The bacterial chemotaxis receptors are transmembrane receptors with a simple signalling pathway which has elements relevant to the general understanding of signal recognition and transduction across membranes, how signals are relayed between molecules in a pathway, and how adaptation to a persistent signal is achieved. In contrast to many mammalian receptors which signal by oligomerizing upon ligand binding, the chemotaxis receptors are dimeric even in the absence of their ligands, and their signalling does not depend on a monomer-dimer equilibrium. Bacterial chemotaxis receptors are composed of a ligand-binding domain, a transmembrane domain consisting of two helices TM1 and TM2, and a cytoplasmic domain. All known bacterial chemotaxis receptors have a highly conserved cytoplasmic domain, which unites signals from different ligand domains into a single signalling pathway to flagella motors. Here we report the crystal structure of the cytoplasmic domain of a serine chemotaxis receptor of Escherichia coli, which reveals a 200 A-long coiled-coil of two antiparallel helices connected by a 'U-turn'. Two of these domains form a long, supercoiled, four-helical bundle in the cytoplasmic portion of the receptor.

Many small bacterial, archaebacterial, and eukaryotic genomes have been sequenced, and the larger eukaryotic genomes are predicted to be completely sequenced within the next decade. In all genomes sequenced to date, a large portion of these organisms' predicted protein coding regions encode polypeptides of unknown biochemical, biophysical, and͞or cellular functions. Three-dimensional structures of these proteins may suggest biochemical or biophysical functions. Here we report the crystal structure of one such protein, MJ0577, from a hyperthermophile, Methanococcus jannaschii, at 1.7-Å resolution. The structure contains a bound ATP, suggesting MJ0577 is an ATPase or an ATP-mediated molecular switch, which we confirm by biochemical experiments. Furthermore, the structure reveals different ATP binding motifs that are shared among many homologous hypothetical proteins in this family. This result indicates that structure-based assignment of molecular function is a viable approach for the large-scale biochemical assignment of proteins and for discovering new motifs, a basic premise of structural genomics.As of October 1998, 16 microbial genomes had been completely sequenced (Web site: www.tigr.org). These genomes are from all three branches of life: four from the Archaea, one from Eukarya, and the rest from Bacteria. To predict a function for each of their predicted protein coding regions or ORFs, the amino acid sequence of the ORF is compared against all functionally assigned sequences in protein sequence databases. If there is significant sequence or motif identity between the ORF and a functionally assigned sequence, then it is assumed that the two sequences share the same function. Unfortunately, up to 62% of the ORFs from these genomes share little or no sequence identity with any assigned sequence and hence are of unknown function (1-15). A major challenge, therefore, is to find ways to reliably and rapidly predict or determine the molecular (biochemical and biophysical) functions as well as cellular functions of these proteins.One approach for assigning the molecular function of a protein with unknown function is first to determine the three-dimensional structure of the protein by either x-ray crystallography or NMR. The structure, instead of the amino acid sequence, then is compared against those of the protein structure database (Protein Data Bank). If there are one or more significant structural homologs, the hypothetical protein is predicted to have molecular properties similar to the homologs. The predictions then can be tested experimentally. The molecular function then can provide a basis for searching for the cellular function of the protein. This method, structural genomics (16,17), is far more sensitive than primary sequence comparisons because proteins having insignificant sequence similarity often adopt similar tertiary structures with similar or related molecular functions. With the increasing advances in computer hardware and software associated with structure determination, this approach ...

The crystal structure of MJ PSP was determined at 1.8 A resolution. Critical residues were assigned based on the active site structure and ligand binding geometry. The PSP structure is in a closed conformation that may resemble the phosphoserine bound state or the state after autodephosphorylation. Compared to a P-type ATPase (Ca(2+)-ATPase) structure, which is in an open state, this PSP structure appears also to be a good model for the closed conformation of P-type ATPase.