MUTATIONS IN THE MEMBRANE-SPANNING DOMAIN OF THE HUMAN-IMMUNODEFICIENCY-VIRUS ENVELOPE GLYCOPROTEIN THAT AFFECT FUSION ACTIVITY

A chimeric protein consisting of the human immunodeficiency virus type 1 (HIV-1) envelope protein (Env) ectodomain joined to the transmembra ne and cytoplasmic-tail domains of vesicular stomatitis virus G protei n lost the ability to fuse CD4(+) HeLa cells yet was transported to th e cell surface and cleaved normally. These results suggested some crit ical role of the HIV gp41 transmembrane or cytoplasmic domain in fusio n. Subsequent mutagenic analysis of the HIV-1 Env transmembrane domain revealed that the sequence of amino acid residues from positions 696 to 707 of the transmembrane domain was important for fusion function b ut was not required for anchoring of the Env protein in the lipid bila yer or for transport to the cell surface. Further analysis indicated t hat the basic residues at positions 696 and 707 were critical for memb rane fusion activity, as was the spacing between these residues. These results demonstrate that in addition to providing an anchoring functi on, the specific amino acid sequence in the transmembrane domain plays a crucial role in the membrane fusion process.