The envelope glycoprotein of human immunodeficiency virus type 1 (HIV- 1) consists of a complex of two noncovalently associated subunits, gp 120 and gp41. Formation of gp 120/gp41 oligomers is thought to be dependent on a 4-3 hydrophobic (heptad) repeat located in the amino-terminal region of the gp41 molecule. We have investigated the role of this heptad repeat in determining the oligomeric structure of gp41 by introducing its buried core residues into the first (a) and fourth (d) positions of the GCN4 leucine-zipper dimerization domain. The mutant peptides fold into trimeric, helical structures, as shown by circular dichroism and equilibrium sedimentation centrifugation. The 2.4 Å resolution crystal structure of one such trimer reveals a parallel three-stranded, α-helical coiled coil. Thus, the buried core residues from the gp41 heptad repeat direct trimer formation. We suggest that the conserved amino-terminal heptad repeat within the gp41 ectodomain possesses trimerization specificity.
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