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Review
, 38 (7), 597-603

New Insights Into the Protein Turnover Regulation in Ethylene Biosynthesis

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Review

New Insights Into the Protein Turnover Regulation in Ethylene Biosynthesis

Gyeong Mee Yoon. Mol Cells.

Abstract

Biosynthesis of the phytohormone ethylene is under tight regulation to satisfy the need for appropriate levels of ethylene in plants in response to exogenous and endogenous stimuli. The enzyme 1-aminocyclopropane-1-carboxylic acid synthase (ACS), which catalyzes the rate-limiting step of ethylene biosynthesis, plays a central role to regulate ethylene production through changes in ACS gene expression levels and the activity of the enzyme. Together with molecular genetic studies suggesting the roles of post-translational modification of the ACS, newly emerging evidence strongly suggests that the regulation of ACS protein stability is an alternative mechanism that controls ethylene production, in addition to the transcriptional regulation of ACS genes. In this review, recent new insight into the regulation of ACS protein turnover is highlighted, with a special focus on the roles of phosphorylation, ubiquitination, and novel components that regulate the turnover of ACS proteins. The prospect of cross-talk between ethylene biosynthesis and other signaling pathways to control turnover of the ACS protein is also considered.

Keywords: 14-3-3; ACS; ethylene; phosphorylation; protein turnover.

Figures

Fig. 1.
Fig. 1.
The ethylene biosynthetic pathway and its intermediates. S-adenosyl methionine (SAM) synthase uses methionine as a precursor for SAM. SAM is subsequently converted to 1-aminocyclopropane-1-carboxylic acid (ACC) in the first committed step in the ethylene biosynthesis by a family of ACC synthase (ACS) proteins, resulting in the release of methylthioadenosine (MTA). ACC is then finally converted to ethylene by ACC oxidase (ACO). MTA is recycled back to Yang cycles. Dashed line indicates additional enzymatic steps in the Yang cycle.
Fig. 2.
Fig. 2.
Structures of the different subgroups of ACC synthases and their regulatory motifs. Each type of ACS protein consists of a short N-terminus, followed by a conserved catalytic domain and a C-terminal domain with regulatory motifs, including phosphorylation sites. The target serine residue for calcium-dependent protein kinase (CDPK) is shown as a black bar and the target serine residues for mitogen-activated protein kinase (MAPK) are shown as grey bars. Target of ETO1 (TOE) motif in type-2 ACS is highlighted in red box. Type-3 ACS does not contain any regulatory motifs. The cartoon does not accurately represent the scale between the domains of ACS.
Fig. 3.
Fig. 3.
Regulatory elements determine the stability of ACS in Arabidopsis. Mitogen-activated protein kinase 3/6 (MPK3/6) and Ethylene Overproducer 1/ETO1-Like (ETO1/EOL) E3 ligases target a specific type of ACS, whereas 14-3-3, Roots curl in 1-N-naphthyl-phthalamic acid 1 (RCN1), XBAT32, and casein kinase 1.8 (CK1.8) could regulate the protein stability of more than one type of ACS. Positive or negative regulation of the different types of ACS protein stability is shown as black or red lines, respectively. Solid lines indicate interactions for which there is direct experimental evidence. Dashed lines indicate hypothetical interactions suggested by indirect evidence.

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