The aspartate receptor cytoplasmic domain: in situ chemical analysis of structure, mechanism and dynamics

Attractant regulation of the aspartate receptor-kinase complex: limited cooperative interactions between receptors and effects of the receptor modification state

The manner by which the bacterial chemotaxis system responds to a wide range of attractant concentrations remains incompletely understood. In principle, positive cooperativity between chemotaxis receptors could explain the ability of bacteria to respond to extremely low attractant concentrations. By utilizing an in vitro receptor-coupled kinase assay, the attractant-dependent response curve has been measured for the Salmonella typhimurium aspartate chemoreceptor. The attractant chosen, alpha-methyl aspartate, was originally used to quantitate high receptor sensitivity at low attractant concentrations by Segall, Block, and Berg [(1986) Proc. Natl. Acad. Sci. U.S.A. 83, 8987-8991]. The attractant response curve exhibits limited positive cooperativity, yielding a Hill coefficient of 1.7-2.4, and this Hill coefficient is relatively independent of both the receptor modification state and the mole ratio of CheA to receptor. These results disfavor models in which there are strong cooperative interactions between large numbers of receptor dimers in an extensive receptor array. Instead, the results are consistent with cooperative interactions between a small number of coupled receptor dimers. Because the in vitro receptor-coupled kinase assay utilizes higher than native receptor densities arising from overexpression, the observed positive cooperativity may overestimate that present in native receptor populations. Such positive cooperativity between dimers is fully compatible with the negative cooperativity previously observed between the two symmetric ligand binding sites within a single dimer. The attractant affinity of the aspartate receptor is found to depend on the modification state of its covalent adaptation sites. Increasing the the level of modification decreases the apparent attractant affinity at least 10-fold in the in vitro receptor-coupled kinase assay. This observation helps explain the ability of the chemotaxis pathway to respond to a broad range of attractant concentrations in vivo.

Structure of a conserved receptor domain that regulates kinase activity: the cytoplasmic domain of bacterial taxis receptors

Many bacteria are motile and use a conserved class of transmembrane sensory receptor to regulate cellular taxis toward an optimal living environment. These conserved receptors are typically stimulated by extracellular signals, but also undergo adaptation via covalent modification at specific sites on their cytoplasmic domains. The function of the cytoplasmic domain is to integrate the extracellular and adaptive signals, and to use this integrated information to regulate an associated histidine kinase. The kinase, in turn, triggers a cytoplasmic phosphorylation pathway of the two-component class. The high-resolution structure of a receptor cytoplasmic domain has recently been determined by crystallographic methods and is largely consistent with a structural model independently generated by chemical studies of the domain in the full-length, membrane-bound receptor. These results represent an important step toward a mechanistic understanding of receptor-to-kinase information transfer.

The aspartate chemoreceptor Tar is effectively methylated by binding to the methyltransferase mainly through hydrophobic interaction

In the chemotaxis of Escherichia coli, adaptation requires the methylation and demethylation of transmembrane receptors, which are catalysed by the methyltransferase CheR and the methylesterase CheB respectively. CheR binds to major chemoreceptors through their C-terminal motif NWETF, which is distinct from the methylation sites. In this study, we carried out a systematic mutagenesis of the pentapeptide sequence of Tar. Receptor methylation and adaptation were severely impaired by the alanine substitution of residue W550 and, to a lesser extent, by that of F553. Substitution of residues N549, E551 and T552 had only a slight or little effect. The defects of the W550A and F553A mutations were suppressed by high- and low-level overproduction of CheR respectively. Expression of a fusion protein containing the NWETF sequence, but not its W550A and F553A versions, inhibited chemotaxis of the Che+ strain. In an in vitro assay, CheR bound to the wild-type version but not to the mutant versions. These results and further mutagenesis suggest that the hydrophobicity and the size of residues W550 and F553 are critical in the interaction with CheR, a conclusion that is consistent with the crystal structure of a CheR-NWETF complex. On the other hand, the negatively charged side chain of E551 and the polar side chains of N549 and T552 may not be strictly required, although the presence of a salt bridge and hydrogen bonds between these residues and residues from CheR has been noted in the co-crystal.