Functional analysis of the disulfide-bonded loop/chain reversal region of human immunodeficiency virus type 1 gp41 reveals a critical role in gp120-gp41 association.
Human immunodeficiency virus type 1 (HIV-1) entry into cells is mediated by the surface-exposed envelope protein (SU) gp120, which binds to cellular CD4 and chemokine receptors, triggering the membrane fusion activity of the transmembrane (TM) protein gp41. The core of gp41 comprises an N-terminal triple-stranded coiled coil and an antiparallel C-terminal helical segment which is packed against the exterior of the coiled coil and is thought to correspond to a fusion-activated conformation. The available gp41 crystal structures lack the conserved disulfide-bonded loop region which, in human T-lymphotropic virus type 1 (HTLV-1) and murine leukemia virus TM proteins, mediates a chain reversal, connecting the antiparallel N- and C-terminal regions. Mutations in the HTLV-1 TM protein gp21 disulfide-bonded loop/chain reversal region adversely affected fusion activity without abolishing SU-TM association (A. L. Maerz, R. J. Center, B. E. Kemp, B. Kobe, and P. Poumbourios, J. Virol. 74:6614-6621, 2000). We now report that in contrast to our findings with HTLV-1, conservative substitutions in the HIV-1 gp41 disulfide-bonded loop/chain reversal region abolished association with gp120. While the mutations affecting gp120-gp41 association also affected cell-cell fusion activity, HIV-1 glycoprotein maturation appeared normal. The mutant glycoproteins were processed, expressed at the cell surface, and efficiently immunoprecipitated by conformation-dependent monoclonal antibodies. The gp120 association site includes aromatic and hydrophobic residues on either side of the gp41 disulfide-bonded loop and a basic residue within the loop. The HIV-1 gp41 disulfide-bonded loop/chain reversal region is a critical gp120 contact site; therefore, it is also likely to play a central role in fusion activation by linking CD4 plus chemokine receptor-induced conformational changes in gp120 to gp41 fusogenicity. These gp120 contact residues are present in diverse primate lentiviruses, suggesting conservation of function.
Maerz
A L
AL
St. Vincent's Institute of Medical Research, Fitzroy, Victoria 3065, Australia.
Drummer
H E
HE
Wilson
K A
KA
Poumbourios
P
P
D000595:Amino Acid Sequence; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D004220:Disulfides; D015699:HIV Envelope Protein gp120 / Q000737:chemistry* / Q000378:metabolism; D015700:HIV Envelope Protein gp41 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D015497:HIV-1 / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008561:Membrane Fusion; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D013329:Structure-Activity Relationship
D004220:Disulfides; D015699:HIV Envelope Protein gp120; D015700:HIV Envelope Protein gp41