Mapping the epitope of MAb NC-1. (A) The sgp140(-) glycoproteins from the JR-FL, YU2, and HXBc2 strains of HIV-1 were immunoprecipitated by PS, MAb 126-6, or MAb NC-1. (B) The HXBc2 sgp140(-) glycoproteins were either untreated or cross-linked with 10 mM GA before being precipitated by PS, MAb 126-6, or MAb NC-1. (C) The HXBc2 sgp140(-) truncated immediately C-terminal to the indicated amino acid residues were either precipitated by PS or MAb NC-1 (top two panels) or cross-linked with 10 mM GA before precipitation by PS (bottom panel). (D) The wild-type (WT) HXBc2 sgp140(-) glycoproteins or mutants containing the indicated changes were precipitated by PS, MAb NC-1, or MAb 126-6. The precipitates were analyzed by SDS-PAGE.