Signal transduction proteins such as bacterial sensor histidine kinases, designed to transition between multiple conformations, are often ruled by unstable transient interactions making structural characterization of all functional states difficult. This study explored the inactive and signal-activated conformational states of the two catalytic domains of sensor histidine kinases, HisKA and HATPase. Direct coupling analyses, a global statistical inference approach, was applied to >13,000 such domains from protein databases to identify residue contacts between the two domains. These contacts guided structural assembly of the domains using MAGMA, an advanced molecular dynamics docking method. The active conformation structure generated by MAGMA simultaneously accommodated the sequence derived residue contacts and the ATP-catalytic histidine contact. The validity of this structure was confirmed biologically by mutation of contact positions in the Bacillus subtilis sensor histidine kinase KinA and by restoration of activity in an inactive KinA(HisKA):KinD(HATPase) hybrid protein. These data indicate that signals binding to sensor domains activate sensor histidine kinases by causing localized strain and unwinding at the end of the C-terminal helix of the HisKA domain. This destabilizes the contact positions of the inactive conformation of the two domains, identified by previous crystal structure analyses and by the sequence analysis described here, inducing the formation of the active conformation. This study reveals that structures of unstable transient complexes of interacting proteins and of protein domains are accessible by applying this combination of cross-validating technologies.

Bacteria use two-component signal transduction systems (TCS) extensively to sense and react to external stimuli. In these, a membrane-bound sensor histidine kinase (SK) autophosphorylates in response to an environmental stimulus and transfers the phosphoryl group to a transcription factor/response regulator (RR) that mediates the cellular response. The complex between these two proteins is ruled by transient interactions, which provides a challenge to experimental structure determination techniques. The functional and structural homolog of an SK/RR pair Spo0B/Spo0F, however, has been structurally resolved. Here, we describe a method capable of generating structural models of such transient protein complexes. By using existing structures of the individual proteins, our method combines bioinformatically derived contact residue information with molecular dynamics simulations. We find crystal resolution accuracy with existing crystallographic data when reconstituting the known system Spo0B/Spo0F. Using this approach, we introduce a complex structure of TM0853/TM0468 as an exemplary SK/RR TCS, consistent with all experimentally available data.

Two-component signal transduction systems with membrane-embedded sensor histidine kinases are believed to recognize environmental signals and transduce this information over the cellular membrane to influence the activity of a transcription factor to which they are mated. The YycG sensor kinase of Bacillus subtilis, containing two transmembrane helices, is subject to a complicated activity-control circuit involving two other proteins with N-terminal transmembrane helices, YycH and YycI. Truncation studies of YycH and YycI demonstrated that the individual transmembrane helices of these proteins are sufficient to adjust YycG activity, indicating that this control is achieved at the membrane level. A replica exchange molecular dynamics computational approach generated in silico structural models of the transmembrane helix complex that informed mutagenesis studies of the YycI transmembrane helix supporting the accuracy of the in silico model. The results predict that signal recognition by any of the extracellular domains of the sensor histidine kinase YycG or the associated proteins YycH and YycI is transmitted across the cellular membrane by subtle alterations in the positions of the helices within the transmembrane complex of the three proteins.