Different effects of a single amino acid substitution on three adjacent epitopes in the gp41 C-terminal tail of a neutralizing antibody escape mutantof human immunodeficiency virus type 1

The envelope protein of human immunodeficiency virus type I (HIV-1) compris es the outer gp 120 SU domain and the anchoring gp41 TM domain, and the con ventional view is that it has a single transmembrane region with the follow ing C-terminal sequence situated entirely within the virion. However, we ha ve recently proposed that the gp41 C-terminal region comprises three transm embrane regions and an external loop structure. Part of this loop is the pe ptide (731) PRGPDRPEGIEEEGGERDRDRS(752) that carries three antibody epitope s, (734)PDRPEG(739), (IEEE743)-I-740, and (ERDRD750)-E-746. PDRPEG is not d etected topes, in virions but reacts with its cognate MAb (C8) in Western b lots, IEEE is a linear and non-neutralizing epitope, and ERDRD is a conform ational and neutralizing epitope. Here we show that escape mutants selected with neutralizing ERDRD specific antibody had a single 732R--> G substitut ion, 14 residues upstream of the cognate epitope, and no longer bound the s electing antibody. The same amino acid substitution altered epitope PDRPEG in the virion so that it now reacted with MAb C8, but left epitope IEEE una ffected. Introduction of 732R-->G by site-specific mutagenesis into the gp4 1 of cloned HIV-1 NL4-3 virions allowed them to escape neutralization by ER DRD-specific IgG, and confirms that 732R makes a major contribution to the neutralizing conformation of the 731-752 region of the C-terminal tail of g p41.