Cytokinin inhibits the proteasome-mediated degradation of carbonylated proteins in Arabidopsis leaves

Authors: JAIN, VANITA - IARI, NEW DELHI; KAISER, WERNER - UNIVERSITY OF WUERZBURG; Huber, Steven

Submitted to: Plant Cell Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2008
Publication Date: 5/15/2008
Citation: Jain, V., Kaiser, W., Huber, S.C. 2008. Cytokinin inhibits the proteasome-mediated degradation of carbonylated proteins in Arabidopsis leaves. Plant Cell Physiology. 49:843-852.

Interpretive Summary: Plant cells contain reactive oxygen species that can damage proteins, lipids and nucleic acids. A marker for oxidative damage to proteins is the appearance of carbonyl groups that can react chemically with dinitrophenyl hydrazine. Even when grown under optimal conditions, plants contain numerous proteins with reactive carbonyls, indicating that oxidative damage to cellular constituents is a normal part of life even under ideal conditions. It is generally assumed that protein carbonylation interferes with protein function, and that these damaged proteins must be degraded. However, the pathway involved and the mechanisms that control degradation are not known. There are several potential degradation pathways, but we found that carbonylated proteins were degraded by a multi-protein complex known as the proteasome and that the plant hormone, cytokinin, controls the process. These findings have implications for control of protein mobilization in plants subjected to changes in nitrogen availability. In response to a decrease in soil nitrogen, it is known that cytokinin production in roots is decreased and protein is mobilized from older leaves. Our results suggest that under these conditions, carbonylated proteins may be selectively degraded by proteasomes to provide amino acids to support essential functions.

Technical 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 µM 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