Insights into the regulation of ethylene receptor signaling by RTE1

Abstract

Ethylene is an important regulator of plant growth, development and responses to environmental stresses. The higher plant Arabidopsis thaliana perceives ethylene through five homologous receptors, which negatively regulate ethylene responses. The molecular mechanism by which these receptors signal to their next downstream component remains elusive. Genetic analyses have shown that the RTE1 locus is a positive regulator of ETR1. RTE1 encodes a novel protein of unknown molecular function, and is conserved in plants, animals and some protists. The goal of this research was to analyze the mechanisms involved in the regulation of ethylene receptor signaling by RTE1 and to enhance our understanding of the conserved cellular role of RTE1. Here we tested hypotheses for how RTE1 affects ETR1 and is specific to only ETR1, not the other ethylene receptor isoforms. We show that ETR1 and RTE1 gene expression patterns partially overlap and that the ETR1 receptor co-localizes with RTE1 within the cell. Moreover, RTE1 has no effect on ETR1 protein abundance or subcellular localization suggesting other mechanisms to regulate ETR1. We provide supporting evidence that RTE1 affects ETR1 signaling by restoring signaling of a non-functional ETR1 in an rte1 null through changes in ETR1 conformation(s). We next addressed the question of RTE1 specificity to ETR1. We discovered that ETR1 is surprisingly distinct from the other four ethylene receptor genes; in that RTE1-dependent mutations only confer insensitivity in ETR1 and not in the other ethylene receptors when the same mutations are introduced. In contrast, the RTE1-independent ETR1 insensitive mutations do give insensitivity in the closest receptor to ETR1, ERS1. Furthermore, we uncover that the ethylene binding domains are not completely interchangeable between ETR1 and ERS1. Our data point to a model in which RTE1 specifically promotes ETR1 signaling via conformational changes in a unique way that does not occur in other ethylene receptors. These findings highlight the importance and uniqueness of ETR1 signaling conformation(s) with respect to the other ethylene receptors, as well as advance our knowledge of RTE1 at the molecular and cellular level.