HIV gp41 protein catalyzes fusion between viral and host cell membranes, and its apolar N-terminal region or "fusion peptide" binds to host cell membranes and plays a key role in viral and host cell membrane fusion. Gp41 fusion can be dominantly inhibited by dilute amounts of V2E mutant gp41, but a structural basis for this inhibition has not been demonstrated. "HFP" is a construct containing the fusion peptide sequence that induces membrane vesicle fusion, and V2E mutant HFP (V2E-HFP) has reduced membrane vesicle fusion rates. Earlier solid-state NMR (SSNMR) studies showed that when HFP or V2E-HFP are associated with membranes with ~30 mol% cholesterol (mHFP or mV2E-HFP), the apolar N-terminal regions of these constructs have predominant β strand secondary structure. In mHFP, a fraction of the strands form antiparallel β sheet structure with residue 161/116 or 171/117 registries of adjacent strands (i.e t = 16 and t = 17 registries). Other SSNMR and infrared studies have been interpreted to support a large fraction of approximately in-register parallel registry of adjacent strands. However, the samples had many isotopic labels and other structural models were also consistent with the data. The tertiary structure of mHFP was studied using SSNMR with the rotational-echo double resonance (REDOR) pulse sequence to measure a sample's average 13CO-15N dipolar couplings. Experimental data were collected for samples with sparser 13CO and 15N labeling and were compared to simulated NMR data. The in-register parallel β sheet fraction was ≤ 0.15, and a much greater fraction of antiparallel registries were identified. The accuracy of the quantitative measurements was enhanced by inclusion of "long range" natural abundance contributions in the data analysis, and the validity of this approach was supported by a negative control sample. Furthermore, mHFP samples were prepared with a single 13CO and a single 15N label for which the closest 13CO-15N interstrand proximity resulted from a distinct registry. These experimental data were compared to simulated data that incorporated fractional populations, ft, of 17 different registries. These ft, were globally fit using a χ2 metric which identified a broad distribution of antiparallel β sheet registries (11 < t < 21). Sequential hydrophobic residues in HFP result in intrastrand hydrophobic patches and interstrand overlap of these patches result in interstrand hydrophobic regions. These regions may insert into the vesicle membranes, and a hydrophobicity or insertion energy metric, , was developed to quantify each registry's insertion energy. In general, registries present in our NMR samples had a negative while registries that were not present generally had a positive . A similar set of experiments were run with mV2E-HFP, and mV2E-HFP had a narrower distribution of registries where the t = 20 registry was significantly more populated in mV2E-HFP than in mHFP. The hydrophobic residues of HFP are located within the first 12 N-terminal amino acids, and the t = 12 registry was more populated in mHFP than mV2E-HFP. The t = 12 registry localizes hydrophobic residues which may result in deeper membrane insertion and increased vesicle fusion rates compared to the t = 20 registry. The t = 20 registry delocalizes the interstrand proximity of N-terminal hydrophobic residues which may result in shallower membrane insertion and reduced membrane fusion rates. These results provide a new, experimentally-based structural model for transdominant inhibition where co-mixing of V2E mutant gp41 and wild type gp41 may energetically favor a non-native registry distribution shifted toward longer registries for the FP region of wild type gp41.
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