Receiver domain of sensor histidine kinase CKI1
Receiver domain of sensor histidine kinase CKI1 (CKI1RD) catalyses the transphosphorylation reaction during hormonal and abiotic signalling in plants. Membrane-bound histidine kinase Cytokinin-independet 1 (CKI1) is a member of the Multistep phosphorelay (MSP) signalling pathway in Arabidopsis. CKI1 was found to be constitutively active activator of a cytokinin-like response. Intracellularly located C-terminal CKI1RD is responsible for the recognition of CKI1 downstream signalling partners from family of Arabidopsis histidine-containing phosphotransfer proteins (AHP) and triggers the cytokinin-like signal transmission. Divalent magnesium ion bound in the active site of CKI1RD is essential for the transphosphorylation reaction. Crystal structure of CKI1RD was determined as magnesium free and magnesium bound form. Magnesium binding induces the rearrangement of residues around the active site of CKI1RD, as was determined by both X-ray crystallography and NMR spectroscopy.
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Biological FunctionBiological Function
CKI1 as member of Multistep phosphorelay signaling in Arabidopsis The sensor histidine kinase CKI1 was identified as an activator of a cytokinin-like response when overexpressed in hypocotyl explants (Kakimoto, 1996) and it is essential for the female gametophyte development (Hejatko, Pischke). Cytokinin response in Arabidopsis involves shoot and root growth regulation, leaf senesce, circadian rhythms etc. No cytokinin binding to CKI1 has been detected, and in contrast to the genuine cytokinin receptors of Arabidopsis, CKI1 was found to be constitutively active in bacteria and yeast or Arabidopsis protoplasts (Yamada et al., 2001; Hwang & Sheen, 2001). Cytokinin signalling in plants is triggered by MSP, which was adopted by plants from bacterial Two-component system (citace). The signalling molecule is bound to the sensory histidin-kinase and then is transferred via AHPs (AHP1-AHP5) to nuclear response regulators (ARRs). ARRs act as transcription factors or interact with other effector proteins (citace) to perform specific cellular response to initial environmental stimuli. In contrast to ancestral two-component signalling in bacteria, protein interactions in plant MSP are supposed to be rather nonspecific. The current MSP interaction maps from Arabidopsis are based on yeast two-hybrid system (Urao et al., 2000; Tanaka et al., 2004; Dortay et al., 2006; Dortay et al., 2008) and show the AHPs as highly promiscuous, able to interact with all cytokinin receptors, number of other histidine-kinases and number of ARRs. Recently was shown that CKI1RD is responsible for a specific interaction with individual subset of second messengers from AHP family (AHP2, AHP3, AHP5) (Pekárová et al, 2011).