A trimeric subdomain of the simian immunodeficiency virus envelope glycoprotein.

Previous attempts to define the oligomeric state of the HIV and SIV envelope glycoproteins have yielded conflicting results. We have produced in Escherichia coli a recombinant model for the ectodomain of the SIV envelope protein gp41 and have identified a small, trimeric subdomain by proteolytic digestion of this gp41 fragment. The subdomain assembles from two peptide fragments, spanning residues 28-80 (N28-80) and residues 107-149 (C107-149) of SIV gp41. Each of these peptides contains a 4,3-hydrophobic repeat, the hallmark of coiled-coil sequences. Upon mixing, the peptides form a highly helical, trimeric complex [3(N+C)] that resists proteolysis and has a melting temperature (Tm) above 90 degrees C in physiological buffer. The N- and C-terminal fragments are antiparallel to each other in the complex, as judged by the observation that digestion of a variant recombinant protein truncated at the amino terminus yields a C-terminal fragment shortened at its carboxy terminus. The N28-80 peptide contains more positions within the heptad repeat than C107-149 that are predominantly hydrophobic, suggesting that N28-80 is buried in the interior of the complex. We propose that the complex consists of a parallel, trimeric coiled-coil of the N-terminal peptide, encircled by three C-terminal peptide helices arranged in an antiparallel fashion, and that this complex forms a core within the gp41 extracellular domain.