Catalytically-inactive beta-amylase BAM4 required for starch breakdown in Arabidopsis leaves is a starch-binding-protein.
Of the four chloroplast beta-amylase (BAM) proteins identified in Arabidopsis, BAM3 and BAM4 were previously shown to play the major roles in leaf starch breakdown, although BAM4 apparently lacks key active site residues and beta-amylase activity. Here we tested multiple BAM4 proteins with different N-terminal sequences with a range of glucan substrates and assay methods, but detected no alpha-1,4-glucan hydrolase activity. BAM4 did not affect BAM1, BAM2 or BAM3 activity even when added in 10-fold excess, nor the BAM3-catalysed release of maltose from isolated starch granules in the presence of glucan water dikinase. However, BAM4 binds to amylopectin and to amylose-Sepharose whereas BAM2 has very low beta-amylase activity and poor glucan binding. The low activity of BAM2 may be explained by poor glucan binding but absence of BAM4 activity is not. These results suggest that BAM4 facilitates starch breakdown by a mechanism involving direct interaction with starch or other alpha-1,4-glucan.
Li
Jing
J
Centres of Excellence for Plant Metabolomics, Plant Energy Biology, Crawley, WA 6009, Australia.
Francisco
Perigio
P
Zhou
Wenxu
W
Edner
Christoph
C
Steup
Martin
M
Ritte
Gerhard
G
Bond
Charles S
CS
Smith
Steven M
SM
D000687:Amylopectin / Q000737:chemistry* / Q000378:metabolism; D017360:Arabidopsis / Q000201:enzymology*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D018515:Plant Leaves / Q000201:enzymology*; D011485:Protein Binding / Q000502:physiology; D013213:Starch / Q000737:chemistry* / Q000378:metabolism; D001614:beta-Amylase / Q000737:chemistry* / Q000378:metabolism
D029681:Arabidopsis Proteins; D013213:Starch; D000687:Amylopectin; D001614:beta-Amylase