Role of HAMP domains in chemotaxis signaling by bacterial chemoreceptors

Abstract

Bacterial chemoreceptors undergo conformational changes in response to variations in the concentration of extracellular ligands. These changes in chemoreceptor structure initiate a series of signaling events that ultimately result in regulation of rotation of the flagellar motor. Here we have used cryo-electron tomography combined with 3D averaging to determine the in situ structure of chemoreceptor assemblies in Escherichia coli cells that have been engineered to overproduce the serine chemoreceptor Tsr. We demonstrate that chemoreceptors are organized as trimers of receptor dimers and display two distinct conformations that differ principally in arrangement of the HAMP domains within each trimer. Ligand binding and methylation alter the distribution of chemoreceptors between the two conformations, with serine binding favoring the ''expanded'' conformation and chemoreceptor methylation favoring the ''compact'' conformation. The distinct positions of chemoreceptor HAMP domains within the context of a trimeric unit are thus likely to represent important aspects of chemoreceptor structural changes relevant to chemotaxis signaling. Based on these results, we propose that the compact and expanded conformations represent the ''kinase-on'' and ''kinaseoff'' states of chemoreceptor trimers, respectively.cryo-electron tomography ͉ molecular architecture ͉ signal transduction M otile bacteria such as Escherichia coli respond to changes in their chemical environment by recognizing variations in the ligand occupancy of a series of transmembrane chemoreceptors (also known as methyl-accepting chemotaxis proteins, or MCPs). Ligand binding to chemoreceptors initiates a signaling cascade that modulates the activity of the histidine kinase CheA, which ultimately regulates the rotation of the flagellar motor through a diffusible intracellular signal (1, 2). Chemoreceptors exist as homodimers and have been shown to function as trimers of chemoreceptor dimers both in vitro (3) and in vivo (4-6).Chemoreceptor homodimers can be divided into three functional modules, each of which plays a critical role in receptormediated signaling (7). The transmembrane-sensing module is composed of both the sensing domain, which resides in the periplasm and contains the ligand binding site (8), and the transmembrane portion of the chemoreceptor. It has been deduced that ligand binding to the periplasmic domain results in a piston-like sliding motion of one of the four transmembrane helices (7, 9). The highly conserved cytoplasmic region of chemoreceptors can be divided into two functional modules: the signal conversion module, which contains a HAMP (histidine kinase, adenylyl cyclases, methyl-binding proteins, and phosphatases) domain, and a kinase control module, which contains an adaptation region and a protein interaction region (7, 9). The HAMP domain is the proposed site of signal conversion between the transmembrane-sensing and kinase control modules (7). The recent solution structure of an archaeal HAMP domain demonstrates that it adopts a ho...

Propulsion and Chemotaxis in Bacteria‐Driven Microswimmers

“…Chemotaxis Signaling Pathway Model : The chemotaxis signaling pathway of E. coli was modeled with three major components, and their corresponding models were adapted from recent studies 37, 38, 39, 40, 41, 42, 43. The first component, MCP complex, was represented with a Monod–Wyman–Changeux (MWC) model52 to describe the allosteric effects of receptor clusters with identical receptors.…”

“…Thus far, the taxis‐based guiding method constitutes the major way to regulate the otherwise highly stochastic motion of bacteria‐driven microswimmers, which has been studied both in vitro17, 20, 21, 22, 23, 24, 25 and in vivo 14, 16. Chemotaxis, one of the most common taxis behaviors in bacteria, has been well understood36 and its signaling pathway has been mathematically modeled 37, 38, 39, 40, 41, 42, 43. In general, the chemotaxis of free‐swimming bacteria associates with a biased random walk, enabled by preferentially suppressed tumble tendency when the bacteria travel up a chemoattractant gradient; whereas in an uniform medium, the tumble tendency is isotropic over all swimming directions.…”

Different Conformations of the Kinase-On and Kinase-Off Signaling States in the Aer HAMP Domain

“…Within the functional unit of chemoreceptors, the trimer of dimers, HAMP dimers may alternate between an expanded trimer-of-dimer conformation in the kinase-off state and a compact conformation in the kinase-on state (20). Such conformational changes would require flexibility at the junction of the HAMP and signaling domains, which could be accommodated by the presence of a short unstructured sequence between the two domains (36).…”

Universal architecture of bacterial chemoreceptor arrays

“…It was further proposed that, in E. coli, trimers of receptor dimers form a hexagonal array with a lattice spacing of 20 nm (13). A subsequent electron cryotomography (ECT) study showed that overexpressed Tsr chemoreceptors in E. coli pack into a hexagonal lattice with a center-to-center spacing of 7.5 nm (14)(15)(16)(17). In these overexpression strains, the receptors surprisingly form a ''zipper-like'' double layer, in which large invaginations of the inner membrane allow the cytoplasmic tips of one layer to interact with the cytoplasmic tips of a second, facing layer.…”