tyrosine rediuses 10 and 14 in the amino-terminal segment of CCR5, persumed tyrosyl sulfation sites, markedly reduced coreceptor activity. A chimera containing the entire extracellular surface of CCR5 grafted onto the bR scaffold exhibited all-trans retinal dependent coreceptor activity. Thus, the approach of using bR as a seven transmembrane helix scaffold may offer an alternative strategy for studying specific receptor-ligand/viral interactions in other G-protein coupled receptors.
Pub Type: Journals
AIDS, chemokine, GPCR, membrane protein, receptor