NMR structure of the pseudo‐receiver domain of CikA

Abstract

The circadian input kinase (CikA) is a major element of the pathway that provides environmental information to the circadian clock of the cyanobacterium Synechococcus elongatus. CikA is a polypeptide of 754 residues and has three recognizable domains: GAF, histidine protein kinase, and receiver-like. This latter domain of CikA lacks the conserved phospho-accepting aspartyl residue of bona fide receiver domains and is thus a pseudo-receiver (PsR). Recently, it was shown that the PsR domain (1) attenuates the autokinase activity of CikA, (2) is necessary to localize CikA to the cell pole, and (3) is necessary for the destabilization of CikA in the presence of the quinone analog 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). The solution structure of the PsR domain of CikA, CikAPsR, is presented here. A model of the interaction between the PsR domain and HPK portion of CikA provides a potential explanation for how the PsR domain attenuates the autokinase activity of CikA. Finally, a likely quinone-binding surface on CikAPsR is shown here.

The KaiA protein of the cyanobacterial circadian oscillator is modulated by a redox-active cofactor

“…2B), involving sections of α2, α3, β3, β4, and preceding β5. DBMIB-induced chemical shift perturbations do not map identically onto the structures of KaiA135N and CikAPsR, suggesting that there are differences in the way DBMIB binds to these PsRs (20,22). In contrast, a 10-fold higher concentration of DBMIB caused no changes in the peaks of CheY, Fig.…”

“…An unexpected finding was that DBMIB decreases the stability of LdpA, CikA, and, to a lesser degree, KaiA (17). The pseudoreceiver (PsR) domain of CikA was shown to bind to DBMIB and other quinone analogs directly (20,22). The data suggest that the cyanobacterial clock obtains information regarding the light environment indirectly, through redox changes in a CikA-bound quinone (20).…”

Structure, function, and mechanism of the core circadian clock in cyanobacteria

“…This metabolic entrainment of the cyanobacterial clock occurs in two ways: through sensitivity of the phosphorylation cycle to ATP/ADP ratios within the cell (75) and through the presence of nighttimeassociated photosynthetic metabolites (71,72,(76)(77)(78). The former entrainment cue occurs as a result of enzymatic sensitivity of the core oscillator to ATP/ADP ratios 72 , causing it to align with the maximal photosynthetic period at midday when this ratio is highest (75,79).…”

“…As a result, the concentration of oxidized quinones rises rapidly but briefly when photosynthesis shuts down with the onset of darkness before other pathways restore the redox steady state (76). Ectopic introduction of oxidized quinones can entrain the clock both in vivo and in vitro, and both KaiA and CikA play roles related to entrainment through binding of their PsR domains to oxidized quinones (71,72,(76)(77)(78). The aggregation state and/or stability of the proteins is affected by quinone binding in vivo (71,77).…”

Integrating the Circadian Oscillator into the Life of the Cyanobacterial Cell

“…The receiver-like domain is recognizable in sequence as a member of that class, but it lacks the crucial aspartate residue needed to perform receiver phosphoryltransfer activity. Thus, like N-KaiA, the carboxy-terminal domain of CikA is a pseudo-receiver (PsR) (Mutsuda et al 2003;Gao et al 2007). …”