Gateway mechanism confers ultrasensitivity on motor′s function

In bacteria such as , information from the chemotaxis system to the flagellar motor is transduced by a single protein, CheY. The motor responds to CheY by switching its direction of rotation from the default direction to the other. This response is ultrasensitive, but the mechanism underlying this ultrasensitive switching is unknown. Here we resolved this mechanism. We show that the motor employs three sequential steps of CheY binding to distinct sites at the motor, each with a different outcome. Binding to the first site activates CheY at the motor. Binding to the second site generates transient motor switching and enables binding to the third site, which stabilizes the switched state. We demonstrate by mathematical modeling that such a gateway mechanism provides ultrasensitivity. The unique combination of tight regulation and flexibility provided by this mechanism raises the possibility that similar mechanisms may be functional in other systems as well.