MIT Chemistry Directory

Mei Hong

Professor

The Hong group develops and applies high-resolution solid-state NMR spectroscopy to elucidate the structure and dynamics of biological macromolecules, with an emphasis on membrane proteins. We design magic-angle-spinning multidimensional NMR correlation techniques to measure inter-atomic distances and molecular motions, and apply these techniques to problems in biology, pharmacology, and biomaterials. We have a long-standing interest in ion channels and curvature-inducing membrane proteins. We are also investigating the structures of energy-rich plant cell walls and amyloid proteins involved in disease and catalysis. These noncrystalline and insoluble macromolecules are studied in their native environments, at atomic resolution, with both structural and dynamical details, from which we obtain crucial mechanistic insights that are rarely available from other structural techniques. 

 

Influenza virus M2 proteins

Ion channels and transporters provide conduits for the passage of ions and polar compounds across the hydrophobic barrier of lipid membranes. The M2 protein of influenza viruses acts as a proton channel that enables virus uncoating; it also mediates membrane scission in the last step of virus budding. As such influenza M2 is an excellent model for larger ion channels, synthetic proton conductors, as well as curvature-inducing membrane proteins. The M2 protein of influenza A viruses, which are responsible for all flu pandemics in history, is the target of one of two classes of antiviral drugs, but currently circulating seasonal flu viruses have evolved drug-resistant mutations. Influenza B virus M2 is so far not druggable, thus structure elucidation of AM2 and BM2 is important for drug development. We investigate the structure and dynamics of the M2 protein to understand how it conducts protons, how it binds drugs, and how it induces membrane curvature. Solid-state NMR techniques are applied to 1) measure the oligomeric assembly and three-dimensional structure of the protein; 2) reveal protein-drug and protein-lipid interactions; 3) elucidate the proton-transfer equilibria, kinetics and motions of functional residues; 4) characterize water dynamics in the channel. The influence of the ectodomain and cytoplasmic domain on the structure and channel activity of the central transmembrane domain is investigated. The effect of the membrane environment on the protein dynamics and drug binding is probed. We also study how drug-resistant mutations and channel-modifying mutations impact the protein structure. Finally, with our collaborators we study novel drugs that target mutant M2 proteins. 

 

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. For example, cationic antimicrobial peptides cause membrane curvature to disrupt the integrity of lamellar phospholipid bilayers. 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, the current framework of virus-cell fusion largely excludes two key hydrophobic domains: the N-terminal fusion peptide domain and the C-terminal transmembrane domain, which are suspected to play important roles in destabilizing the lamellar structures of the two lipid membranes. Using solid-state NMR and complementary techniques such as small-angle X-ray scattering, we seek to elucidate the conformations and oligomeric structures of the hydrophobic fusion peptide and transmembrane domains in biologically relevant lipid membranes. By coupling protein structure measurements with observation of membrane morphology by 31P NMR and detection of water-lipid and water-protein interactions, we couple protein structure information with membrane morphology and membrane hydration information, thus obtaining a comprehensive view of the protein and membrane structural transformations along the fusion pathway. The fusion proteins of parainfluenza and HIV viruses are currently studied. 

 

Plant cell walls 

Plant cell walls provide mechanical strength to cells while at the same time allowing plants to grow. This energy-rich material consists of a mixture of polysaccharides and proteins, whose composition varies between primary and secondary cell walls and between different plant organisms. Although the composition of plant cell walls is relatively well known, the three-dimensional structures and interactions of cell wall polymers have long remained elusive due to the lack of high-resolution structural techniques for characterizing the insoluble and noncrystalline cell wall. We have pioneered the use of multidimensional correlation solid-state NMR to elucidate the structures and dynamics of the polysaccharides of intact primary cell walls. By enriching whole plants during growth with 13C, we are able to employ 2D and 3D SSNMR techniques to detect and resolve the signals of cellulose, hemicellulose and pectins, determine their intermolecular contacts and their mobilities. Moreover, by sensitivity-enhancing dynamic nuclear polarization and paramagnetic relaxation enhancement approaches, we can elucidate how proteins bind polysaccharides to loosen the cell wall. Using model plants of both dicot (Arabidopsis thaliana) and grass (Brachypodium distachyon and Zea mays) families, we have shown that cellulose, hemicellulose and pectins form a single three-dimensional network instead of two separate networks, thus changing the long-held view of the cell wall architecture. With our collaborators, we investigate the structural polymorphism of cellulose microfibrils, hydration of wall polysaccharides, interactions of cellulose with matrix polysaccharides, and the effects of genetic mutations on the cell-wall network structure. 

 

Solid-state NMR techniques for biomolecular structure determination 

To address complex and cutting-edge biological questions, we continue to expand the capability of solid-state NMR spectroscopy. Of particular interest to us are multinuclear distance-measuring techniques (e.g. 19F-19F, 13C-2H, 13C-31P and 13C-1H distances); intermolecular correlation experiments that probe protein-lipid, protein-water and polysaccharide-water interactions; anisotropic-isotropic correlation techniques to measure molecular motion with high angular resolution; facile isotopic labeling schemes for proteins and other biomacromolecules; and computational methods for resonance assignment of NMR spectra. 

 

Contact Information

t: 6172535521
e: MEIHONG@MIT.EDU

Office

NW14-3212

Education

B.A. 1992, MOUNT HOLYOKE COLLEGE
Ph.D. 1996, UNIVERSITY OF CALIFORNIA-BERKELEY

Administrative Assistant

Jillian Haggerty

Email:jhaggert@mit.edu
Tel: (617) 253-5478
Room NW14-3218
77 Massachusetts Ave.
Cambridge, MA 02139