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.