TRIMERIC SUBDOMAIN OF THE SIMIAN IMMUNODEFICIENCY VIRUS GLYCOPROTEIN

Previous attempts to define the oligomeric state of the HIV and SIV en velope glycoproteins have yielded conflicting results. We have produce d in Escherichia coli a recombinant model for the ectodomain of the SN envelope protein gp41 and have identified a small, trimeric subdomain by proteolytic digestion of this gp41 fragment. The subdomain assembl es from two peptide fragments, spanning residues 28-80 (N-28-80) and r esidues 107-149 (C-107-149) Of SIV gp41. Each of these peptides contai ns a 4,3-hydrophobic repeat, the hallmark of coiled-coil sequences. Up on mixing, the peptides form a highly helical, trimeric complex [3(N C)] that resists proteolysis and has a melting temperature (T-m) abov e 90 degrees C in physiological buffer. The N- and C-terminal fragment s are antiparallel to each other in the complex, as judged by the obse rvation that digestion of a variant recombinant protein truncated at t he amino terminus yields a C-terminal fragment shortened at its carbox y terminus. The N-28-80 peptide contains more positions within the hep tad repeat than C-107-149 that are predominantly hydrophobic, suggesti ng that N-28-80 is buried in the interior of the complex. We propose t hat the complex consists of a parallel, trimeric coiled-coil of the N- terminal peptide, encircled by three C-terminal peptide helices arrang ed in an antiparallel fashion, and that this complex forms a core with in the gp41 extracellular domain.