Conformation and Trimer Association of the Transmembrane Domain of the Parainfluenza Virus Fusion Protein in Lipid Bilayers from Solid-State NMR: Insights into the Sequence Determinants of Trimer Structure and Fusion Activity

Enveloped viruses enter cells by using their fusion proteins to merge the virus lipid envelope and the cell membrane. While crystal structures of the water-soluble ectodomains of many viral fusion proteins have been determined, the structure and assembly of the C-terminal transmembrane domain (TMD) remains poorly understood. Here we use solid-state NMR to determine the backbone conformation and oligomeric structure of the TMD of the parainfluenza virus 5 fusion protein. 13C chemical shifts indicate that the central leucine-rich segment of the TMD is α-helical in POPC/cholesterol membranes and POPE membranes, while the Ile- and Val-rich termini shift to the β-strand conformation in the POPE membrane. Importantly, lipid mixing assays indicate that the TMD is more fusogenic in the POPE membrane than in the POPC/cholesterol membrane, indicating that the β-strand conformation is important for fusion by inducing membrane curvature. Incorporation of para-fluorinated Phe at three positions of the α-helical core allowed us to measure interhelical distances using 19F spin diffusion NMR. The data indicate that, at peptide:lipid molar ratios of ~ 1:15, the TMD forms a trimeric helical bundle with inter-helical distances of 8.2–8.4 Å for L493F and L504F and 10.5 Å for L500F. These data provide high-resolution evidence of trimer formation of a viral fusion protein TMD in phospholipid bilayers, and indicate that the parainfluenza virus 5 fusion protein TMD harbors two functions: the central α-helical core is the trimerization unit of the protein, while the two termini are responsible for inducing membrane curvature by transitioning to a β-sheet conformation. Keywords: magic-angle-spinning NMR; trimer formation; conformational plasticity; spin diffusionNational Institutes of Health (U.S.) (Grant GM066976

Cholesterol-binding site of the influenza M2 protein in lipid bilayers from solid-state NMR

The influenza M2 protein not only forms a proton channel but also mediates membrane scission in a cholesterol-dependent manner to cause virus budding and release. The atomic interaction of cholesterol with M2, as with most eukaryotic membrane proteins, has long been elusive. We have now determined the cholesterol-binding site of the M2 protein in phospholipid bilayers using solid-state NMR spectroscopy. Chain-fluorinated cholesterol was used to measure cholesterol proximity to M2 while sterol-deuterated cholesterol was used to measure bound-cholesterol orientation in lipid bilayers. Carbon–fluorine distance measurements show that at a cholesterol concentration of 17 mol%, two cholesterol molecules bind each M2 tetramer. Cholesterol binds the C-terminal transmembrane (TM) residues, near an amphipathic helix, without requiring a cholesterol recognition sequence motif. Deuterium NMR spectra indicate that bound cholesterol is approximately parallel to the bilayer normal, with the rough face of the sterol rings apposed to methyl-rich TM residues. The distance- and orientation-restrained cholesterol-binding site structure shows that cholesterol is stabilized by hydrophobic interactions with the TM helix and polar and aromatic interactions with neighboring amphipathic helices. At the 1:2 binding stoichiometry, lipid31P spectra show an isotropic peak indicative of high membrane curvature. This M2–cholesterol complex structure, together with previously observed M2 localization at phase boundaries, suggests that cholesterol mediates M2 clustering to the neck of the budding virus to cause the necessary curvature for membrane scission. The solid-state NMR approach developed here is generally applicable for elucidating the structural basis of cholesterol’s effects on membrane protein function. Keywords: membrane; ¹⁹F-NMR; deuterium NMR; docking; membrane scissio

Characterization of structure and dynamics of membrane proteins from solid-state NMR

Thesis: Ph. D. in Physical Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2018.Cataloged from PDF version of thesis.Includes bibliographical references.Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is an essential tool for elucidating the structure, dynamics, and function of biomolecules. ssNMR is capable of studying membrane proteins in near-native lipid bilayers and is thus preferred over other biophysical techniques for characterizing the structure and dynamics of membrane proteins. This thesis primarily focuses on the study of the following membrane proteins: 1) the N-terminal ectodomain and C-terminal cytoplasmic domain of influenza A virus M2 and 2) HIV-1 glycoprotein gp4l membrane-proximal external region and transmembrane domain (MPER-TMD) in a near native membrane environment. The cytoplasmic domain of M2 is necessary for membrane scission and virus shedding. The M2(22-71) construct shows random-coil chemical shifts, large motional amplitudes, and a membrane surface-bound location with close proximity to water, indicating the post-amphipathic helix (AH) cytoplasmic domain is a dynamic random coil near the membrane surface. The influenza M2 ectodomain contains highly conserved epitopes but its structure is largely unknown. The M2(1-49) construct containing both the ectodomain and transmembrane domain exhibits an entirely unstructured ectodomain with a motional gradient in which the motion is slower for residues near the TM domain, which attributed to the formation of a tighter helical bundle in the presence of drug that should cause the more tightened C-terminal ectodomain, thereby slowing its local motions. HIV-1 virus gp4l is directly involved in virus-cell membrane fusion. However, the structural topologies of the gp4l MPER-TMD are still controversial and the biologically-relevant intrinsic conformational state of MPER has not yet been determined. In order to obtain near native structural information of gp4l, we have studied gp41 (665-704) and found a primarily a-helical conformation, membrane-anchored trimeric TMD and water-exposed membrane surface-bound MPER. Intra- and intermolecular distances measured using ¹⁹C-¹⁹F REDOR and ¹⁹F-¹⁹F CODEX revealed that MPER-TMD has a significant kink between MPER and TMD, which has aided a deeper understanding of the HIV virus entry mechanism and the design of vaccines.by Byungsu Kwon.Ph. D. in Physical Chemistr

The Influenza M2 Ectodomain Regulates the Conformational Equilibria of the Transmembrane Proton Channel: Insights from Solid-State Nuclear Magnetic Resonance

The influenza M2 protein is the target of the amantadine family of antiviral drugs, and its transmembrane (TM) domain structure and dynamics have been extensively studied. However, little is known about the structure of the highly conserved N-terminal ectodomain, which contains epitopes targeted by influenza vaccines. In this study, we synthesized an M2 construct containing the N-terminal ectodomain and the TM domain, to understand the site-specific conformation and dynamics of the ectodomain and to investigate the effect of the ectodomain on the TM structure. We incorporated ¹³C- and ¹⁵N-labeled residues into both domains and measured their chemical shifts and line widths using solid-state nuclear magnetic resonance. The data indicate that the entire ectodomain is unstructured and dynamic, but the motion is slower for residues closer to the TM domain. ¹³C line shapes indicate that this ecto-TM construct undergoes fast uniaxial rotational diffusion, like the isolated TM peptide, but drug binding increases the motional rates of the TM helix while slowing the local motion of the ectodomain residues that are close to the TM domain. Moreover, ¹³C and ¹⁵N chemical shifts indicate that the ectodomain shifts the conformational equilibria of the TM residues toward the drug-bound state even in the absence of amantadine, thus providing a molecular structural basis for the lower inhibitory concentration of full-length M2 compared to that of the ectodomain-truncated M2. We propose that this conformational selection may result from electrostatic repulsion between negatively charged ectodomain residues in the tetrameric protein. Together with the recent study of the M2 cytoplasmic domain, these results show that intrinsically disordered extramembrane domains in membrane proteins can regulate the functionally relevant conformation and dynamics of the structurally ordered TM domains.National Institutes of Health (U.S.) (Grant GM088204

Interactions of HIV gp41's membrane-proximal external region and transmembrane domain with phospholipid membranes from 31P NMR

HIV-1 entry into cells requires coordinated changes of the conformation and dynamics of both the fusion protein, gp41, and the lipids in the cell membrane and virus envelope. Commonly proposed features of membrane deformation during fusion include high membrane curvature, lipid disorder, and membrane surface dehydration. The virus envelope and target cell membrane contain a diverse set of phospholipids and cholesterol. To dissect how different lipids interact with gp41 to contribute to membrane fusion, here we use 31P solid-state NMR spectroscopy to investigate the curvature, dynamics, and hydration of POPE, POPC and POPS membranes, with and without cholesterol, in the presence of a peptide comprising the membrane proximal external region (MPER) and transmembrane domain (TMD) of gp41. Static 31P NMR spectra indicate that the MPER-TMD induces strong negative Gaussian curvature (NGC) to the POPE membrane but little curvature to POPC and POPC:POPS membranes. The NGC manifests as an isotropic peak in the static NMR spectra, whose intensity increases with the peptide concentration. Cholesterol inhibits the NGC formation and stabilizes the lamellar phase. Relative intensities of magic-angle spinning 31P cross-polarization and direct-polarization spectra indicate that all three phospholipids become more mobile upon peptide binding. Finally, 2D 1H-31P correlation spectra show that the MPER-TMD enhances water 1H polarization transfer to the lipids, indicating that the membrane surfaces become more hydrated. These results suggest that POPE is an essential component of the high-curvature fusion site, and lipid dynamic disorder is a general feature of membrane restructuring during fusion

Two-dimensional 19F–13C correlation NMR for 19F resonance assignment of fluorinated proteins

© 2020, Springer Nature B.V. 19F solid-state NMR is an excellent approach for measuring long-range distances for structure determination and for studying molecular motion. For multi-fluorinated proteins, assignment of 19F chemical shifts has been traditionally carried out using mutagenesis. Here we show 2D 19F–13C correlation experiments that allow efficient assignment of the 19F chemical shifts. We have compared several rotational-echo double-resonance-based pulse sequences and 19F–13C cross polarization (CP) for 2D 19F–13C correlation. We found that direct transferred-echo double-resonance (TEDOR) transfer from 19F to 13C and vice versa outperforms out-and-back coherence transfer schemes. 19F detection gives twofold higher sensitivity over 13C detection for the 2D correlation experiment. At MAS frequencies of 25–35 kHz, double-quantum 19F–13C CP has higher coherence transfer efficiencies than zero-quantum CP. The most efficient TEDOR transfer experiment has higher sensitivity than the most efficient double-quantum CP experiment. We demonstrate these 2D 19F–13C correlation experiments on the model compounds t-Boc-4F-phenylalanine and GB1. Application of the 2D 19F–13C TEDOR correlation experiment to the tetrameric influenza BM2 transmembrane peptide shows intermolecular 13C–19F cross peaks that indicate that the BM2 tetramers cluster in the lipid bilayer in an antiparallel fashion. This clustering may be relevant for the virus budding function of this protein

Elucidating Relayed Proton Transfer through a His–Trp–His Triad of a Transmembrane Proton Channel by Solid-State NMR

© 2019 Elsevier Ltd Proton transfer through membrane-bound ion channels is mediated by both water and polar residues of proteins, but the detailed molecular mechanism is challenging to determine. The tetrameric influenza A and B virus M2 proteins form canonical proton channels that use an HxxxW motif for proton selectivity and gating. The BM2 channel also contains a second histidine (His), H27, equidistant from the gating tryptophan, which leads to a symmetric H19xxxW23xxxH27 motif. The proton-dissociation constants (pKa's) of H19 in BM2 were found to be much lower than the pKa's of H37 in AM2. To determine if the lower pKa's result from H27-facilitated proton dissociation of H19, we have now investigated a H27A mutant of BM2 using solid-state NMR. 15N NMR spectra indicate that removal of the second histidine converted the protonation and tautomeric equilibria of H19 to be similar to the H37 behavior in AM2, indicating that the peripheral H27 is indeed the origin of the low pKa's of H19 in wild-type BM2. Measured interhelical distances between W23 sidechains indicate that the pore constriction at W23 increases with the H19 tetrad charge but is independent of the H27A mutation. These results indicate that H27 both accelerates proton dissociation from H19 to increase the inward proton conductance and causes the small reverse conductance of BM2. The proton relay between H19 and H27 is likely mediated by the intervening gating tryptophan through cation–π interactions. This relayed proton transfer may exist in other ion channels and has implications for the design of imidazole-based synthetic proton channels