POLARIS is a copper-binding peptide that interacts with ETR1 to negatively regulate ethylene signaling in Arabidopsis

Ethylene signaling is one of the classic hormonal pathways in plants, with diverse roles in development and stress responses. The dimeric ethylene receptor localizes to the endoplasmic reticulum and contains Cu(I) ions essential for ethylene binding and signal transduction. We previously discovered that mutants of the Arabidopsis gene POLARIS (PLS), encoding a 36-amino-acid peptide, exhibit enhanced ethylene signaling responses suggestive of reduced receptor activity, but the role and activity of the PLS peptide in this signaling cascade have not been defined. Here, we report that Arabidopsis PLS binds copper as a 1:2 thiol-dependent Cu(I):PLS₂ complex with an affinity of 3.79 (±1.5) × 10¹⁹ M^-2 via two cysteine residues conserved in the related species Camelina sativa. These residues are also essential for biological function. This affinity precludes a role for PLS as a cytosolic Cu chaperone. We demonstrate that PLS localizes to endomembranes and interacts with the transmembrane domain of the receptor protein ETR1. PLS-ETR1 binding is increased in the presence of copper, and this interaction provides a Cu-dependent mechanism for mediating the repression of ethylene responses. Because PLS transcription is upregulated by auxin and downregulated by ethylene, PLS-ETR1 interactions also provide a mechanism for modulation of ethylene responses in high-auxin tissues.