Abstract
Bacterial
chemoreceptors cluster in highly ordered, cooperative,
extended arrays with a conserved architecture, but the principles
that govern array assembly remain unclear. Here we show images of
cellular arrays as well as selected chemoreceptor complexes reconstituted in vitro that reveal new principles of array structure and
assembly. First, in every case, receptors clustered in a trimers-of-dimers
configuration, suggesting this is a highly favored fundamental building
block. Second, these trimers-of-receptor dimers exhibited great versatility
in the kinds of contacts they formed with each other and with other
components of the signaling pathway, although only one architectural
type occurred in native arrays. Third, the membrane, while it likely
accelerates the formation of arrays, was neither necessary nor sufficient
for lattice formation. Molecular crowding substituted for the stabilizing
effect of the membrane and allowed cytoplasmic receptor fragments
to form sandwiched lattices that strongly resemble the cytoplasmic
chemoreceptor arrays found in some bacterial species. Finally, the
effective determinant of array structure seemed to be CheA and CheW,
which formed a “superlattice” of alternating CheA-filled
and CheA-empty rings that linked receptor trimers-of-dimer units into
their native hexagonal lattice. While concomitant overexpression of
receptors, CheA, and CheW yielded arrays with native spacing, the
CheA occupancy was lower and less ordered, suggesting that temporal
and spatial coordination of gene expression driven by a single transcription
factor may be vital for full order, or that array overgrowth may trigger
a disassembly process. The results described here provide new insights
into the assembly intermediates and assembly mechanism of this massive
macromolecular complex.