HIV-1 RSGP41 DEPENDS ON CALCIUM FOR BINDING OF HUMAN C1Q BUT NOT FOR BINDING OF GP120

Human immunodeficiency virus type 1 activates the complement cascade v ia the classical pathway by direct binding of Clq through specific sit es in the TM surface protein, gp41. In this paper we investigated the divalent cation dependence of the interaction between HIV-1 gp41 and C 1q or gp120. A solid phase radioimmunoassay was used to investigate th e interaction between a recombinant soluble form of HIV-1 gp41 (rsgp41 ) and C1q and an enzyme linked immunoassay was used to investigate the interaction between rsgp41 and gp120. The interaction between C1q and rsgp41, but not between C1q and immune complexes, was dependent upon the presence of calcium. Calcium could not be replaced by larger catio ns such as strontium, barium, lead or smaller ions such as magnesium a nd manganese. Zinc increased binding to 22% of binding achieved with c alcium. The interaction between rsgp41 and gp120 was not dependent upo n the presence of divalent ions. Thus, calcium is required for the int eraction between rsgp41 and C1q, whereas the interaction between rsgp4 1 and C1q is independent of divalent cations.