Rotational On-off Switching of a Hybrid Membrane Sensor Kinase Tar-ArcB in Escherichia coli *

Signal transduction in biological systems typically involves receptor proteins that possess an extracytosolic sensory domain connected to a cytosolic catalytic domain. Relatively little is known about the mechanism by which the signal is transmitted from the sensory site to the catalytic site. At least in the case of Tar (methyl-accepting chemotaxis protein for sensing aspartate) ofEscherichia coli, vertical piston-like displacements of one transmembrane segment relative to the other within the monomer induced by ligand binding has been shown to modulate the catalytic activity of the cytosolic domain. The ArcB sensor kinase of E. coli is a transmembrane protein without a significant periplasmic domain. Here, we explore how the signal is conveyed to the catalytic site by analyzing the property of various Tar-ArcB hybrids. Our results suggest that, in contrast to the piston-like displacement that operates in Tar, the catalytic activity of ArcB is set by altering the orientation of the cytosolic domain of one monomer relative to the other in the homodimer. Thus, ArcB represents a distinct family of membrane receptor proteins whose catalytic activity is determined by rotational movements of the cytosolic domain.