Structural and functional analysis of the HIV gp41 core containing an Ile573 to Thr substitution: Implications for membrane fusion

The envelope glycoprotein of HIV-1 consists of the surface subunit gp120 an d the transmembrane subunit gp41. Binding of gp120 to target cell receptors induces a conformational change in gp41, which then mediates the fusion of viral and cellular membranes. A buried isoleucine (Ile573) in a central tr imeric coiled coil within the fusion-active gp41 ectodomain core is thought to favor this conformational activation. The role of Ile573 in determining the structure and function of the gp120-gp41 complex was investigated by m utating this residue to threonine, a nonconservative substitution in HIV-1 that occurs naturally in SIV. While the introduction of Thr573 markedly des tabilized the gp41 core, the three-dimensional structure of the mutant trim er of hairpins was very similar to that of the wild-type molecule. A new hy drogen-bonding interaction between the buried Thr573 and Thr569 residues ap pears to allow formation of the trimer-of-hairpins structure at physiologic al temperature. The mutant envelope glycoprotein expressed in 293T cells an d incorporated within pseudotyped virions displayed only a moderate reducti on in syncytium-inducing capacity and virus infectivity, respectively. Our results demonstrate that the proper folding of the gp41 core underlies the membrane fusion properties of the gp120-gp41 complex. An understanding of t he gp41 activation process may suggest novel strategies for vaccine and ant iviral drug development.