【正文】 t to play a fundamental role in this proteinmetalhormone interaction. ● In planta, the relevance of this interaction was further confirmed with the identification of RESPONSIVE TO ANTAGONIST1 (RA\N1). RAN1 was isolated using a screening for mutants with altered specificity in hormone binding by employing the ethylene antagonist transcyclooctene (TCO). ran1 plants are defective in a copper transporter similar to Ptype ATPases. ● The lack of the metal cofactor impairs receptor function in ran1 mutants by causing altered ligand specificity and thus rendering the plants responsive to the antagonist TCO. Furthermore, a strong LOF ran1 allele results in a constitutive ethylene response phenotype in the absence of the gaseous hormone. ● Autophosphorylation activity has been demonstrated for ETR1 and all other members of the receptor family. However, while ETR1 autophosphorylates in the predicted conserved histidine residue, ERS1 and all of the subfamily II members display a serinekinase activity in vitro. ERS1 also possesses a histidinekinase activity, whereas its serine autophosphorylation is thought not to be significant in vivo. ● Another interesting study addressed the roles of the kinase activity and of the carboxylterminal receiver domain of ETR1, ETR2, and EIN4 in ethylene signaling by looking at the effect of expressing truncated versions of ETR1 in a triple LOF etr1。ein4 mutant background. ● Transformation of the triple mutant with a truncated version of ETR1 that lacks both the histidinekinase and the receiver domain failed to restore ethylene responsiveness to the triple mutant. Conversely, a second truncated ETR1 construct in which only the receiver domain was missing did rescue the partial constitutive triple response phenotype of the triple knockout line. Moreover, transgenic plants harboring this particular construct displayed hypersensitivity to ethylene. These observations implied that the kinase domain was necessary for signal transmission by the receptors and that the receiver domain was not essential for restoring ethylene responsiveness. ● Modulation of the ethylene responses by the receiver domain could be achieved, for instance, through its reported interaction with another negative regulator of the pathway, CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) (see below), assuming that in the absence of the receiver domain the function of CTR1 is impaired. Interestingly, a detailed kiic analysis of seedling growth response in the presence of exogenous ethylene and of the consecutive recovery after ethylene withdrawal also indicated the importance of both the kinase and receiver domains in specific aspects of the ethylene response. ● These results suggest that both the kinase and the receiver domain are important for the hypocotyl recovery from short ethylene treatment and indicate that, despite the large functional overlap, some specificity can be attributed to the different receptors. ● The accumulated data have provided a model for the role of the receptors in ethylene signaling. (1) In the absence of the hormone, the receptors actively repress downstream ponents of the pathway and inhibit ethylene responses. (2) Ethylene gas binds to all five receptors, causing the receptors to bee inactive and releasing the pathway from their repression. Although still controversial, (3) receptor kinase activity possibly plays an important role in signaling. 3. A Raflike kinase acts downstream of the receptors repressing ethylene responses ● A subset of those was further characterized as defective in ethylene biosynthesis, namely ethylene overproducer (eto). Conversely, the ctr1 mutant did not respond to inhibitors of ethylene biosynthesis indicative of an alteration in signal transduction. CTR1 encodes a serine/threonine kinase whose carboxylterminus shares sequence similarity with the Raf family of protein kinases. The constitutive response phenotype exhibited by LOF mutants indicates that CTR1 negatively regulates the ethylene signaling pathway. ● Interestingly, CTR1 was shown to physically interact with the receptors. Subsequently, CTR1 was shown to colocalize with the receptors at the ER membranes. Moreover, copurification of an affinitytagged CTR1 and endogenous ETR1 in transgenic Arabidopsis lines strongly supported the in vivo interaction of these proteins. The significance of this interaction in signal transmission is corroborated by the observation that double and triple LOF receptor muta