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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

The Role of Cytosolic -Glucan Phosphorylase in Maltose Metabolism and the Comparison of Amylomaltase in Arabidopsis and Escherichia coli¹

Department of Botany (Y.L., T.D.S.), Department of Biochemistry (J.M.S.), and Department of Plant Pathology (J.Y.), University of Wisconsin, Madison, Wisconsin 53706

Transitory starch of leaves is broken down hydrolytically, making maltose the predominant form of carbon exported from chloroplasts at night. Maltose metabolism in the cytoplasm of Escherichia coli requires amylomaltase (MalQ) and maltodextrin phosphorylase (MalP). Possible orthologs of MalQ and MalP in the cytosol of Arabidopsis (Arabidopsis thaliana) were proposed as disproportionating enzyme (DPE2, At2g40840) and -glucan phosphorylase (AtPHS2, At3g46970). In this article, we measured the activities of recombinant DPE2 and AtPHS2 proteins with various substrates; we show that maltose and a highly branched, soluble heteroglycan (SHG) are excellent substrates for DPE2 and propose that a SHG is the in vivo substrate for DPE2 and AtPHS2. In E. coli, MalQ and MalP preferentially use smaller maltodextrins (G₃–G₇) and we suggest that MalQ and DPE2 have similar, but nonidentical, roles in maltose metabolism. To study this, we complemented a MalQ^– E. coli strain with DPE2 and found that the rescue was not complete. To investigate the role of AtPHS2 in maltose metabolism, we characterized a T-DNA insertion line of the AtPHS2 gene. The nighttime maltose level increased 4 times in the Atphs2-1 mutant. The comparison of maltose metabolism in Arabidopsis with that in E. coli and the comparison of the maltose level in plants lacking DPE2 or AtPHS2 indicate that an alternative route to metabolize the glucan residues in SHG exists. Other plant species also contain SHG, DPE2, and -glucan phosphorylase, so this pathway for maltose metabolism may be widespread among plants.

² Present address: Department of Biochemistry, Michigan State University, East Lansing, MI 48824.

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: Thomas D. Sharkey (tsharkey{at}wisc.edu).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.086850

^* Corresponding author; e-mail tsharkey{at}wisc.edu; fax 608–262–7509.

Received July 16, 2006; accepted September 4, 2006; published September 15, 2006.

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