Hybrid histidine kinase activation by cyclic di-GMP- mediated domain liberation
Cytosolic hybrid histidine kinases (HHKs) constitute major signalling nodes that control various biological processes, but their input signals and how these are processed are largely unknown. In Caulobacter crescentus, the HHK ShkA is essential for accurate timing of the G1-S cell cycle transition and is regulated by the corresponding increase in the level of the second messenger c-di-GMP. Here, we use a combination of X-ray crystallography, NMR spectroscopy, functional analyses and kinetic modelling to reveal the regulatory mechanism of ShkA. In the absence of c-di-GMP, ShkA predominantly adopts a compact domain arrangement that is catalytically inactive. C-di-GMP binds to the dedicated pseudo-receiver domain Rec1 thereby liberating the canonical Rec2 domain from its central position where it obstructs the large-scale motions required for catalysis. Thus, c-di-GMP cannot only stabilize domain interactions, but also engage in domain dissociation to allosterically control activity. Enzyme kinetics data are consistent with conformational selection of the ensemble of active domain constellations by the ligand.\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=86 SRC=\"FIGDIR/small/675454v2_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (11K):\norg.highwire.dtl.DTLVardef@8304borg.highwire.dtl.DTLVardef@19a9d61org.highwire.dtl.DTLVardef@1d27000org.highwire.dtl.DTLVardef@b74d75_HPS_FORMAT_FIGEXP M_FIG C_FIG