Regulation of ethylene biosynthesis through protein degradation

Abstract

The function of hormones during plant growth, development and response to environmental stresses relies heavily upon the actions of the ubiquitin proteasome system (UPS), which selectively degrades numerous proteins. Synthesis of ethylene, a growth and stress hormone, is regulated in part by the ubiquitin-dependent degradation of the rate-limiting enzymatic protein aminocyclopropane-1-carboxylic acid synthase (ACS). Regulation of ACS protein stability, and therefore ethylene production, is mediated by non-catalytic sequences within the C-terminal extension of many ACS proteins. In this review we provide a brief overview of the E3 ligases that target ACS proteins for degradation and discuss how post-translational modification of the C-terminal extensions influence protein stability.

Keywords: ETO1; XBAT32; aminocyclopropane-1-carboxylic acid synthase; ethylene; phosphorylation; proteasome; ubiquitin.

Figure 1. A. Classification of Arabidopsis and tomato (Lycopersicon esculentum) ACS proteins based on phosphorylation sites within the C-terminal extension. Type 1 ACS proteins contain one CDPK and three MAPK phosphorylation sites. Type 2 ACS proteins contain one CDPK phosphorylation site and a target of ETO1 (TOE) sequence. Type 3 ACS proteins contain neither phosphorylation sites nor TOE sequence. B. Factors that influence the stability of Arabidopsis ACS proteins. Type 1 ACS protein are phosphorylated by MPK3/6 in response to external stimuli such as Botrytis cinerea infection and are possibly phosphorylated by CDPKs. Phosphorylated type 1 ACS proteins are more stable than unphosphorylated forms. Dephosphorylation by PP2A renders type 1 ACS proteins unstable. Type 2 ACS proteins are ubiquitinated by ETO1 and EOL1/2 ubiquitin ligases and are stabilized by cytokinin or brassinosteroid treatment. It is unclear how CDPK phosphorylation affects protein stability. The type 3 ACS protein is ubiquitinated and targeted for degradation by the E3 ligase XBAT32. Factors that influence the stability of type 3 ACS proteins are unknown. P, phosphate group; S, serine residue