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β-AMYLASE4, a Noncatalytic Protein Required for Starch Breakdown, Acts Upstream of Three Active β-Amylases in Arabidopsis Chloroplasts^[W],[OA]

^a Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh EH9 3JH, United Kingdom
^b Institute of Plant Sciences, ETH Zurich, CH-8092 Zurich, Switzerland
^c School of Crystallography, Birkbeck College, University of London, London WC1E 7HX, United Kingdom
^d Centre of Excellence for Plant Metabolomics and Australian Research Council Centre of Excellence in Plant Energy Biology, University of Western Australia, Crawley WA 6009, Australia
^e Institute for Computational Science, ETH Zurich, CH-8092 Zurich, Switzerland
^f John Innes Centre, Norwich NR4 7UH, United Kingdom

² Address correspondence to szeeman{at}ethz.ch.

This work investigated the roles of β-amylases in the breakdown of leaf starch. Of the nine β-amylase (BAM)–like proteins encoded in the Arabidopsis thaliana genome, at least four (BAM1, -2, -3, and -4) are chloroplastic. When expressed as recombinant proteins in Escherichia coli, BAM1, BAM2, and BAM3 had measurable β-amylase activity but BAM4 did not. BAM4 has multiple amino acid substitutions relative to characterized β-amylases, including one of the two catalytic residues. Modeling predicts major differences between the glucan binding site of BAM4 and those of active β-amylases. Thus, BAM4 probably lost its catalytic capacity during evolution. Total β-amylase activity was reduced in leaves of bam1 and bam3 mutants but not in bam2 and bam4 mutants. The bam3 mutant had elevated starch levels and lower nighttime maltose levels than the wild type, whereas bam1 did not. However, the bam1 bam3 double mutant had a more severe phenotype than bam3, suggesting functional overlap between the two proteins. Surprisingly, bam4 mutants had elevated starch levels. Introduction of the bam4 mutation into the bam3 and bam1 bam3 backgrounds further elevated the starch levels in both cases. These data suggest that BAM4 facilitates or regulates starch breakdown and operates independently of BAM1 and BAM3. Together, our findings are consistent with the proposal that β-amylase is a major enzyme of starch breakdown in leaves, but they reveal unexpected complexity in terms of the specialization of protein function.