Structural features discriminating hybrid histidine kinase Rec domains from response regulator homologs

In two-component systems, the information gathered by histidine kinases (HKs) are relayed to cognate response regulators (RRs). Thereby, the phosphoryl group of the auto-phosphorylated HK is transferred to the receiver (Rec) domain of the RR to allosterically activate its effector domain. In contrast, multi-step phosphorelays comprise at least one additional Rec (Rec(inter)) domain that is typically part of the HK and acts as an intermediary for phosphoryl-shuttling. While RR Rec domains have been studied extensively, little is known about discriminating features of Rec(inter) domains. Here we study the Rec(inter) domain of the hybrid HK CckA by X-ray crystallography and NMR spectroscopy. Strikingly, all active site residues of the canonical Rec-fold are pre-arranged for phosphoryl-binding and BeF3- binding does not alter secondary or quaternary structure, indicating the absence of allosteric changes, the hallmark of RRs. Based on sequence-covariation and modeling, we analyze the intra-molecular DHp/Rec association in hybrid HKs. Bacterial multi-step phospho-relays employ receiver domains that act as intermediaries in phosphoryl shuttling. Here, the authors show that such a domain does not experience structural changes upon pseudo-phosphorylation, in contrast to the well-studied homologs in response regulators.

Automated summary

In two-component systems, histidine kinases (HKs) relay information to response regulators (RRs) by transferring the phosphoryl group from HK to RR's Rec domain. Multi-step phosphorelays involve an additional Rec (Rec(inter)) domain in HK that acts as an intermediary for phosphoryl shuttling. This study focuses on the Rec(inter) domain of the hybrid HK CckA and reveals that it does not undergo structural changes upon pseudo-phosphorylation, unlike the well-studied homologs in response regulators.