Membrane-enveloped viruses have protein spikes that include a “fusion peptide” (Fp) segment that binds the target membrane of a host cell and plays a critical role in fusion (joining) of viral and target membranes. For influenza virus, this is subunit 2 of hemagglutinin which has a ~20-residue N-terminal fusion peptide region that binds target membrane. The Fp of human immunodeficiency virus (HIV) is the ~23 N-terminal residues of the glycoprotein 41 kD (gp41) subunit of the gp160 spike complex. Although the fusion mechanism of class I enveloped virus is relatively well-understood, researchers continue making efforts to reveal the full detailed picture of the fusion.My studies related to influenza fusion peptide aims to provide answer to an outstanding question which is whether there are associated membrane changes important for fusion. Several computational studies have found increased “protrusion” of lipid acyl chains near Fp, i.e. one or more chain carbons are closer to the aqueous region than the headgroup phosphorus. Protrusion may accelerate initial joining of outer leaflets of the two membranes into a stalk intermediate. In this study, higher protrusion probability in membrane with vs. without Fp is convincingly detected by larger Mn2+-associated increases in chain 13C NMR transverse relaxation rates (2’s). Data analysis provides a ratio 2,neighbor/2,distant for lipids neighboring vs. more distant from the Fp. The calculated ratio depends on the number of Fp-neighboring lipids and the experimentally-derived range of 4 to 24 matches the range of increased protrusion probabilities from different simulations. For samples either with or without Fp, the 2 values are well-fitted by an exponential decay as the 13C site moves closer to the chain terminus. The decays correlate with free-energy of protrusion proportional to the number of protruded CH2 groups, with free energy per CH2 of ~0.25 kBT. The NMR data support one major fusion role of the Fp to be much greater protrusion of lipid chains, with highest protrusion probability for chain regions closest to the headgroups. Unlike Fp of influenza virus adopting "α" helical structure, the Fp of HIV adopts predominant intermolecular antiparallel b sheet structure when mole fraction cholesterol » 0.3 which is comparable to host cell fractions. The V2E engineered mutation near the N-terminus of the Fp greatly reduces gp160-mediated cell-cell fusion and gp41-induced vesicle fusion. To explore the broad population distribution of HIV Fp, REDOR NMR was applied to determine the registries (alignments) of adjacent Fp molecules in membrane-bound Fp. REDOR dephasing probed proximity between a backbone 13CO label at a specific residue in one Fp molecule and backbone 15N labels in adjacent Fp’s at a different residue. For both WT and V2E, REDOR was measured for 17 differently-labeled Fp’s by Dr. Scott Schmick and Dr. Li Xie and the data then analyzed by me to quantitatively-determine the fractional populations, f(t)’s, of individual antiparallel registries indexed by t, the number of Fp residues in the sheet starting from the N-terminus. Both the WT and V2E sheets contained broad distributions of populated registries that included t=11-20,22,23 for WT and t=15-21,23 for V2E, with WT = 16.2 and V2E = 18.5. The f(t)WT values were well-fitted to free energies, G(t)WT, that were sums of favorable contributions including one proportional to sheet length, another for registries in which Leu’s were aligned in adjacent Fp’s, and a third proportional to free energy of sidechain membrane insertion. The f(t)V2E’s were similarly well-fitted except there wasn’t the insertion contribution. Non-inserted V2E Fp is one basis for reduced fusion, and another is that longer V2E sheets result in shorter C-terminal hairpins, with consequent larger distances between initial apposed membranes. The structural information of inclusion bodies (IBs) of recombinant protein (Rp) grown in bacterial host is another interest of my research. IBs is intracellular solid aggregates, a byproduct of growing cells in bacterial systems. The IBs fraction is often discarded because solubilization and subsequent refolding is difficult. There is little information about the structure of any Rp in IBs and such information may be useful for developing better solubilization and refolding. Because of this gap in knowledge, solid-state NMR was used to obtain structural information about a 109-residue “HM” Rp produced in bacteria. Several 2D and 3D 15N-13C NMR correlation spectra of 13C and 15N labeled HM have been recorded and the assignment of the spectral crosspeaks based on amino acid type supported that there exist major α helical and minor β sheet structure in HM lBs.
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