| Two component signal transduction systems, consisting of conserved histidine kinase and receiver domains, are present in over 30 genera of prokaryotic organisms. Recently, two-component systems have also been identified in plants, fungi, and slime mold. In Arabidopsis thaliana, the ethylene and cytokinin signaling pathways utilize the two-component ethylene receptor ETR1, and cytokinin receptor CKI1, respectively. While the transmembrane N-terminal domain of each protein binds to the signaling hormone, the cytoplasmic domains show significant sequence homology to the histidine kinase and receiver domains of archetypal two-component systems.; Described herein is the first crystal structure of the plant receiver domain ETR1RD (residues 604–738 of ETR1), as well as the preliminary characterization of the receiver domain of CKI1, CKI1RD (residues 978–1122). The monomeric form of ETR1RD resembles the known structure of the bacterial receiver domain. ETR1RD forms a homodimer in solution and in the crystal, an interaction that has not been described previously. Dimerization is mediated by the carboxy terminus of the receiver domain, which forms an extended β sheet with the dimer-related β-strand core. Given that the dimer interface of ETR1RD coincides with the phosphorylation-dependent interfaces of characterized receiver domains, such as CheY, CheB, and FixJ, we suggest that the monomerization of ETR1RD is phosphorylation-dependent, too. Furthermore, the loop immediately following the active site adopts an exceptional conformation in ETR1RD. In the Mg2+-free form of ETR1RD, the γ-loop conformation does not allow a comparable interaction as observed in the active-site architectures of Mg 2+-bound CheY from Escherichia coli and Salmonella typhimurium. Preliminary characterization of the receiver domain of CKI1 illustrates its divergence from typical receiver domains. These structures will open the door to explore the structural biology of plant signaling pathways in general. |
