Mapping of the interaction between the immunodominant loop of the ectodomain of HIV-1 gp41 and human complement protein C1q.

The human immunodeficiency virus type 1 transmembrane envelope glycoprotein gp41 has been previously shown to activate the C1 complex of human complement through direct interaction with its C1q subunit. The major interaction site has been located within the gp41 immunodominant region (residues 590-620), and a synthetic peptide overlapping residues 601-613 of gp41 (sequence GIWGCSGKLICTT) was shown to inhibit binding of gp41 to C1q in vitro (Thielens, N.M., Bally, I.M., Ebenbichler, C.F., Dierich, M.P. & Arlaud, G.J. (1993) J. Immunol. 151, 6583-6592). The ectodomain of gp41 (s-gp41) was secreted from the methylotrophic yeast Pichia pastoris and purified by immunoaffinity chromatography. Enzymatic deglycosylation of the recombinant s-gp41 was necessary to allow its in vitro interaction with C1q. A solid-phase competition assay was used to monitor the effect of mutant peptides derived from segment 601-613 of gp41 on the binding of deglycosylated s-gp41 to C1q. Whereas mutation of Ser606 had no effect, replacement of Ile602, Trp603, Lys608, Leu609 and Ile610 by Ala abolished the ability of the resulting peptides to inhibit binding of s-gp41 to C1q, suggesting that these residues participate in the interaction between gp41 and C1q. These findings are discussed in the light of a structural model of the immunodominant loop of gp41. It is proposed that the recognition of gp41 by C1q is driven by hydrophobic interactions, and that the sites of gp41 responsible for interaction with gp120 and C1q partly overlap.