Department of Cell Biology
Molecular electron microscopy (EM) is a versatile tool to analyze the structure of molecules that are not easily amenable to structure determination by the more established techniques of X-ray crystallography and NMR spectroscopy. Our group uses single-particle EM to visualize macromolecular complexes and electron crystallography of two-dimensional crystals to determine the structure of membrane proteins.
We are interested in understanding how membrane proteins interact with their surrounding, annual lipids, but direct structural information on the position and conformation of lipids surrounding membrane proteins has been hard to obtain. Much of our knowledge derives from crystal structures of membrane proteins with specifically bound lipid molecules. Our electron crystallographic analysis of aquaporin-0 (AQP0) reconstituted with the lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) into double-layered 2D crystals at 1.9-Å resolution revealed nine DMPC molecules (see the figure). Since AQP0 has no specific lipid binding sites and DMPC is not a native lipid, our structure shows a nonspecific mode of protein-lipid interactions. AQP0 forms 2D crystals with many different lipids, offering an unusual opportunity to ask fundamental questions about how lipids interact with membrane proteins. By visualizing the structure of AQP0 2D crystals obtained with lipids with different acyl chains and different headgroups, we hope to learn how the bilayer structure adapts to accommodate the hydrophobic belt of membrane proteins and which lipid characteristics are most important for interactions with membrane proteins.
Complexes involved in vesicular transport are a second focus of our group. Eukaryotic cells have to moveproteins and other cellular components between their organelles. This intracellular trafficking is mediated by transport vesicles that bud off from the membrane of the donor compartment and dock at the membrane of the target compartment, with which they fuse in a process catalyzed by SNARE (soluble NSF [N-ethylmaleimide sensitive factor] attachment protein receptor) proteins. The individual steps in vesicular transport are catalyzed by a large collection of proteins that form elaborate complexes that, in turn, interact with each other and drive the process. We are currently focusing on the so-called “multisubunit tethering complexes” (MTCs), which play a central role in organizing the events that occur when a vesicle arrives at its target membrane. Through interactions with small GTPases, primarily those in the Rab family, as well as with vesicle coat proteins, and phospholipids, MTCs mediate the initial, reversible interaction of a transport vesicle with its target membrane. In addition, MTCs interact with SNARE and SM proteins, thus coupling vesicle capture to the membrane fusion machinery. MTCs can be subdivided into three groups: MTCs functioning in the secretory pathway (Dsl1, COG, GARP, and exocyst), MTCs of the endolysosomal pathway (HOPS and CORVET), and transport protein particle (TRAPP) complexes. MTCs in the secretory and endolysosomal pathways are Rab effectors and are thought to promote tethering by interacting with Rabs and SNAREs. In contrast, TRAPP complexes function as GEFs for the Rab GTPase Ypt1/Rab1 and combine tethering with coat recognition. To establish a basis for understanding how MTCs can orchestrate these diverse events and to obtain mechanistic insights into these processes, we use single-particle EM to establish the subunit organization of the various MTCs, to characterize their structural dynamics, and to visualize how they interact with Rabs, coat proteins and SNAREs.
Hite, R. K., Li, Z., and Walz, T. (2010) Principles of membrane protein interactions with annular lipids deduced from aquaporin-0 2D crystals. EMBO J. 29: 1652-1658.
Yip, C. K., Berscheminski, J., and Walz, T. (2010) Molecular architecture of the TRAPPII complex and implications for vesicle tethering. Nat. Struct. Mol. Biol. 17: 1298-1304.
Lees JA, Yip CK, Walz T, Hughson FM. (2010) Molecular organization of the COG vesicle tethering complex. Nat. Struct. Mol. Biol. 17: 1292-1297.
Aponte-Santamaría, C., Briones, R., Schenk, A. D., Walz, T., and de Groot, B. L. (2012) Molecular driving forces defining lipid positions around aquaporin-0. Proc. Natl. Acad. Sci. U.S.A. 109: 9887-9892.
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