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To study the binding of human complement proteins to gp41 and gp120 of HIV-1.The interaction of complement proteins with gp41 and gp120 and their effect on the gp41-gp120 complex in enzyme-linked immunosorbent assays (ELISA) and on stably transfected Schneider-2 cells expressing a gp41-gp120 complex was investigated. The molecular basis of these interactions was analysed by computer-supported sequence analysis.gp41 strongly binds human complement regulatory proteins factors H and properdin, and weakly binds factors I and B. The binding occurs with recombinant soluble (rs) gp41 fixed on ELISA plates as well as gp41-gp120 complex expressed on Schneider-2 cells. The basis for this binding potential might be an amino-acid (aa) sequence of gp41 displaying homologies to sites in human C3. rgp120 also binds C3(H20), a C3b-like form of C3, and C4b. These binding features of gp120 can be explained by homology of constant region (CR) 4 in gp120 to sites in C4b binding protein. Additionally, CR1 in gp120 exhibits a weak similarity to human properdin. Preincubation of rsgp41 with either factor H or properdin, and of rgp120 with C3b or C4b affected the interaction between rsgp41 and rgp120. Incubation of Schneider-2 cells, expressing a functional gp41-gp120 complex, with factor H reduced the detectable amount of gp120. This effect was similar to that induced by soluble CD4.These results strongly suggest that HIV-1 envelope proteins interact with human complement proteins. Additionally, C3b-like features of gp41 and the C3b/C4b binding structures in gp120 may affect the non-covalent association between gp41 and gp120.