Identification of key residues in subgroup A avian leukosis virus envelopedetermining receptor binding affinity and infectivity of cells expressing chicken or quail Tva receptor

To better understand retroviral entry, we have characterized the interactio ns between subgroup A avian leukosis virus [ALV(A)] envelope glycoproteins and Tva, the receptor for ALV(A), that result in receptor interference. Ne have recently shown that soluble forms of the chicken and quail Tva recepto r (sTva), expressed from genes delivered by retroviral vectors, block ALV(A ) infection of cultured chicken cells (similar to 200-fold antiviral effect ) and chickens (>98% of the birds were not infected). We hypothesized that inhibition of viral replication by sTva would select virus variants with mu tations in the surface glycoprotein (SU) that altered the binding affinity of the subgroup A SU for the sTva protein and/or altered the normal recepto r usage of the virus. Virus propagation in the presence of quail sTva-mIgG, the quail Tva extracellular region fused to the constant region of the mou se immunoglobulin G (IgG) protein, identified viruses with three mutations in the subgroup A hr1 region of SU, E149K, Y142N, and Y142N/E149K These mut ations reduced the binding affinity of the subgroup A envelope glycoprotein s for quail sTva-mIgG (32-, 324-, and 4,739-fold, respectively) but did not alter their binding affinity for chicken sTva-mIgG. The ALV(A) mutants eff iciently infected cells expressing the chicken Tva receptor but sere 2-fold (E149K), 10-fold (Y142N), and 600-fold (Y142N/E149K) less efficient at inf ecting cells expressing the quail Tva receptor. These mutations identify ke y determinants of the interaction between the ALV(A) glycoproteins and the Tva receptor. We also conclude from these results that, at least for the wi ld-type and variant ALV(A)s tested, the receptor binding affinity was direc tly related to infection efficiency.