14-3-3 Regulates 1-Aminocyclopropane-1-Carboxylate Synthase Protein Turnover in Arabidopsis    

Abstract

14-3-3 proteins are a family of conserved phospho-specific binding proteins involved in diverse physiological processes. Plants have large 14-3-3 gene families, and many binding partners have been identified, though relatively few functions have been defined. Here, we demonstrate that 14-3-3 proteins interact with multiple 1-aminocyclopropane-1-carboxylate synthase (ACS) isoforms in Arabidopsis thaliana. ACS catalyzes the generally rate-limiting step in the biosynthesis of the phytohormone ethylene. This interaction increases the stability of the ACS proteins. 14-3-3s also interact with the ETHYLENE-OVERPRODUCER1 (ETO1)/ETO1-LIKE (EOLs), a group of three functionally redundant proteins that are components of a CULLIN-3 E3 ubiquitin ligase that target a subset of the ACS proteins for rapid degradation by the 26S proteasome. In contrast with ACS, the interaction with 14-3-3 destabilizes the ETO1/EOLs. The level of the ETO1/EOLs in vivo plays a role in mediating ACS protein turnover, with increased levels leading to a decrease in ACS protein levels. These studies demonstrate that regulation of ethylene biosynthesis occurs by a mechanism in which 14-3-3 proteins act through a direct interaction and stabilization of ACS and through decreasing the abundance of the ubiquitin ligases that target a subset of ACS proteins for degradation.

Producing the Ethylene Signal: Regulation and Diversification of Ethylene Biosynthetic Enzymes

“…hormone levels, developmental transitions). Light affects ACS stability both positively and negatively; for instance, turnover of ACS5 is decreased (Yoon and Kieber, 2013a), whereas ACS7 turnover is increased (Xiong et al, 2014) by light. Exogenous cytokinin and brassinosteroid treatments increase ethylene production via posttranslational stabilization (Woeste et al, 1999;Hansen et al, 2009), and pathogen infection increases ACS abundance at both transcriptional and posttranslational levels (Han et al, 2010;Li et al, 2012).…”

“…Ethylene overproduction due to loss of ETO/EOL-mediated regulation is dramatic in etiolated seedlings but is also observed in light-grown seedlings and adult plants (Christians et al, 2009;Yoon and Kieber, 2013a).…”

“…However, several newly identified regulatory factors target ACSs of different types (e.g. Yoon and Kieber, 2013a), highlighting the layering and integration of mechanisms that control ACS turnover. Overall, the stability control mechanisms clearly link ethylene production to exogenous (e.g.…”

ACC synthase and its cognate E3 ligase are inversely regulated by light

“…Recently, we have shown that the 14-3-3 proteins bind to both the ACS proteins and to the ETO1/EOL E3 ligases to regulate their stability. 14 14-3-3s generally interact specifically with phosphorylated proteins to control their function by changing their localization, activity or stability. [15][16][17][18] We demonstrated that 14-3-3 stabilizes ACS proteins via 2 distinct mechanisms.…”

“…Consistent with this, there is a quantitative effect of disruption of the 6 copies of the ETO1/ EOL1/EOL2 genes on ethylene production in etiolated and light-grown seedlings. 10,14 If modulation of ETO1/EOL levels is a mechanism by which plants regulate ethylene biosynthesis, then the level of ETO1/EOL proteins should be regulated by a subset of factors known to regulate ethylene biosynthesis. To test this, we examined the effect of light on ETO1/EOL proteins as light has a substantial effect on ethylene biosynthesis in various plant species, including Arabidopsis.…”