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National Biomedical Center
for Advanced Electron Spin Resonance Technology

Our research is supported by a grant from the National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health.

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Probing the Structure of Flagellar Switch Complex from Thermotoga Maritima with Pulse-Dipolar ESR Spectroscopy

Bacterial chemotaxis, the ability of cells to adapt their motion to external stimuli, has long stood as a model system for understanding signal transduction. Furthermore, such an understanding could lead to the eventual development of new anti-bacterial agents which can attack this mechanism so essential to the survival of bacterial cells. Chemotaxis relies on two large transmembrane complexes to convert environmental signals into changes in cell motility. We have been intensively studying by our pulse-dipolar electron-spin resonance (ESR) methods, the structure and function on a molecular level, of these complexes. One is the polar receptor array, which we studied in the past, and the other is the flagella motor, which controls the motion of the flagella to take the bacterium toward a nutritious milieu (by counterclockwise rotation) or to swim away from a harmful environment (by clockwise rotation) of the motor according to signals received from the receptor via the messenger protein CheY-P. The component of the motor responsible for rotation is the rotor, composed of proteins, FLiG, FLiM, and FLiN, which together form the switch complex of the cytoplasmic "C-ring" (cf. Fig.on the right). The functional form of the cylindrical C-ring has yet to be established, although several models have been proposed based on crystallographic and biochemical data. Our ESR studies have focused primarily on distinguishing between these models. We have found from studies of soluble complexes of FLiG and FLiM, that they mostly interact through their middle domains in a 1:1 ratio in a parallel fashion (cf. Fig. on left) consistent with one of the proposed models. This interaction would produce a chain-like structure in the C-ring (cf. Fig. on right, insert) consistent with prior results from the other methods. Such an arrangement would explain the high degree of cooperativity observed on switching. We find that CheY-P destabilizes the interface between aligned FLiM modules, and in the absence of the membrane assemblies, produces the anti-parallel arrangement observed in several crystal structures.
Relevant Publication: Ria Sircar and Brian R. Crane, Structural Studies on Flagellar Rotor (Abstract from GRC: Signal Transduction in Microorganisms).

Ria Sircar, Michael Lynch, and Brian R. Crane (Department of Chemistry and Chemical Biology, Cornell University)
Peter P. Borbat and Jack H. Freed (ACERT)
June 2014