Self-Assembly of Receptor/Signaling Complexes in Bacterial Chemotaxis

Escherichia coli chemotaxis is mediated by membrane receptor/histidine kinase signaling complexes. Fusing the cytoplasmic domain of the aspartate receptor, Tar, to a leucine zipper dimerization domain produces a hybrid,$IzTar_{c}$, that forms soluble complexes with CheA and CheW. The three-dimensional reconstruction of these complexes was different from that anticipated based solely on structures of the isolated components. We found that analogous complexes self-assembled with a monomeric cytoplasmic domain fragment of the serine receptor without the leucine zipper dimerization domain. These complexes have essentially the same size, composition, and architecture as those formed from$IzTar_{c}$. Thus, the organization of these receptor/signaling complexes is determined by conserved interactions between the constituent chemotaxis proteins and may represent the active form in vivo. To understand this structure in its cellular context, we propose a model involving parallel membrane segments in receptor-mediated CheA activation in vivo.