Single domain response regulators: molecular switches with emerging roles in cell organization and dynamics

Single domain response regulators (SD-RRs) are signaling components of two-component phosphorylation pathways that harbor a phosphoryl receiver domain but lack a dedicated output domain. The Escherichia coli protein CheY, the paradigm member of this family, regulates chemotaxis by relaying information between chemoreceptors and the flagellar motor switch. New data provide a more complex picture of CheY-mediated motility control in several bacteria and suggest diverging mechanisms in control of cellular motors. Moreover, advances have been made in understanding cellular functions of SD-RRs beyond chemotaxis. We review recent reports indicating that SD-RRs constitute a family of versatile molecular switches that contribute to cellular organization and dynamics as spatial organizer and/or as allosteric regulators of histidine protein kinases.