BOTH THE V2 AND V3 REGIONS OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1SURFACE GLYCOPROTEIN FUNCTIONALLY INTERACT WITH OTHER ENVELOPE REGIONS IN SYNCYTIUM FORMATION

To map the regions of the external envelope glycoproteins of human imm unodeficiency virus type 1 (HIV-1) involved in the process of membrane fusion, we determined the syncytium-inducing capacity of a panel of t ransiently expressed chimeric envelope genes. This panel was generated by exchanging gene fragments between four previously studied envelope genes that exhibited a high degree of sequence homology yet displayed marked differences in syncytium-inducing capacity when expressed by r ecombinant vaccinia virus. The results demonstrate that multiple regio ns of the HIV-1 envelope glycoproteins are involved in syncytium forma tion. Some fragments, most notably those containing the V2 or V3 regio n, can transfer syncytium-inducing capacity to envelope proteins previ ously not capable of inducing syncytia. Moreover, it is shown that suc h regions functionally interact with other envelope regions, especiall y one encompassing the V4 and V5 regions of gp120 or a region encompas sing part of gp41, to exert their function in membrane fusion.