Rhesus STRL33 is a less efficient coreceptor than human STRL33.

(A) Schematic diagram of human STRL33. Human and rhesus STRL33 are 94% identical at the amino acid level, with most of the extracellular loop differences (inset) clustered at the amino terminus. Differences between human and rhesus STRL33 are indicated by the shaded residues. The conserved “DRY” motif in chemokine receptors implicated for coupling to G-proteins is modified in human and rhesus STRL33 (filled circle residues). (B) SIVmac251(BK28 clone) env protein–pseudotyped GFP reporter virus was used to infect cells expressing CD4 and the indicated coreceptor (top row). Receptor expression was confirmed by staining with the indicated antibodies (middle row). Note that mAb 699 does not recognize rhesus STRL33, but AU1-tagged rhesus STRL33 indicated that rhesus STRL33 was expressed at similar levels to human STRL33. Infection was seen only on the transfected cell population (bottom row). Results were similar for SIVΔB670 clone 3 (Clone3), SIVmac/17E-Fr (Fred), SIVmac316 (316) and HIV-1 JR-FL and are quantified in panel C, where infection efficiency (% of GFP⁺ cells) was normalized against that obtained for human CCR5.