The ethylene response mediator ETR1 from Arabidopsis forms a disulfide-linked dimer

Mutations in the ETR1 gene of the higher plant Arabidopsis confer insensitivity to ethylene, indicating a role for the gene product in ethylene signal perception and transduction. The ETR1 gene product has an amino-terminal hydrophobic domain and a carboxyl-terminal domain showing homology to the two-component signal transduction proteins of bacteria. We report here that in both its native Arabidopsis and when transgenically expressed in yeast, the ETR1 protein is isolated from membranes as a dimer of 147 kDa. Treatment with the reducing agent dithiothreitol converted the dimer to a monomer of 79 kDa, indicative of a disulfide linkage between monomers. Expression of truncated versions of ETR1 in yeast confirmed that the high molecular mass form is a homodimer and demonstrated that the amino-terminal region of ETR1 is necessary and sufficient for this dimerization. Site-directed mutagenesis of two cysteines near the amino terminus of ETR1 prevented formation of the covalently linked dimer in yeast, consistent with a role in disulfide bond formation. These data indicate that ETR1 may use a dimeric mechanism of signal transduction in a manner similar to its bacterial counterparts but with the additional feature of a disulfide bond between monomers.