Separation anxiety

Since the discovery of the CTR1 protein kinase and the endoplasmic reticulum (ER)-localized EIN2 protein nearly 20 y ago, plant biologists have wondered how these proteins respectively serve as negative and positive regulators of ethylene-mediated signal transduction in plants. Now with the publication of four studies, it can be concluded that in the absence of ethylene (ET) in Arabidopsis thaliana, CTR1 phosphorylates EIN2 thereby inactivating ET signal transduction, while in the presence of ET, CTR1 no longer phosphorylates EIN2 and the cytosolic C-terminus of EIN2 is released from the ER to translocate to the nucleus to promote gene transcription. Chen et al. (2011) showed that EIN2 is differentially phosphorylated at amino acids (a.a.) S(645) and S(924) after ET treatment. Ju et al. (2012) then proved that CTR1 phosphorylates EIN2 at those positions and that the lack of phosphorylation at S(645) and S(924) leads to the translocation of an EIN2 C-terminus peptide. Wen et al. (2012) and Qiao et al. (2012) also demonstrated ET-induced translocation of an EIN2 C-terminus peptide, while Qiao et al. (2012) proved that EIN2 has a nuclear localization signal sequence required for translocation, confirmed phosphorylation at S(645) and said that proteolytic cleavage occurs at S(645) in absence of phosphorylation there. Despite the revelation of this elegant switch, there are contradictory indications for specific cleavage at EIN2 S(645). This article investigates the data and concludes that EIN2 may be cleaved at alternative positions.