Structural studies of two macromolecular complexes involved in bacterial chemotaxis.

摘要

The drive-train of the bacterial flagellum is comprised of an axle, a universal joint, and a propeller. These substructures appear to share a common helical symmetry. Nine proteins make up the drive-train: four rod proteins, the hook protein, two hook-associated proteins, flagellin, and a capping protein. Sequence information from the nine proteins that make up the drive-train suggests the presence of a common alpha-helical motif. In high-resolution reconstructions of the flagellar filament, there are features consistent with an alpha-helical motif. The hypothesis is that these features correspond to the alpha-helical motif suggested by the sequence information. Using helical reconstruction methods on electron micrographs, we have generated a three-dimensional map of the flagellar hook from Salmonella typhimurium . This map was of sufficient quality to confirm a second example of this alpha-helical motif.; Bacterial chemoreceptor complexes in Escherichia coli and Salmonella typhimurium are composed of ligand-specific receptors, a histidine kinase CheA, and an adapter protein CheW. The transmembrane receptor for aspartate, Tar, is representative of a class of receptor molecules. The isolated cytoplasmic domain of Tar fused to a leucine zipper sequence forms a soluble ternary complex with CheA and CheW. The complex is an elongated, bipolar structure composed of approximately 28 receptor signaling domain chains, 6 CheW chains, and 4 CheA chains. Immunoelectron microscopy has provided a general picture of the subunit organization of the complexes. CheA and CheW appear to be in the middle of the complex with the leucine zippers of the receptor construct at the ends. Image analysis of electron micrographs of the undecorated complex suggests that the receptor domains in the complex have 7-fold symmetry along the long axis and 2-fold axes perpendicular to it. These results raise questions about the organization of the complex in the cell. Instead of projecting into the cytoplasm perpendicular to the membrane, the finger-like cytoplasmic domains of the receptors, with the bound CheW and CheA, may dimerize by their tips and run parallel to the membrane.