Hydrogen-Deuterium Exchange Supports Independent Membrane-Interfacial Fusion Peptide and Transmembrane Domains in Subunit 2 of Influenza Virus Hemagglutinin Protein, a Structured and Aqueous-Protected Connection between the Fusion Peptide and Soluble Ectodomain, and the Importance of Membrane Apposition by the Trimer-of-Hairpins Structure

A large HIV gp41 construct with trimer-of-hairpins structure exhibits V2E mutation-dominant attenuation of vesicle fusion and helicity very similar to V2E attenuation of HIV fusion and infection and supports: (1) hairpin stabilization of membrane apposition with larger distance for V2E; and (2) V2E dominance by an antiparallel β sheet with interleaved fusion peptide strands from two gp41 trimers

There is complete attenuation of fusion and infection mediated by HIV gp160 with gp41 subunit with V2E mutation, and also V2E dominance with WT/V2E mixtures. V2E is at the N-terminus of the ∼25-residue fusion peptide (Fp) which likely binds the target membrane. In this study, large V2E attenuation and dominance were observed for vesicle fusion induced by FP_HM, a large gp41 ectodomain construct with Fp followed by hyperthermostable hairpin with N- and C-helices, and membrane-proximal external region (Mper). FP_HM is a trimer-of-hairpins, the final gp41 structure during fusion. Vesicle fusion and helicity were measured for FP_HM using trimers with different fractions (f's) of WT and V2E proteins. Reductions in FP_HM fusion and helicity vs. fV2E were quantitatively-similar to those for gp160-mediated fusion and infection. Global fitting of all V2E data supports 6 WT gp41 (2 trimers) required for fusion. These data are understood by a model in which the ∼25 kcal/mol free energy for initial membrane apposition is compensated by the thermostable hairpin between the Fp in target membrane and Mper/transmembrane domain in virus membrane. The data support a structural model for V2E dominance with a membrane-bound Fp with antiparallel β sheet and interleaved strands from the two trimers. Relative to fV2E = 0, a longer Fp sheet is stabilized with small fV2E because of salt-bridge and/or hydrogen bonds between E2 on one strand and C-terminal Fp residues on adjacent strands, like R22. A longer Fp sheet results in shorter N- and C-helices, and larger separation during membrane apposition which hinders fusion.

Rapid 2H NMR Transverse Relaxation of Perdeuterated Lipid Acyl Chains of Membrane with Bound Viral Fusion Peptide Supports Large-Amplitude Motions of These Chains That Can Catalyze Membrane Fusion

An early step in cellular infection by a membrane-enveloped virus like HIV or influenza is joining (fusion) of the viral and cell membranes. Fusion is catalyzed by a viral protein that typically includes an apolar "fusion peptide" (fp) segment that binds the target membrane prior to fusion. In this study, the effects of nonhomologous HIV and influenza fp's on lipid acyl chain motion are probed with ²H NMR transverse relaxation rates (R₂'s) of a perdeuterated DMPC membrane. Measurements were made between 35 and 0 °C, which brackets the membrane liquid-crystalline-to-gel phase transitions. Samples were made with either HIV "GPfp" at pH 7 or influenza "HAfp" at pH 5 or 7. GPfp induces vesicle fusion at pH 7, and HAfp induces more fusion at pH 5 vs 7. GPfp bound to DMPC adopts an intermolecular antiparallel β sheet structure, whereas HAfp is a monomer helical hairpin. The R₂'s of the no peptide and HAfp, pH 7, samples increase gradually as temperature is lowered. The R₂'s of GPfp and HAfp, pH 5, samples have very different temperature dependence, with a ∼10× increase in R₂^CD2 when temperature is reduced from 25 to 20 °C and smaller but still substantial R₂'s at 10 and 0 °C. The large R₂'s with GPfp and HAfp, pH 5, are consistent with large-amplitude motions of lipid acyl chains that can aid fusion catalysis by increasing the population of chains near the aqueous phase, which is the chain location for transition states between membrane fusion intermediates.

Advances in Hydrogen/Deuterium Exchange Mass Spectrometry and the Pursuit of Challenging Biological Systems

Solution-phase hydrogen/deuterium exchange (HDX) coupled to mass spectrometry (MS) is a widespread tool for structural analysis across academia and the biopharmaceutical industry. By monitoring the exchangeability of backbone amide protons, HDX-MS can reveal information about higher-order structure and dynamics throughout a protein, can track protein folding pathways, map interaction sites, and assess conformational states of protein samples. The combination of the versatility of the hydrogen/deuterium exchange reaction with the sensitivity of mass spectrometry has enabled the study of extremely challenging protein systems, some of which cannot be suitably studied using other techniques. Improvements over the past three decades have continually increased throughput, robustness, and expanded the limits of what is feasible for HDX-MS investigations. To provide an overview for researchers seeking to utilize and derive the most from HDX-MS for protein structural analysis, we summarize the fundamental principles, basic methodology, strengths and weaknesses, and the established applications of HDX-MS while highlighting new developments and applications.

Influenza A H1 and H3 Transmembrane Domains Interact Differently with Each Other and with Surrounding Membrane Lipids

Hemagglutinin (HA) is a class I viral membrane fusion protein, which is the most abundant transmembrane protein on the surface of influenza A virus (IAV) particles. HA plays a crucial role in the recognition of the host cell, fusion of the viral envelope and the host cell membrane, and is the major antigen in the immune response during the infection. Mature HA organizes in homotrimers consisting of a sequentially highly variable globular head and a relatively conserved stalk region. Every HA monomer comprises a hydrophilic ectodomain, a pre-transmembrane domain (pre-TMD), a hydrophobic transmembrane domain (TMD), and a cytoplasmic tail (CT). In recent years the effect of the pre-TMD and TMD on the structure and function of HA has drawn some attention. Using bioinformatic tools we analyzed all available full-length amino acid sequences of HA from 16 subtypes across various host species. We calculated several physico-chemical parameters of HA pre-TMDs and TMDs including accessible surface area (ASA), average hydrophobicity (H_av), and the hydrophobic moment (µ_H). Our data suggests that distinct differences in these parameters between the two major phylogenetic groups, represented by H1 and H3 subtypes, could have profound effects on protein–lipid interactions, trimer formation, and the overall HA ectodomain orientation and antigen exposure.

The role of fusion peptides in depth-dependent membrane organization and dynamics in promoting membrane fusion

Membrane fusion is an important event in the life of eukaryotes; occurs in several processes such as endocytosis, exocytosis, cellular trafficking, compartmentalization, import of nutrients and export of waste, vesiculation, inter cellular communication, and fertilization. The enveloped viruses as well utilize fusion between the viral envelope and host cell membrane for infection. The stretch of 20-25 amino acids located at the N-terminus of the fusion protein, known as fusion peptide, plays a decisive role in the fusion process. The stalk model of membrane fusion postulated a common route of bilayer transformation for stalk, transmembrane contact, and pore formation; and fusion peptide is believed to facilitate bilayer transformation to promote membrane fusion. The peptide-induced change in depth-dependent organization and dynamics could provide important information in understanding the role of fusion peptide in membrane fusion. In this review, we have discussed about three depth-dependent properties of the membrane such as rigidity, polarity and heterogeneity, and the impact of fusion peptide on these three membrane properties.

Viral Membrane Fusion and the Transmembrane Domain

Initiation of host cell infection by an enveloped virus requires a viral-to-host cell membrane fusion event. This event is mediated by at least one viral transmembrane glycoprotein, termed the fusion protein, which is a key therapeutic target. Viral fusion proteins have been studied for decades, and numerous critical insights into their function have been elucidated. However, the transmembrane region remains one of the most poorly understood facets of these proteins. In the past ten years, the field has made significant advances in understanding the role of the membrane-spanning region of viral fusion proteins. We summarize developments made in the past decade that have contributed to the understanding of the transmembrane region of viral fusion proteins, highlighting not only their critical role in the membrane fusion process, but further demonstrating their involvement in several aspects of the viral lifecycle.

2H nuclear magnetic resonance spectroscopy supports larger amplitude fast motion and interference with lipid chain ordering for membrane that contains β sheet human immunodeficiency virus gp41 fusion peptide or helical hairpin influenza virus hemagglutinin fusion peptide at fusogenic pH

Enveloped viruses are surrounded by a membrane which is obtained from an infected host cell during budding. Infection of a new cell requires joining (fusion) of the virus and cell membranes. This process is mediated by a monotopic viral fusion protein with a large ectodomain outside the virus. The ectodomains of class I enveloped viruses have a N-terminal "fusion peptide" (fp) domain that is critical for fusion and binds to the cell membrane. In this study, ²H NMR spectra are analyzed for deuterated membrane with fp from either HIV gp41 (GP) or influenza hemagglutinin (HA) fusion proteins. In addition, the HAfp samples are studied at more fusogenic pH 5 and less fusogenic pH 7. GPfp adopts intermolecular antiparallel β sheet structure whereas HAfp is a monomeric helical hairpin. The data are obtained for a set of temperatures between 35 and 0 °C using DMPC-d54 lipid with perdeuterated acyl chains. The DMPC has liquid-crystalline (L_α) phase with disordered chains at higher temperature and rippled gel (P_β') or gel phase (L_β') with ordered chains at lower temperature. At given temperature T, the no peptide and HAfp, pH 7 samples exhibit similar spectral lineshapes. Spectral broadening with reduced temperature correlates with the transition from L_α to P_β' and then L_β' phases. At given T, the lineshapes are narrower for HAfp, pH 5 vs. no peptide and HAfp, pH 7 samples, and even narrower for the GPfp sample. These data support larger-amplitude fast (>10⁵ Hz) lipid acyl chain motion for samples with fusogenic peptides, and peptide interference with chain ordering. The NMR data of the present paper correlate with insertion of these peptides into the hydrocarbon core of the membrane and support a significant fusion contribution from the resultant lipid acyl chain disorder, perhaps because of reduced barriers between the different membrane topologies in the fusion pathway. Membrane insertion and lipid perturbation appear common to both β sheet and helical hairpin peptides.