Studies of the structural organization of a bacterial chemoreceptor by electron microscopy

We used analysis by electron microscopy to obtain structural information about a transmembrane receptor that mediates chemotaxis in Escherichia coli. Two-dimensional arrays of regularly packed particles of the receptor Trg were obtained by reconstitution of purified, detergent-solubilized 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 central depression. In certain conditions, reconstituted, Trg-containing bilayers associated into membrane stacks. The regular spacing of the stacks provided a value of 15 nm for the dimension of the receptor normal to the membrane. Using these dimensions, the estimated occupied volume of the structure would be sufficient to contain four monomers of Trg. This tetramer form may be a dimer of two antiparallel or parallel homodimers. Our analysis indicates that a receptor monomer is approximately 4.4 nm at the widest point and 15 nm long. Given the dimensions of the periplasmic domain of the closely related receptor Tars, determined by X-ray crystallography, and a minimum bilayer thickness of 3 nm, the cytoplasmic domain would be approximately 5.0 by 4.4 nm. Higher resolution analysis should reveal additional information about receptor structure.