title: Cytokinin inhibits the proteasome-mediated degradation of carbonylated proteins in Arabidopsis leaves.
abstract: Under normal conditions, plants contain numerous carbonylated proteins, which are thought to be indicative of oxidative stress damage. Conditions that promote formation of reactive oxygen species (ROS) enhance protein carbonylation, and protein degradation is required to reverse the damage. However, it is not clear how the degradation of carbonylated proteins is controlled in planta. In this report, we show that detached Arabidopsis leaves rapidly and selectively degrade carbonylated proteins when kept in the dark. The loss of carbonylated proteins corresponded to a loss of soluble protein and accumulation of free amino acids. Degradation of carbonylated proteins and the loss of soluble protein was blocked by MG132 but not 3-methyladenine, suggesting that the 26S proteasome pathway rather than the autophagic pathway was involved. Consistent with this, rpn10 and rpn12 mutants, which are defective in proteasome function, had increased (rather than decreased) levels of carbonylated proteins when detached in the dark. Feeding metabolites (amino acids and sucrose) to detached leaves of wild-type Arabidopsis in the dark had little or no effect on the loss of carbonylated proteins, whereas providing soybean xylem sap via the transpiration stream effectively prevented degradation. The effect of xylem sap was mimicked by feeding 10 muM kinetin. We postulate that disruption of cytokinin flux to detached leaves triggers the selective degradation of carbonylated proteins via the proteasome pathway. The results may have implications for the control of protein mobilization in response to changes in N availability.
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA.
authors.forename: Vanita
authors.initials: V
authors.lastname: Jain
authors.forename: Werner
authors.initials: W
authors.lastname: Kaiser
authors.forename: Steven C
authors.initials: SC
authors.lastname: Huber
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D017360:Arabidopsis / Q000187:drug effects / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001343:Autophagy / Q000187:drug effects; D003583:Cytokinins / Q000494:pharmacology*; D003624:Darkness; D007701:Kinetin / Q000494:pharmacology; D007976:Leupeptins / Q000494:pharmacology; D009154:Mutation / Q000235:genetics; D018515:Plant Leaves / Q000187:drug effects / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D050050:Protein Carbonylation / Q000187:drug effects*; D011499:Protein Processing, Post-Translational / Q000187:drug effects*; D012995:Solubility / Q000187:drug effects; D013025:Glycine max; D052584:Xylem / Q000187:drug effects / Q000378:metabolism
chemical_list: D000596:Amino Acids; D029681:Arabidopsis Proteins; D003583:Cytokinins; D007976:Leupeptins; D061988:Proteasome Inhibitors; D046988:Proteasome Endopeptidase Complex; D007701:Kinetin; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde