LAUSR

Significance The conformational changes that accompany signaling events in transmembrane chemoreceptors are difficult to follow with structural methods that cannot replicate the native cellular environment. We developed in vivo cysteine crosslinking reporters of the Escherichia coli serine chemoreceptor Tsr that distinguished its signal-state structures. Crosslinking treatment of cells containing marked receptors produced substantial amounts of crosslinked forms in the presence of serine but not in its absence. Moreover, crosslink formation during in vivo signaling assays locked receptor output in the serine-induced output state. Output-shifting mutational changes or adaptational modifications also altered crosslinking patterns of Tsr reporters. These in vivo crosslinking assays probably detect transitions in helix-packing arrangements of the receptor molecule and may provide generally applicable tools for receptor signaling studies.