MULTIPLE RESIDUES CONTRIBUTE TO THE INABILITY OF MURINE CCR-5 TO FUNCTION AS A CORECEPTOR FOR MACROPHAGE-TROPIC HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ISOLATES

Infection of CD4-positive cells by human immunodeficiency virus type 1 (HIV-1) requires Functional interaction of the viral envelope protein with a coreceptor belonging to the chemokine receptor family of seven -membrane spanning receptors, For the majority of macrophage-tropic HI V-1 isolates, the physiologically relevant coreceptor is the human CCR -5 (hCCR-5) receptor. Although the murine homolog of CCR-5 (mCCR-5) is unable to mediate HIV-1 infection, chimeric hCCR-5/mCCR-5 molecules c ontaining single extracellular domains derived from hCCR-5 are effecti ve coreceptors for certain macrophaage-tropic HIV-1 isolates, Here, we have sought to identify residues in hCCR-5 critical for HIV-1 infecti on by substitution of mCCR-5-derived residues into the context of func tional chimeric hCCR-J/mCCR-5 receptor molecules. Using this strategy, we demonstrate that residues 7, 13, and 15 in the first extracellular domain and residue 180 in the third extracellular domain of CCR-5 are important for HIV-1 envelope-mediated membrane fusion. Of interest, c ertain substitutions, for example, at residues 184 and 185 in the thir d extracellular domain, have no phenotype when introduced individually but strongly inhibit hCCR-5 coreceptor function when present together , We hypothesize that these changes, which do not preclude chemokine r eceptor function, may inhibit a conformational transition in hCCR-5 th at contributes to HIV-1 infection, Finally, we report that substitutio n of gla cine for valine at residue 5 in CCR-5 can significantly enhan ce the level of envelope-dependent cell fusion by expressing cells, Th e diversity of the mutant phenotypes observed in this mutational analy sis, combined with their wide distribution across the extracellular le gions of CCR-5, emphasizes the complexity of the interaction between H IV-1 envelope and coreceptor.