Conserved determinants of enhanced CCR5 binding in the human immunodeficiency virus subtype D envelope third variable loop.

Human immunodeficiency virus 1 subtype D (HIV-1D) contributes to a significant portion of the HIV-1 disease burden in eastern and central Africa, and is associated with more rapid disease progression. Its viral envelope sequences, particularly in the third variable region (V3), are highly divergent from other major subtypes yet have rarely been studied to date. We evaluated the V3 and select bridging sheet residues of the HIV-1D 94UG114 envelope by alanine-scanning mutagenesis to determine the residues involved in CCR5 usage conservation in the face of sequence variability. We found most single alanine mutations capable of abolishing CCR5 binding, suggesting binding contacts that are highly sensitive to mutation. Despite drastic binding defects across the board, most mutants mediated fusion at or near wild-type levels, demonstrating an ability to accommodate changes in CCR5 affinity while maintaining the ability to complete entry. Three of the alanine mutations did not abolish CCR5 binding but rather resulted in enhanced CCR5 binding. The positions of these residues were found to be conserved between strains of two subtypes, revealing similar V3 elements that suggest a conservation of constraints in V3 loop conformation.