1.G.16 The Human Immunodeficiency Virus Type 1 (HIV-1) Fusion Peptide (HIV-FP) Family
The HIV-FP family consists of an N-terminal segment of the envelop protein, Env. Although the fusion peptide is only 20 aas in length with a single N-terminal TMS, the 68 aa peptide including this segment is 68 aas in length (Apellániz et al. 2014). Pfam/CDD calls this family the Ebola_HIV1-like HR1-HR2 Superfamily.
The caveolin-1 binding domains (CBD), referred to as CBD1 and CBD2, are highly conserved in the transmembrane envelope glycoproteins of various HIV-1 and HIV-2 isolates, respectively. However, HIV-1 neutralizing antibodies raised against the synthetic CBD1 peptide (SLEQIWNNMTWMQWDK) do not cross-react with the CBD2 peptide (SLTPDWNNMTWQEWER) and have no effect on HIV-2 infection. The CBD2 peptide is not immunogenic under similar immunization conditions as the CBD1 peptide, and the CBD1 but not the CBD2 peptide has the capacity to bind caveolin-1. Circular dichroism spectroscopy and fluorimetry analysis indicated that CBD1 but not CBD2 could adopt a defined secondary structure and form a complex with a peptide corresponding to the caveolin-1 scaffolding domain, which is the site of interaction of caveolin-1 (Benferhat et al. 2008). Also, CBD1 but not CBD2 binds cells and forms large aggregates at the plasma membrane by colocalizing with cytofacial caveolin-1. This is dependent on the lipid raft integrity of the membrane. Supporting the suggestion that the ability to penetrate into membranes is sustained by folding at the interface, CBD1 but not CBD2 has the capacity to insert into lipid monolayers, penetrate into artificial membranes and adopt a beta-sheet conformation in the presence of lipid vesicles (Benferhat et al. 2008).
gp41, a single-pass integral viral membrane protein, contains an ~ 170-residue ectodomain located outside the virus that mediates fusion and includes the fusion peptide (FP), the N-helix, loop, the C-helix, and the viral membrane-proximal external region (MPER). The virion initially has noncovalent complexes between three gp41 ectodomains and three gp120 proteins. A gp120 contains approximately 500 residues and functions to identify target T-cells and macrophages via binding to specific protein receptors of the target cell membrane. gp120 moves away from the gp41 ectodomain before the ectodomain binds to the target cell membrane to mediate membrane fusion. The secondary and tertiary structures of the ectodomain are different in the initial complex with gp120 and the final state without gp120. The ectodomain and TMS are sufficient for fusion of both neutral and anionic vesicles at neutral pH, which reflects the expected conditions of HIV/cell fusion (Liang et al. 2018). Fusion is enhanced by the fusion peptide, which may contacts the host membrane, and the MPER and TM, which respectively interfacially contact and traverse the HIV membrane. Initial contact with vesicles is made by protein trimers that are in a native oligomeric state that reflects the initial complex with gp120 and also is commonly observed for the ectodomain without gp120. Circular dichroism data support helical structures for the N-helix, C-helix, and MPER, and nonhelical structure for the FP and loop. Distributions of monomer, trimer, and hexamer states were observed by size-exclusion chromatography, with dependences on solubilizing detergent and construct. The data have been integrated into a working model of HIV/cell fusion that includes dissociation of the ectodomain into gp41 monomers followed by folding into hairpins that appose the two membranes, and subsequent fusion catalysis by trimers and hexamers of hairpins. The monomeric and oligomeric gp41 states may therefore satisfy dual requirements for HIV entry of membrane apposition and fusion (Liang et al. 2018).