The V3 region of the envelope glycoprotein (gp120) of HIV-1 forms the binding site for the chemokine receptors that serve as co-receptor for HIV-1 binding and entry. The sequence of the V3 region determines the phenotype of the virus, i.e. whether it will bind the CCR5 chemokine receptor and infect macrophages or it will bind the CXCR4 chemokine receptor and infect T-cells. Using isotope filtering and isotope editing NMR techniques we determined the structure of V3 peptides in complex with the Fv fragment of anti-gp120 HIV-1 neutralizing antibodies directed against the V3 region. In all cases the V3 peptides bound to the antibodies' Fv forms a beta-hairpin conformation. However, two major conformations were found, differing in the pairing of the residues in the beta-hairpin, the side-chain orientation and the register of the residues occupying the hydrogen-bonding positions in the N-terminal strand (1-3). One conformation is postulated to be that of viruses that use the CCR5 receptor (R5 viruses) while the other is of viruses that use the CXCR4 receptor (X4 viruses) (1). A single mutation at position 322 within the V3 loop of the HIV-1 gp120, from a negatively to a positively charged residue, or a mutation to neutral residue at position 322 accompanied by mutation to positively charged residue at position 306 were found to be sufficient to switch an R5 virus to an X4 virus. Comparison of the V3 structures reveals that electrostatic interaction between R304 and E322 could stabilize the R5 conformation of the V3 while the E322R mutation leads to repulsion between R304 and R322 triggering the observed one register shift of the N-terminal strand relative to the C-terminal strand and the formation of the X4 conformation (3). Mutation to a positively charged residue at position 306 accompanied by a mutation to neutral residue at position 322 could cause a similar one register shift to enhance cation-pi interactions between the positively charged residues in the N-terminal strand and a phenylalanine and tyrosine residues in the C-terminal strand. Other than the interactions involving the residues critical for phenotype switch, most other cross-strand interactions are similar in the R5 and X4 conformation of the V3. This is due to the existence of three pairs of identical (or very similar) amino acids in the V3 C-terminal strand. This similarity in cross strand interactions between the two conformations facilitates the switch between the R5 and X4 conformations. The structure of the V3 peptides bound to the HIV-1 neutralizing antibodies can be used to design constrained V3 analogs and have the potential to serve as immunogens in an anti-HIV-1 vaccine and also as inhibitors of viral entry.

References and Footnotes

1. Sharon, M., Kessler, N., Levy, R., Zolla-Pazner, S., Gorlach, M., and Anglister, J. (2003) Alternative Conformations of HIV-1 V3 Loops Mimic beta Hairpins in Chemokines, Suggesting a Mechanism for Coreceptor Selectivity, Structure (Camb) 11, 225-36 (2003)
2. Rosen, O., Chill, J., Sharon, M., Kessler, N., Mester, B., Zolla-Pazner, S., and Anglister, J. Induced Fit in HIV-Neutralizing Antibody Complexes: Evidence for Alternative Conformations of the gp120 V3 Loop and the Molecular Basis for Broad Neutralization, Biochemistry 44, 7250-8 (2005)
3. Rosen, O., Sharon, M., Quadt-Akabayov, S. R., and Anglister, J. , Molecular switch for alternative conformations of the HIV-1 V3 region: implications for phenotype conversion, Proc Natl Acad Sci U S A 103, 13950-5 (2006)