FIG. 4.

Receptor binding activity of mutant Env proteins. NIH 3T3 cells expressing Tva were biotinylated and lysed as described in the legend to Fig. 2B. Unlabeled lysates of cells expressing Env A, Env C, or mutant Env A proteins were mixed with the biotinylated Tva lysate and then immunoprecipitated with anti-A tail (or anti-C tail) antibodies. Samples were resolved on SDS-PAGE (12% gel) and blotted to nitrocellulose. Biotinylated proteins were detected as described in the legend to Fig. 2B.