This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. During bacterial chemotaxis ligand binding to transmembrane chemoreceptors regulates the activity of the histidine kinase CheA. CheA initiates cytoplasmic signaling by phosphorylating the response regulator CheY, which diffuses from the membrane to modulate the flagellar motor. How receptors regulate CheA activity is a central question in understanding prokaryotic signal transduction. We aim to understand in molecular detail how the receptor:kinase complexes propagate signals and then adapt to those excitations. In order to do so, we determine structures of the individual proteins and their domains with just a few angstrom resolution by measuring a set of distance constraints in the 15-65A range by pulsed ESR. The interactions between receptors and CheA require the binding of the coupling protein CheW. However how CheW binds to CheA was unknown, and thus our first goal was aimed at solving this structural problem.