CELL-FUSION MEDIATED BY INTERACTION OF A HYBRID CD4.CD8 MOLECULE WITHTHE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ENVELOPE GLYCOPROTEIN DOES OCCUR AFTER A LONG LAG TIME

Several domains of CD4 have been suggested to play a critical role in events that follow its binding to the human immunodeficiency virus typ e 1 (HIV-1) envelope glycoprotein (gp120-gp41). It has been reported p reviously that cells expressing a chimeric molecule consisting of the first 177 residues of human CD4 attached to residues from the hinge, t ransmembrane, and cytoplasmic domains of human CD8 did not form syncyt ia with HIV-1-infected cells (L. Poulin, L. A. Evans, S. Tang, A. Barb oza, H. Legg, D. R. Littman, and J. A. Levy, J. Virol. 65:4893-4901, 1 991). In contrast, we found that the hybrid CD4.CD8 molecule expressed in human cells did render them susceptible to fusion with cells expre ssing HIV-1IIIB or HIV-1RF envelope glycoproteins encoded by vaccinia virus recombinants, but only after long lag times. The lag time of mem brane fusion mediated by the hybrid CD4.CD8 molecule was fivefold long er than that for the wild-type CD4 molecule. However, the rate of bind ing to and the affinity of soluble gp120 for membrane-associated CD4.C D8 were the same as for CD4. Both molecules were laterally mobile, as determined by patching experiments. Coexpression of the CD4.CD8 chimer a with wild-type CD4 did not lead to interference in fusion but had an additive effect. Therefore, the proximal membrane domains of CD4 play an important role in determining the kinetics of postbinding events l eading to membrane fusion. We hypothesize that the long lag time is du e to the inability of the CD4.CD8-gp120-gp41 complex to undergo the ra pid conformational changes which occur during the fusion mediated by w ild-type CD4.