Structural Coupling between Autokinase and Phosphotransferase Reactions in a Bacterial Histidine Kinase

Bacterial two-component systems consist of a sensor histidine kinase (HK) and a response regulator (RR). HKs are homodimers that catalyze the autophosphorylation of a histidine residue and the subsequent phosphoryl transfer to its RR partner, triggering an adaptive response. How the HK autokinase and phosphotransferase activities are coordinated remains unclear. Here, we report X-ray structures of the prototypical HK CpxA trapped as a hemi-phosphorylated dimer, and of the receiver domain from the RR partner, CpxR. Our results reveal that the two catalytic reactions can occur simultaneously, one in each protomer of the asymmetric CpxA dimer. Furthermore, the increase of autokinase activity in the presence of phosphotransfer-impaired CpxR put forward the idea of an allosteric switching mechanism, according to which CpxR binding to one CpxA protomer triggers autophosphorylation in the second protomer. The ensuing dynamical model provides a mechanistic explanation of how HKs can efficiently orchestrate two catalytic reactions involving large-scale protein motions. [Display omitted] •X-ray structure of a hemi-phosphorylated CpxA dimer, ready for phosphoryl transfer•Autokinase and phosphotransfer reactions can occur concurrently in a single HK dimer•CpxR allosterically promotes CpxA autokinase activity•A dynamic catalytic cycle coupling both phosphorylation reactions is proposed In this work, Mechaly and colleagues use the paradigmatic Cpx signal transduction system as a model to investigate how autokinase and phosphotransferase activities are coordinated in sensory histidine kinases. X-ray structural data suggest a concerted switch between subunits by which autophosphorylation and phosphotransfer reactions are coupled. The switching mechanism involves large-scale domain motions that might be allosterically promoted by response regulator binding.