Biophysical studies of HIV-1 glycoprotein-41 interactions with peptides and small molecules – Effect of lipids and detergents
Graphical abstract
Introduction
HIV-1 glycoprotein-41 (gp41) is a transmembrane protein that forms part of the spike on the viral surface and that is involved in fusion between HIV-1 and host cells. It consists of an ectodomain, transmembrane domain and cytoplasmic domain [1,2], and exists in a metastable state prior to fusion. Fusion is triggered by viral attachment and release of glycoprotein-120 (gp120), followed by collapse of trimeric N-heptad repeat (NHR) and C-heptad repeat (CHR) helices of the ectodomain into a six helix bundle (6HB) [1,[3], [4], [5], [6]]. Inhibitors targeting gp41 have been studied for their ability to prevent entry and infection. A hydrophobic pocket (HP) defined by two chains of the NHR trimer is contained within the protein-protein interface that mediates 6HB formation [7]. The pocket is considered to be a potential target for low molecular weight fusion inhibitors [[8], [9], [10], [11], [12], [13]]. Nevertheless, small molecule fusion inhibitors with drug-like potency have not been found. The most effective (nM or better) fusion inhibitors discovered to date are peptides based on the CHR domain of gp41 [14], including T20 (enfuvirtide), an FDA approved fusion inhibitor. They act in a dominant-negative fashion by preventing the conformational changes in gp41 required for fusion.
It has proved difficult to study gp41 – small molecule interactions by biophysical methods as an aid to structure based drug design. Isolated gp41 ectodomain adopts the collapsed 6HB form in which the hydrophobic pocket is obscured (Fig. 1A) [15,16]. In order to study binding in the pocket, we designed reverse hairpin proteins in which a truncated CHR preceded the NHR in sequence with a short connecting loop (Fig. 1B) [17,18]. C28(L4)N50, named according to the length of the domains, has an exposed HP in solution. We also prepared CHR peptide containing the HP binding domain (HPbd) attached to the small globular domain GB1, allowing for straightforward bacterial synthesis and labeling for NMR. The resulting 15N-labeled constructs CPi635 (shown in Fig. 1B) was shown by HSQC to bind to C28(L4)N50 [17].
While C28(L4)N50 proved to be trimeric and highly helical [18], it gave broad low quality HSQC spectra (Supplementary Material Fig. S1), evidence of significant dynamics on the NMR chemical shift time scale. Furthermore ligands are hydrophobic and complexes with protein are poorly soluble at concentrations required for structural studies in aqueous solution. Therefore protein-detected ligand binding and structural analysis by NMR was not feasible for this system.
We observed that non-denaturing detergents helped to solubilize some of the more hydrophobic inhibitors and their mixtures with protein. We reasoned that lipids or membrane mimetic components could contribute to the mechanism of action in the milieu of the in situ fusion reaction. Here we examined this possibility using 19F NMR and fluorescence experiments. 19F NMR is a useful probe of molecular interactions due to the sensitivity of 19F-resonances to changes in local environment [19], as is deuterium NMR [20], and the low concentrations required for measurement. Additionally, 19F NMR is readily applicable to the study of proteins in the presence of detergents, lipids or stabilizing agents, because of the absence of background signals from these compounds. We examined both peptide and low molecular weight inhibitor binding to the HP, as well as the effect of phospholipid detergent dodecylphosphocholine (DPC) and Tween-20. Tween-20 is a mild non-ionic non-denaturing detergent that is used to solubilize membrane proteins without disrupting structure [21,22], DPC is a zwitterionic detergent that has been used in NMR studies of membrane-associated peptides and proteins as a membrane mimetic. The small size of DPC micelles together with the availability of DPC in perdeuterated form have favored its use for solution state proton NMR studies [23], with the caveat that functional activity should be checked on a case by case basis [24]. Previous studies of peptides and constructs of the 6HB and associated domains have resulted in disparate interpretations of their interactions with detergent micelles and lipid bilayers [2,25,26]. Some studies have indicated that DPC can alter the monomer-trimer equilibrium in gp41, which would disrupt the HP binding site [[27], [28], [29]]. Other studies have suggested that DPC is a suitable membrane mimetic in which to evaluate structure and fusion-inhibitory agents targeting gp41 domains [[30], [31], [32]]. Interpretation is likely compounded by the difficulties inherent in biophysical studies of the hydrophobic peptides, making the outcomes sensitive to exact composition, concentration and conditions. Overall, it appears that the 6HB, stable in aqueous solution, is destabilized or more dynamic in DPC micelles, while the structure is retained in zwitterionic lipid bilayers representative of the outer leaflet of the plasma membrane [33,34]. Isolated NHR domain but not 6HB has also been shown to disrupt lipid ordering [25,26].
Since our ligands contain indole or indazole groups with intrinsic fluorescence, we additionally studied binding by examining changes in fluorescence spectra. Fluorescence studies were also conducted in the presence of liposomes that could more accurately represent the bilayer interface, giving us the opportunity to examine whether DPC micelles were adequate mimetics for this protein – lipid – small molecule system.
Section snippets
Material and methods
Experimental Procedures are described in Sections S1 – S3 of the Supplementary Material. These include synthetic routes of synthesis of ligands, binding and antiviral assays, NMR and fluorescence experiments and preparation of liposomes.
C-heptad repeat domain binding in the hydrophobic pocket is readily detected by 19F NMR
We first established the sensitivity of 19F NMR to the binding interaction in the HP by studying the validated CPi635 – C28(L4)N50 interaction (Fig. 1) [17]. CPi635 and C28(L4)N50 were labeled with 5-F-tryptophan by addition of 5-F-indole to the growth media [19]. Tryptophan residues predominate in the CHR-NHR interaction in the HP. CPi635 contains three tryptophan residues, two in the HPbd and one within GB1. The NHR receptor C28(L4)N50 contains a single tryptophan residue in the HP. Fluorine
Conclusions
This study demonstrated that HPbd peptide binding and small molecule binding in the hydrophobic pocket were readily detected by 19F NMR and fluorescence. 19F resonances of fluorine-labeled peptides and small molecules shifted downfield upon addition of protein and/or detergent, indicating an increase in hydrophobic environment. In peptides, 19F spectra of 5-F-Trp were very sensitive to binding. However, in the small molecules, sensitivity of 19F chemical shifts was highest for 19F atoms that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by NIH grants GM087998 and AI122847 to MG.
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