Five-Helix protein, which comprises the three N-helices and at least two, but not three, of the three C-helices of the trimer-of-hairpin structure of HIV gp41, separated by linkers, such as amino acid residue linkers, is disclosed. Six-Helix protein, which includes the three N-helices and the three C-helices of the trimer-of-hairpin structure of HIV gp41, separated by linkers, is also disclosed.