Cross-linking evidence for motional constraints within chemoreceptor trimers of dimers

Chemotactic behavior in bacteria relies on the sensing ability of large chemoreceptor clusters that are usually located at the cell pole. In Escherichia coli, chemoreceptors exhibit higher-order interactions within those clusters based on a trimer-of-dimers organization. This architecture is conserved in a variety of other bacteria and archaea, implying that receptors in many microorganisms form trimer-of-dimer signaling teams. To gain further insight into the assembly and dynamic behavior of receptor trimers of dimers, we used in vivo cross-linking targeted to cysteine residues at various positions that define six different levels along the cytoplasmic signaling domains of the aspartate and serine chemoreceptors, Tar and Tsr, respectively. We found that the cytoplasmic domains of these receptors are close to each other near the trimer contact region at the cytoplasmic tip and lie farther apart as the receptor dimers approach the cytoplasmic membrane. Tar and Tsr reporter sites within the same or closely adjacent levels readily formed mixed cross-links, whereas reporters located different distances from the tip did not. These findings indicate that there are no significant vertical displacements of one dimer with respect to the others within the trimer unit. Attractant stimuli had no discernible effect on the cross-linking efficiency of any of the reporters tested, but a strong osmotic stimulus reproducibly enhanced cross-linking at most of the reporter sites, indicating that individual dimers may move closer together under this condition.