FIG. 1.

The effect of deletion of HIS8 (ΔH8) in Fr-MLV SU on receptor binding and infection in comparison with the effect of substitutions for residues in the receptor binding pocket that reduce binding affinity and titer. (A) Infection. End point dilution of virus on NIH 3T3 fibroblasts was used to measure the titers of Fr-MLVs with ΔH8 and/or substitutions for one of three residues (W102G, D86A, or S84I) in the receptor binding pocket of SU that reduce receptor binding affinity (13). Each titer was determined in triplicate by end point dilution from independent stocks of virus, and standard errors are indicated. Incorporation of Env into virions was monitored by immunoblotting with anti-gp70 antibody (below). (B) Binding. Binding of exogenous ¹²⁵I-Fr-RBD to Xenopus oocytes was measured 2 days after injection with capped mRNA encoding mCAT1 alone or in combination with a capped mRNA encoding either Fr-RBD, Fr-RBD (W102G), or Fr-RBD (ΔH8). Each measurement is the mean of the counts per minute from five oocytes ± 1 standard error. +, present; −, absent; WT or wt, wild type.