ABSTRACT
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 Mg(2+)-free form of ETRRD, the gamma-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.
Subject(s)
Arabidopsis/physiology , Plant Proteins/chemistry , Plant Proteins/physiology , Receptors, Cell Surface/chemistry , Receptors, Cell Surface/physiology , Amino Acid Sequence , Crystallography, X-Ray , Histidine Kinase , Models, Molecular , Molecular Sequence Data , Protein Conformation , Protein Kinases/metabolism , Protein Structure, Secondary , Sequence Alignment , Sequence Homology, Amino Acid , Signal Transduction , SoftwareABSTRACT
The signal receiver domain of ETR1, an ethylene receptor from Arabidopsis thaliana, has been subcloned and expressed in E. coli and purified by affinity chromatography. Crystals of both native and a selenomethionine-substituted form of the receiver domain have been obtained. Native crystals grew in 1.6 M Li2SO4 and 0.1 M HEPES pH 7. 5 and once flash-frozen diffract to 2.1 A resolution. They belong to space group P41212 with unit-cell dimensions a = b = 48.4, c = 112.3 A.
Subject(s)
Arabidopsis/chemistry , Peptide Fragments/chemistry , Plant Proteins/chemistry , Receptors, Cell Surface/chemistry , Arabidopsis/genetics , Crystallization , Crystallography, X-Ray , DNA, Complementary/genetics , Hydrogen-Ion Concentration , Peptide Fragments/isolation & purification , Plant Proteins/genetics , Plant Proteins/isolation & purification , Protein Structure, Tertiary , Receptors, Cell Surface/genetics , Receptors, Cell Surface/isolation & purification , Recombinant Fusion Proteins/chemistryABSTRACT
A cDNA clone encoding ascorbate free radical (AFR) reductase (EC 1.6.5.4) was isolated from tomato (Lycopersicon esculentum Mill.) and its mRNA levels were analyzed. The cDNA encoded a deduced protein of 433 amino acids and possessed amino acid domains characteristic of flavin adenine dinucleotide- and NAD(P)H-binding proteins but did not possess typical eukaryotic targeting sequences, suggesting that it encodes a cytosolic form of AFR reductase. Low-stringency genomic DNA gel blot analysis indicated that a single nuclear gene encoded this enzyme. Total ascorbate contents were greatest in leaves, with decreasing amounts in stems and roots and relatively constant levels in all stages of fruit. AFR reductase activity was inversely correlated with total ascorbate content, whereas the relative abundance of AFR reductase mRNA was directly correlated with enzyme activity in tissues examined. AFR reductase mRNA abundance increased dramatically in response to wounding, a treatment that is known to also induce ascorbate-dependent prolyl hydroxylation required for the accumulation of hydroxyproline-rich glycoproteins. In addition, AFR reductase may contribute to maintaining levels of ascorbic acid for protection against wound-induced free radical-mediated damage. Collectively, the results suggest that AFR reductase activity is regulated at the level of mRNA abundance by low ascorbate contents or by factors that promote ascorbate utilization.
