title: Identification of multiple phosphorylation sites on maize endosperm starch branching enzyme IIb, a key enzyme in amylopectin biosynthesis.
abstract: Starch branching enzyme IIb (SBEIIb) plays a crucial role in amylopectin biosynthesis in maize endosperm by defining the structural and functional properties of storage starch and is regulated by protein phosphorylation. Native and recombinant maize SBEIIb were used as substrates for amyloplast protein kinases to identify phosphorylation sites on the protein. A multidisciplinary approach involving bioinformatics, site-directed mutagenesis, and mass spectrometry identified three phosphorylation sites at Ser residues: Ser(649), Ser(286), and Ser(297). Two Ca(2+)-dependent protein kinase activities were partially purified from amyloplasts, termed K1, responsible for Ser(649) and Ser(286) phosphorylation, and K2, responsible for Ser(649) and Ser(297) phosphorylation. The Ser(286) and Ser(297) phosphorylation sites are conserved in all plant branching enzymes and are located at opposite openings of the 8-stranded parallel β-barrel of the active site, which is involved with substrate binding and catalysis. Molecular dynamics simulation analysis indicates that phospho-Ser(297) forms a stable salt bridge with Arg(665), part of a conserved Cys-containing domain in plant branching enzymes. Ser(649) conservation appears confined to the enzyme in cereals and is not universal, and is presumably associated with functions specific to seed storage. The implications of SBEIIb phosphorylation are considered in terms of the role of the enzyme and the importance of starch biosynthesis for yield and biotechnological application.
authors.affiliation: From the Department of Molecular and Cellular Biology, College of Biological Science.
authors.forename: Amina
authors.initials: A
authors.lastname: Makhmoudova
authors.forename: Declan
authors.initials: D
authors.lastname: Williams
authors.forename: Dyanne
authors.initials: D
authors.lastname: Brewer
authors.forename: Sarah
authors.initials: S
authors.lastname: Massey
authors.forename: Jenelle
authors.initials: J
authors.lastname: Patterson
authors.forename: Anjali
authors.initials: A
authors.lastname: Silva
authors.forename: Kenrick A
authors.initials: KA
authors.lastname: Vassall
authors.forename: Fushan
authors.initials: F
authors.lastname: Liu
authors.forename: Sanjeena
authors.initials: S
authors.lastname: Subedi
authors.forename: George
authors.initials: G
authors.lastname: Harauz
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Tetlow
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Emes
mesh_terms: D015061:1,4-alpha-Glucan Branching Enzyme / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000687:Amylopectin / Q000096:biosynthesis*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D056625:Endosperm / Q000201:enzymology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000494:pharmacology; D010766:Phosphorylation; D011487:Protein Conformation; D011494:Protein Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D003313:Zea mays / Q000201:enzymology*
chemical_list: D004791:Enzyme Inhibitors; D010455:Peptides; D011994:Recombinant Proteins; D000687:Amylopectin; D015061:1,4-alpha-Glucan Branching Enzyme; D011494:Protein Kinases; D002118:Calcium
keywords: Branching Enzyme; Carbohydrate Biosynthesis; Maize; Molecular Modeling; Plant Biochemistry; Protein Kinases; Protein Phosphorylation; Starch