Electron microscopic analysis of membrane assemblies formed by the bacterial chemotaxis receptor Tsr

The serine receptor (Tsr) from Escherichia coli is representative of a large family of transmembrane receptor proteins that mediate bacterial chemotaxis by influencing cell motility through signal transduction pathways. Tsr and other chemotaxis receptors form patches in the inner membrane that are often localized at the poles of the bacteria. In an effort to understand the structural constraints that dictate the packing of receptors in the plane of the membrane, we have used electron microscopy to examine ordered assemblies of Tsr in membrane extracts isolated from cells engineered to overproduce the receptor. Three types of assemblies were observed: ring-like "micelles" with a radial arrangement of receptor subunits, two-dimensional crystalline arrays with approximate hexagonal symmetry, and "zippers," which are receptor bilayers that result from the antiparallel interdigitation of cytoplasmic domains. The registration among Tsr molecules in the micelle and zipper assemblies was sufficient for identification of the receptor domains and for determination of their contributions to the total receptor length. The overall result of this analysis is compatible with an atomic model of the receptor dimer that was constructed primarily from the X-ray crystal structures of the periplasmic and cytoplasmic domains. Significantly, the micelle and zipper structures were also observed in fixed, cryosectioned cells expressing the Tsr receptor at high abundance, suggesting that the modes of Tsr assembly found in vitro are relevant to the situation in the cell.