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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Light and Metabolic Signals Control the Selective Degradation of Sucrose Synthase in Maize Leaves during Deetiolation^1,[OA]

Department of Plant Biology, University of Illinois, Urbana, Illinois 61801 (Q.-S.Q., S.C. Hardin, S.C. Huber); United States Department of Agriculture, Agricultural Research Service, Photosynthesis Research Unit, Urbana, Illinois 61801 (S.C. Huber); and Boyce Thompson Institute, Cornell University, Ithaca, New York 14853 (J.M., T.P.B.)

The content and activity of Suc (Suc) synthase (SUS) protein is high in sink organs but low in source organs. In this report, we examined light and metabolic signals regulating SUS protein degradation in maize (Zea mays) leaves during deetiolation. We found that SUS protein accumulated in etiolated leaves of the dark-grown seedlings but was rapidly degraded upon exposure to white, blue, or red light. This occurred concurrent with the accumulation of photosynthetic enzymes, such as Rubisco and Rubisco activase, and enzymes of Suc biosynthesis such as Suc-phosphate synthase. Deetiolation-induced SUS degradation was not inhibited by the proteasome inhibitor MG132. Moreover, neither full-length nor truncated SUS phosphorylated at the serine-170 site was found in the crude 26S proteasome fraction (150,000g postmicrosomal pellet) isolated in the presence of MG132. However, SUS degradation was strongly inhibited by feeding cycloheximide or amino acids to detached leaves, while Suc feeding had no effect. Of the amino acids tested, exogenous glutamate had the greatest effect. Collectively, these results demonstrate that SUS protein degradation during deetiolation: (1) is selective; (2) can be triggered by either blue- or red light-mediated signaling pathways; (3) does not involve the 26S proteasome; and (4) is inhibited by free amino acids. These findings suggest that SUS degradation is important to supply residues for the synthesis of other proteins required for autotrophic metabolism.

² Present address: BASF Plant Science, 26 Davis Drive, Research Triangle Park, NC 27709.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Steven C. Huber (schuber1{at}life.uiuc.edu).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.095182

^* Corresponding author; e-mail schuber1{at}life.uiuc.edu; fax 217–244–4419.

Received December 21, 2006; accepted March 26, 2007; published March 30, 2007.

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