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Published on September 15, 2006; 10.1104/pp.106.086850

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Received July 16, 2006
Accepted September 4, 2006

The Role of Cytosolic fÑ-Glucan Phosphorylase in Maltose Metabolism and the Comparison of Amylomaltase in Arabidopsis and E. coli

Yan Lu , Jon M. Steichen , Jian Yao , and Thomas D. Sharkey *

Department of Botany, University of Wisconsin, Madison, WI 53706, USA
Department of Biochemistry, University of Wisconsin, Madison, WI 53706, USA
Department of Plant Pathology, University of Wisconsin, Madison, WI 53706 USA

* Corresponding author; email: tsharkey{at}wisc.edu.

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 {alpha}-glucan phosphorylase (AtPHS2, At3g46970). In this work, 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 soluble heteroglycan (SHG) is the in vivo substrate for DPE2 and AtPHS2. In E. coli, MalQ and MalP preferentially use smaller maltodextrins (G3 to G7) and we suggest that MalQ and DPE2 have similar but non-identical 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 to four 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 {alpha}-glucan phosphorylase so this pathway for maltose metabolism may be widespread among plants.




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