Ethylene is a plant gaseous hormone which acts as a critical growth regulator. Its synthesis is also rapidly evoked in response to a variety ofbiotic and abiotic stresses. The rate-limiting step in ethylene biosynthesis from SAM to ACC is catalyzed by a family of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) isozymes that are finely controlled at the transcriptional level. A growing body of results suggests for them an additional post-trascriptional regulation level, which implies interaction of ACS proteins with other proteins. Here we report the interaction of the AtACS2 isoform with four different mitogen activated protein kinases (MAPKs) in the yeast two hybrid (Y2H) assay and in vitro phosphorylation by at least of one of them, the stress-responsive AtMPK6. Indeed all Arabidopsis ACS sequences contain MAPK docking motifs and potential phosphorylation sites. Furthermore, a still-under-way Y2H screening with AtACS2 as bait pulled down several proteases and an F-box protein (a proteasome component), as potential interactors. Since F-box proteins interaction with their targets can be phosphorylation-dependent, and based on the model recently proposed by Wang et al. (2004, Nature 428:945) in which phosphorylation of AtACS5 prevents its ETO1-dependent degradation by the proteasome, we suggest that AtACS2 is phosphorylated by AtMPK6. This post-transcriptional modification could affect AtACS2 stability by altering its affinity with proteins responsible for its degradation, such as the isolated F-box protein. Immunoblot analysis of AtACS2 content and phosphorylation level, ethylene production in F-box knockout lines as well as transient expression studies are under way to further investigate this hypothesis.
A possible role for MAPKs in proteasome-dependent degradation of ACC synthase in Arabidopsis
CARDINALE, Francesca
2004-01-01
Abstract
Ethylene is a plant gaseous hormone which acts as a critical growth regulator. Its synthesis is also rapidly evoked in response to a variety ofbiotic and abiotic stresses. The rate-limiting step in ethylene biosynthesis from SAM to ACC is catalyzed by a family of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) isozymes that are finely controlled at the transcriptional level. A growing body of results suggests for them an additional post-trascriptional regulation level, which implies interaction of ACS proteins with other proteins. Here we report the interaction of the AtACS2 isoform with four different mitogen activated protein kinases (MAPKs) in the yeast two hybrid (Y2H) assay and in vitro phosphorylation by at least of one of them, the stress-responsive AtMPK6. Indeed all Arabidopsis ACS sequences contain MAPK docking motifs and potential phosphorylation sites. Furthermore, a still-under-way Y2H screening with AtACS2 as bait pulled down several proteases and an F-box protein (a proteasome component), as potential interactors. Since F-box proteins interaction with their targets can be phosphorylation-dependent, and based on the model recently proposed by Wang et al. (2004, Nature 428:945) in which phosphorylation of AtACS5 prevents its ETO1-dependent degradation by the proteasome, we suggest that AtACS2 is phosphorylated by AtMPK6. This post-transcriptional modification could affect AtACS2 stability by altering its affinity with proteins responsible for its degradation, such as the isolated F-box protein. Immunoblot analysis of AtACS2 content and phosphorylation level, ethylene production in F-box knockout lines as well as transient expression studies are under way to further investigate this hypothesis.
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