Conformational flexibility and strand arrangements of the membrane-associated HIV fusion peptide trimer probed by solid-state NMR spectroscopy

The human immunodeficiency virus ( HIV) fusion peptide ( HFP) is the N-terminal apolar region of the HIV gp41 fusion protein and interacts with target cell membranes and promotes membrane fusion. The free peptide catalyzes vesicle fusion at least to the lipid mixing stage and serves as a useful model fusion system. For gp41 constructs which lack the HFP, high-resolution structures show trimeric protein and suggest that at least three HFPs interact with the membrane with their C-termini in close proximity. In addition, previous studies have demonstrated that HFPs which are cross-linked at their C-termini to form trimers ( HFPtr) catalyze fusion at a rate which is 15-40 times greater than that of non-cross-linked HFP. In the present study, the structure of membrane-associated HFPtr was probed with solid-state nuclear magnetic resonance ( NMR) methods. Chemical shift and intramolecular (CO)-C-13-N-15 distance measurements show that the conformation of the Leu-7 to Phe-11 region of HFPtr has predominant helical conformation in membranes without cholesterol and, strand conformation in membranes containing similar to 30 mol % cholesterol. Interstrand (CO)-C-13-(CO)-C-13 and (CO)-C-13-N-15 distance measurements were not consistent with an in-register parallel strand arrangement but were consistent with either ( 1) parallel arrangement with adjacent strands two residues out-of-register or ( 2) antiparallel arrangement with adjacent strand crossing between Phe-8 and Leu-9. Arrangement 1 could support the rapid fusion rate of HFPtr because of placement of the apolar N-terminal regions of all strands on the same side of the oligomer while arrangement 2 could support the assembly of multiple fusion protein trimers.