The defective effect of starch branching enzyme IIb from weak to strong induces the formation of biphasic starch granules in amylose-extender maize endosperm

AbstractStarch from cereal endosperm is a major energy source for many mammals. The synthesis of this starch involves a number of different enzymes whose mode of action is still not completely understood. ADPglucose pyrophosphorylase is involved in the synthesis of starch monomer (ADP-glucose), a process, which almost exclusively takes place in the cytosol. ADPglucose is then transported into the amyloplast and incorporated into starch granules by starch synthase, starch-branching enzyme and debranching enzyme. Additional enzymes, including starch phosphorylase and disproportionating enzyme, may be also involved in the formation of starch granules, although their exact functions are still obscure. Interactions between these enzymes in the form of functional complexes have been proposed and investigated, resulting more complicated starch biosynthetic pathways. An overall picture and recent advances in understanding of the functions of these enzymes is summarized in this review to provide insights into how starch granules are synthesized in cereal endosperm.

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Glucan affinity of starch synthase IIa determines binding of starch synthase I and starch-branching enzyme IIb to starch granules

Biochemical Journal ◽

10.1042/bj20120573 ◽

2012 ◽

Vol 448 (3) ◽

pp. 373-387 ◽

Cited By ~ 50

Author(s):

Fushan Liu ◽

Nadya Romanova ◽

Elizabeth A. Lee ◽

Regina Ahmed ◽

Martin Evans ◽

...

Keyword(s):

Catalytic Activity ◽

Protein Interactions ◽

Starch Granule ◽

Protein Complexes ◽

Starch Synthase ◽

Starch Granules ◽

Branching Enzyme ◽

Wild Type ◽

Protein Protein Interactions ◽

Starch Branching Enzyme

The sugary-2 mutation in maize (Zea mays L.) is a result of the loss of catalytic activity of the endosperm-specific SS (starch synthase) IIa isoform causing major alterations to amylopectin architecture. The present study reports a biochemical and molecular analysis of an allelic variant of the sugary-2 mutation expressing a catalytically inactive form of SSIIa and sheds new light on its central role in protein–protein interactions and determination of the starch granule proteome. The mutant SSIIa revealed two amino acid substitutions, one being a highly conserved residue (Gly522→Arg) responsible for the loss of catalytic activity and the inability of the mutant SSIIa to bind to starch. Analysis of protein–protein interactions in sugary-2 amyloplasts revealed the same trimeric assembly of soluble SSI, SSIIa and SBE (starch-branching enzyme) IIb found in wild-type amyloplasts, but with greatly reduced activities of SSI and SBEIIb. Chemical cross-linking studies demonstrated that SSIIa is at the core of the complex, interacting with SSI and SBEIIb, which do not interact directly with each other. The sugary-2 mutant starch granules were devoid of amylopectin-synthesizing enzymes, despite the fact that the respective affinities of SSI and SBEIIb from sugary-2 for amylopectin were the same as observed in wild-type. The data support a model whereby granule-bound proteins involved in amylopectin synthesis are partitioned into the starch granule as a result of their association within protein complexes, and that SSIIa plays a crucial role in trafficking SSI and SBEIIb into the granule matrix.

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