CTR1 phosphorylates the central regulator EIN2 to control ethylene hormone signaling from the ER membrane to the nucleus in Arabidopsis

Authors: JU, CHUANLI - University Of Maryland; YOON, GYEONG MEE - University Of North Carolina; SHEMANSKY, JENNIFER - University Of Maryland; LIN, DAVID - University Of Maryland; YING, IRENE - University Of Maryland; CHANG, JIANHONG - University Of Maryland; Garrett, Wesley; KESSENBROCK, MARIEKE - University Of Dusseldorf; GROTH, GEORGE - University Of Dusseldorf; Tucker, Mark; Cooper, Bret; KIEBER, JOSEPH - University Of North Carolina; CHANG, CAREN - University Of Maryland

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2012
Publication Date: 11/12/2012
Publication URL: http://handle.nal.usda.gov/10113/56698
Citation: Ju, C., Yoon, G., Shemansky, J., Lin, D., Ying, I.S., Chang, J., Garrett, W.M., Kessenbrock, M., Groth, G., Tucker, M.L., Cooper, B., Kieber, J.J., Chang, C. 2012. CTR1 phosphorylates the central regulator EIN2 to control ethylene hormone signaling from the ER membrane to the nucleus in Arabidopsis. Proceedings of the National Academy of Sciences. 109:19486-19491.

Interpretive Summary: Ethylene is an important gas that induces fruit ripening and plant development. The receptors in plant cells that bind ethylene have been identified, and it is known that the receptors affect two other proteins,CTR1 and EIN2,that transmit a molecular signal that tells the plant cell to respond to ethylene, but how the signal is transmitted in terms of biochemistry has been unknown. Research was performed to show that CTR1 and EIN2 proteins physically interact in plant cells. It was found that when this happens, CTR1 adds a phosphate molecule (phosphorylation) to EIN2, at which point EIN2 is degraded. Thus, if ethylene is not present, the EIN2 signaling mechanism is degraded to keep the signal turned off. When the molecular sites for phosphorylation were blocked in EIN2, it was discovered that EIN2 moved away from the ethylene receptors to the nucleus where gene transcription important for plant development was initiated. Therefore, the results show how the absence of ethylene leads to turning EIN2 off and how the presence of ethylene leads to turning EIN2 on. These data are most likely to influence scientists at universities, government agencies and companies who are studying fruit ripening and plant development and who are trying to better control these processes in order to produce higher quality fruits and vegetables.

Technical Abstract: The gaseous phytohormone ethylene (C2H4) mediates numerous aspects of growth and development. Genetic analysis has identified a number of critical elements in the ethylene signaling (1), but how these elements interact biochemically to transduce the signal from the ethylene receptor complex at the endoplasmic reticulum (ER) membrane to transcription factors in the nucleus is unknown. In order to close this gap in our understanding of the ethylene signaling pathway, the challenge has been to identify the target of the CTR1 Raf-like protein kinase, as well as the molecular events surrounding EIN2, an ER membrane-localized Nramp homolog that positively regulates ethylene responses. Here we demonstrate that CTR1 interacts with and directly phosphorylates the cytosolic C-terminal domain of EIN2. Mutations that block the EIN2 phosphorylation sites result in constitutive nuclear localization of the EIN2 C-terminus, together with constitutive activation of ethylene responses in Arabidopsis. Our results suggest that phosphorylation of EIN2 by CTR1 prevents EIN2 from signaling in the absence of ethylene, whereas inhibition of CTR1 upon ethylene perception is a signal for nuclear localization of the EIN2 C-terminus, allowing the ethylene signal to reach the downstream transcription factors. These findings significantly advance our understanding of the mechanisms underlying ethylene signal transduction.