Helical interactions in the HIV-1 gp41 core reveal structural basis for the inhibitory activity of gp41 peptides

The HIV-1 gp41 envelope protein mediates membrane fusion that leads to viru s entry into the cell. The core structure of fusion-active gp41 is a six-he lix bundle in which an N-terminal three-stranded coiled coil is surrounded by a sheath of antiparallel C-terminal helices. A conserved glutamine (Gln 652) buried in this helical interface replaced by leucine increases HIV-1 i nfectivity. To define the basis for this enhanced membrane fusion activity, we investigate the role of the Gin 652 to Leu substitution on the conforma tion, stability, and biological activity of the N34(L6)C28 model of the gp4 1 ectodomain core. The 2.0 Angstrom resolution crystal structure of the mut ant molecule shows that the Leu 652 side chains make prominent contacts wit h hydrophobic grooves on the surface of the central coiled coil. The Gin 65 2 to Leu mutation leads to a marginal stabilization of the six-helix bundle by -0.8 kcal/mol, evaluated from thermal unfolding experiments. Strikingly , the mutant N34(L6)C28 peptide is a potent inhibitor of HIV-1 infection, w ith 10-fold greater activity than the wild-type molecule. This inhibitory p otency can be traced to the corresponding C-terminal mutant peptide that li kely has greater potential to interact with the coiled-coil trimer. These r esults provide strong evidence that conserved interhelical packing interact ions in the gp41 core are important determinants of HIV-1 entry and its inh ibition. These interactions also offer a test-bed for the development of mo re potent analogues of gp41 peptide inhibitors.