Viral Fusion Proteins

Membrane curvature is essential to many biological processes such as endocytosis, vesicle trafficking, and cell division. Proteins can sense, stabilize, and induce membrane curvature. An important class of membrane proteins that induce membrane curvature is viral fusion proteins, which merge the virus envelope and the target membrane to enable virus entry into cells. They accomplish this task by undergoing complex conformational rearrangements, as seen in X-ray crystal structures of water-soluble ectodomains of these proteins. These protein conformational changes presumably lower the free energy barriers for membrane dehydration and membrane structural changes from the lamellar state to hemifused intermediates to the final fused state. However, this conceptual framework excludes two key hydrophobic domains: the N-terminal fusion peptide (FP) domain and the C-terminal transmembrane domain (TMD), which play important roles in destabilizing the lamellar structures of the two lipid membranes.

Using solid-state NMR spectroscopy and complementary techniques such as small-angle X-ray scattering (SAXS), we investigate the conformations and oligomeric structures of the hydrophobic FP and TMD of viral fusion proteins in biologically relevant lipid membranes. Studies of the parainfluenza virus fusion protein indicate that both the FP and the TMD have membrane-dependent structures, and the Beta-sheet conformation is responsible for generating saddle-splay curvature to the membrane. The latter is manifested in 31P NMR spectra and SAXS data. We also measure water-lipid and water-protein interactions to obtain information about membrane dehydration during fusion. By coupling these protein structure measurements with membrane morphology experiments, we obtain comprehensive information about the protein and membrane structural changes along the fusion pathway. We are also investigating the structure of the HIV fusion protein, gp41.