Details

Autor(en) / Beteiligte

• Liu, Fushan
• Romanova, Nadya
• Lee, Elizabeth A
• Ahmed, Regina
• Evans, Martin
• Gilbert, Elliot P
• Morell, Matthew K
• Emes, Michael J
• Tetlow, Ian J

Titel

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

Ist Teil von

• Biochemical journal, 2012-12, Vol.448 (3), p.373-387

Ort / Verlag

England

Erscheinungsjahr

2012

Quelle

MEDLINE

Beschreibungen/Notizen

• 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.