Submitted to: Journal of Biological Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/10/2006
Publication Date: 6/1/2006
Citation: Chalivendra, S.C., Palaniappan, A., Duncan, K., Rhoads, D.M., Huber, S.C., Sachs, M.M. 2006. Mitochondrial localization and a putative signaling function of sucrose synthase in maize. Journal of Biological Chemistry. 281:15625-15635. Interpretive Summary: 1) Rationale: It was previously shown that the enzyme sucrose synthase can be localized to membranes of cells during flooding-stress. The goal here was to have a fuller understanding of where sucrose synthase is localized during its role in the response to low oxygen conditions in maize during flooding. 2) Accomplishments: It was determined that sucrose synthase appears to have an important role other than its known role as an enzyme in carbohydrate metabolism A cell organelle (mitochondria) appears to be a site that sucrose synthase localizes to and there plays a novel alternative role, perhaps in signaling during flooding stress. 3) Significance: The understanding of sucrose synthase’s role in a plant’s response to low oxygen-stress conditions will allow for a greater understanding of how these conditions cause a plant to alter protein synthesis in response to flooding and may allow for effective methods to produce crop plants that are tolerant to flooding.
Technical Abstract: In many organisms, an increasing number of proteins seem to play two or more unrelated roles. Here, we report that maize sucrose synthase (SUS) is distributed in organelles and may have additional roles besides sucrose catabolism. Bioinformatics analysis predicts that among the three maize SUS isoforms, SH1 protein has a putative mitochondrial targeting peptide (mTP). We validated this prediction by the immunodetection of SUS in mitochondria. Analysis with isoform-specific antisera revealed that both SH1 and SUS1 occur in mitochondria, although the latter lacks a strong mTP. The SUS2 isoform is not detectable in mitochondria, despite its presence in the cytosol. In maize primary roots, the mitochondrion-associated SUS (mitoSUS) is present mostly in the root tip, indicating a developmental regulation of SUS-compartmentation. Unlike the glycolytic enzymes that occur attached to the outside of mitochondria, SUS is intramitochondrial. The low abundance of SUS in mitochondria, its high Km for sucrose and low or lack of sucrose in mitochondria, suggest that mitoSUS plays a non-sucrolytic role. Co- immunoprecipitation studies indicate that SUS interacts with the voltage-dependent anion channel (VDAC) and thus may be involved in the regulation of solute fluxes between mitochondria and the cytosol. In several plant species, at least one of the SUS proteins possesses a putative mTP, indicating the conservation of the non- catalytic function across plant species. In summary, the interaction with VDAC, isoform-specific association with compartments not involved in sucrose catabolism and an apparent conservation of mitochondrial targeting among plants indicate a novel signaling function for SUS in plant cells.