THE 2ND EXTRACELLULAR LOOP OF CXCR4 DETERMINES ITS FUNCTION AS A RECEPTOR FOR FELINE IMMUNODEFICIENCY VIRUS

The feline homolog of the alpha-chemokine receptor CXCR4 has recently been shown to support cell-cell fusion mediated by CXCR4-dependent str ains of human immunodeficiency virus (HIV) and strains of feline immun odeficiency virus (FIV) that have been selected for growth in the Cran dell feline kidney (CrFK) cell line. In this report we demonstrate tha t expression of CXCR4 alone is sufficient to render cells from diverse species permissive for fusion with FIV-infected cells, suggesting tha t CXCR4 is the sole receptor for CrFK-tropic strains of FN, analogous to CD4-independent strains of HIV-2. To identify the regions of CXCR4 involved in fusion mediated by FIV, we screened panels of chimeric CXC R4 molecules for the ability to support fusion with FIV-infected cells . Human CXCR4 supported fusion more efficiently than feline CXCR4 and feline/human CXCR4 chimeras, suggesting that the second and third extr acellular loops of human CXCR4 contain a critical determinant for rece ptor function. Rat/human CXCR4 chimeras suggested that the second extr acellular loop contained the principal determinant for receptor functi on; however, chimeras constructed between human CXCR2 and CXCR4 reveal ed that the first and third loops of CXCR4 contribute to the FIV Env b inding site, as replacement of these domains with the corresponding do mains of CXCR2 rendered the molecule nonfunctional in fusion assays. M utation of the DRY moth and the C-terminal cytoplasmic tail of CXCR4 d id not affect the ability of the molecule to support fusion, suggestin g that neither signalling via G proteins nor receptor internalization was required for fusion mediated by FIV; similarly, truncation of the N terminus of CXCR4 did not affect the function of the molecule as a r eceptor for FIV. CXCR4-transfected feline cells mere rendered permissi ve for infection with both the CrFK-tropic PET isolate of FIV and the CXCR4-dependent RF strain of HIV-1, and susceptibility to infection co rrelated well with ability to support fusion. The data suggest that th e second extracellular loop of CXCR4 is the major determinant of CXCR4 usage by FIV.