The structure of the signal receiver domain of the Arabidopsis thaliana ethylene receptor ETR1

Background: In Arabidopsis thaliana, ethylene perception and signal transduction into the cell are carried out by a family of membrane-bound receptors, one of which is ethylene resistant 1 (ETR1). The large cytoplasmic domain of the receptor showed significant sequence homology to the proteins of a common bacterial regulatory pathway, the two-component system. This system consists of a transmitter histidine kinase and a response regulator (or signal receiver). We present the crystal structures of the first plant receiver domain ETRRD (residues 604-738) of ETR1 in two conformations. Results: The monomeric form of ETRRD resembles the known structure of the bacterial receiver domain. ETRRD forms a homodimer in solution and in the crystal, an interaction that has not been described previously. Dimerization is mediated by the C terminus, which forms an extended beta sheet with the dimer-related beta-strand core. Furthermore, the loop immediately following the active site adopts an exceptional conformation. Conclusions: The three-dimensional structure of ETRRD shows the expected conformational conservation to prokaryotic receiver proteins, such as CheY and CheB, both of which are part of the chemotaxis signaling pathway. ETRRD provides the first detailed example of a dimerized receiver domain. Given that the dimer interface of ETRRD coincides with the phosphorylation-dependent interfaces of CheY and CheB, we suggest that the monomerization of ETRRD is phosphorylation-dependent too. In the Mg2+-free form of ETRRD, the gamma-loop conformation does not allow a comparable interaction as observed in the active-site architectures of Mg2+-bound CheY from Escherichia coli and Salmonella typhimurium.