TCBG Seminar

Crystallization and Structural Determination of Bacteriorhodopsin in Lipidic Cubic Phases Towards Elucidating the Proton Translocation Pathway

Dr. Ehud Landau
University of Basel

Tuesday, October 21, 1997
2:00 pm (CT)
3269 Beckman Institute


A molecular level understanding of the mechanisms of action of membrane proteins requires elucidation of their structures to high resolution. To date, only a few high resolution structures of membrane proteins have been solved, reflecting the major stumbling block in this endeavor - the routine production of well-ordered three-dimensional crystals. We have devised a novel concept for the crystallization of membrane proteins by exploiting the properties of bicontinuous lipidic cubic phases. This membrane-mimetic system, consisting of lipid, water and protein in appropriate proportions, forms a transparent 3-D curved bilayer matrix, which is pervaded by two non-connected aqueous channels. Membrane proteins, once inserted into this complex array, diffuse laterally to nucleate, and eventually to yield well-ordered crystals. Hexagonal bacteriorhodopsin microcrystals diffract isotropically to 2.0 A resolution, with a space group P 63 and unit cell dimensions of a=b=61.76 A, c=104.16 A, a=b=90 deg and g=120 deg and one monomer per asymmetric unit. The crystal structure was solved at a resolution of 2.5 A by molecular replacement, using previous results from electron crystallographic studies as a model. The earlier structure is confirmed overall, but several significant differences are revealed. Our structure identifies, for the first time, the locations of water molecules within this membrane protein. Thus, the structural basis for the mechanism of proton translocation, which hitherto remained largely speculative is set, at least in the extracellular part of the channel.

Tea and coffee will be served in R3151 Beckman Institute at 2:15pm.

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