Temperature Dependence of Switching of the Bacterial Flagellar Motor by the Protein CheY13DK106YW

The behavior of the bacterium Escherichia coli is controlled by switching of the flagellar rotary motor between the two rotational states, clockwise (CW) and counterclockwise (CCW). The molecular mechanism for switching remains unknown, but binding of the response regulator CheY-P to the motor component FliM enhances CW rotation. This effect is mimicked by the unphosphorylated double mutant CheY^13DK106YW (CheY**). To learn more about switching, we measured the fraction of time that a motor spends in the CW state (the CW bias) at different concentrations of CheY** and at different temperatures. From the CW bias, we computed the standard free energy change of switching. In the absence of CheY, this free energy change is a linear function of temperature (Turner et al., 1996. Biophys. J. 71:2227–2233). In the presence of CheY**, it is nonlinear. However, the data can be fit by models in which binding of each molecule of CheY** shifts the difference in free energy between CW and CCW states by a fixed amount. The shift increases linearly from ∼0.3kT per molecule at 5°C to ∼0.9kT at 25°C, where k is Boltzmann's constant and T is 289 Kelvin (= 16°C). The entropy and enthalpy contributions to this shift are about −0.031kT/°C and 0.10kT, respectively.