STUDIES OF THE STRUCTURAL ORGANIZATION OF A BACTERIAL CHEMORECEPTOR BY ELECTRON-MICROSCOPY

We used analysis by electron microscopy to obtain structural informati on about a transmembrane receptor that mediates chemotaxis in Escheric hia coli. Two-dimensional arrays of regularly packed particles of the receptor Trg were obtained by reconstitution of purified, detergent-so lubilized protein into lipid bilayers. Preliminary image processing of negatively stained arrays revealed an almost square 8.8 x 8.8-nm unit cell and resolved the particles into four peaks of density around a c entral depression. In certain conditions, reconstituted, Trg-containin g bilayers associated into membrane stacks. The regular spacing of the stacks provided a value of 15 nm for the dimension of the receptor no rmal to the membrane. Using these dimensions, the estimated occupied v olume of the structure would be sufficient to contain four monomers of Trg. This tetramer form may be a dimer of two antiparallel or paralle l homodimers. Our analysis indicates that a receptor monomer is approx imately 4.4 nm at the widest point and 15 nm long. Given the dimension s of the periplasmic domain of the closely related receptor Tar(s), de termined by X-ray crystallography, and a minimum bilayer thickness of 3 nm, the cytoplasmic domain would be approximately 5.0 by 4.4 nm. Hig her resolution analysis should reveal additional information about rec eptor structure. (C) 1994 Academic Press, Inc.