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Journal of Virology, July 2001, p. 6635-6644, Vol. 75, No. 14
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.14.6635-6644.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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

Anne L. Maerz, Heidi E. Drummer, Kirilee A. Wilson, and Pantelis Poumbourios*

St. Vincent's Institute of Medical Research, Fitzroy, Victoria, Australia

Received 2 February 2001/Accepted 24 April 2001

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.

* Corresponding author. Mailing address: St. Vincent's Institute of Medical Research, 41 Victoria Pde., Fitzroy, VIC 3065, Australia. Phone: 61-3-9288-2480. Fax: 61-3-9416-2676. E-mail: apoum{at}ariel.its.unimelb.edu.au.

Journal of Virology, July 2001, p. 6635-6644, Vol. 75, No. 14
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.14.6635-6644.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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