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 complem ent through direct interaction with its C1q subunit. The major interaction site has been located within the gp41 immunodominant region (residues 590-6 20), and a synthetic peptide overlapping residues 601-613 of gp41 (sequence GIWGCSGKLICTT) was shown to inhibit binding of gp41 to C1q in vitro (Thiel ens, N.M., Bally, I.M., Ebenbichler, C.F., Dierich, M.P. & Arlaud, G.J. (19 93) J. Immunol. 151, 6583-6592). The ectodomain of gp41 (s-gp41) was secret ed from the methylotrophic yeast Pichia pastoris and purified by immunoaffi nity chromatography. Enzymatic deglycosylation of the recombinant s-gp41 wa s necessary to allow its in vitro interaction with C1q. A solid-phase compe tition 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. Wh ereas mutation of Ser606 bad no effect, replacement of Ile602, Trp603, Lys6 08, Leu609 and Ile610 by Ala abolished the ability of the resulting peptide s to inhibit binding of s-gp41 to C1q, suggesting that these residues parti cipate in the interaction between gp41 and C1q. These findings are discusse d 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 i nteractions, and that the sites of gp41 responsible for interaction with gp 120 and C1q partly overlap.