Naturally occurring CCR5 extracellular and transmembrane domain variants affect HIV-1 co-receptor and ligand binding function

Analysis of CCR5 variants in human immunodeficiency virus, type 1 (HIV-1), high risk cohorts led to the identification of multiple single amino acid s ubstitutions in the amino-terminal third of the HIV-1 co-receptor CCR5 sugg esting the possibility of protective and permissive genotypes; unfortunatel y, the low frequency of these mutations did not led to correlation with fun ction. Therefore, we used analytical methods to assess the functional and s tructural significance of six of these variant receptors in vitro. These st udies showed three categories of effects on CCR5 function. 1) Mutations in the first extracellular domain of CCR5 severely reduce specific ligand bind ing and chemokine-induced chemotaxis. 2) An extracellular domain variant, A 29S, when co-expressed with CD4, supported HIV-1 infection whereas the othe rs do not. 3) The transmembrane region variants of CCR5 support monotropic HIV-1 infection that is blocked by addition of some receptor agonists, Muta tions in the first and second transmembrane domains increase RANTES (regula ted on activation normal T-cell expressed) binding affinity but did not aff ect MIP1 beta binding affinity presumably based on differences in ligand-re ceptor interaction sites. Furthermore, the CCR5 transmembrane mutants do no t respond to RANTES with the classical bell-shaped chemotactic response cur ve, suggesting that they are resistant to RANTES-induced desensitization. T hese data demonstrate that single amino acid changes in the extracellular d omains of CCR5 can have profound effects on both HIV-1 coreceptor and speci fic ligand-induced functions, whereas mutations in the transmembrane domain only affect the response to chemokine ligands.