Activation of ethylene signaling is mediated by nuclear translocation of the cleaved EIN2 carboxyl terminus

Dear Editor, Ethylene gas is a classic phytohormone regulating many aspects of plant development and defense. Molecu- lar and genetic studies have revealed a highly conserved signaling pathway starting from the ER membrane-as- sociated receptors to transcription factors in the nucleus [1]. EIN2 （ETHYLENE INSENSITIVE2） is an essential positive regulator of ethylene signaling in Arabidopsis thaliana, as loss-of-function ein2 mutants are completely insensitive to ethylene [2]. EIN2 encodes a 1 294 amino acid protein that comprises a membrane-spanning amino terminus and a functionally unknown carboxyl terminus [3]. E1N2 was reported to localize at the ER membrane when transiently expressed in tobacco leaf cells [4]. EIN3 （ETHYLENE INSENSITIVE3） and its homolog EIL 1 （EIN3-LIKE 1） are two nuclear-localized transcrip- tion factors genetically acting downstream of EIN2 [5, 6]. However, it remains a long mystery on how the ethyl- ene signal is transmitted from ER-located EIN2 into the nucleus to modulate EIN3/EILl-directed transcription. Here we report that the carboxyl end of E1N2 （CEND） is a trafficking signal translocating from ER membrane to the nucleus. Ethylene signal promotes the cleavage of CEND from ER-located E1N2, and facilitates its nuclear localization to stabilize E1N3 protein.