title: Phosphorylation of Arabidopsis transketolase at Ser428 provides a potential paradigm for the metabolic control of chloroplast carbon metabolism.
abstract: Calcium is an important second messenger in eukaryotic cells that regulates many different cellular processes. To elucidate calcium regulation in chloroplasts, we identified the targets of calcium-dependent phosphorylation within the stromal proteome. A 73 kDa protein was identified as one of the most dominant proteins undergoing phosphorylation in a calcium-dependent manner in the stromal extracts of both Arabidopsis and Pisum. It was identified as TKL (transketolase), an essential enzyme of both the Calvin-Benson-Bassham cycle and the oxidative pentose phosphate pathway. Calcium-dependent phosphorylation of both Arabidopsis isoforms (AtTKL1 and AtTKL2) could be confirmed in vitro using recombinant proteins. The phosphorylation is catalysed by a stroma-localized protein kinase, which cannot utilize GTP. Phosphorylation of AtTKL1, the dominant isoform in most tissues, occurs at a serine residue that is conserved in TKLs of vascular plants. By contrast, an aspartate residue is present in this position in cyanobacteria, algae and mosses. Characterization of a phosphomimetic mutant (S428D) indicated that Ser428 phosphorylation exerts significant effects on the enzyme's substrate saturation kinetics at specific physiological pH values. The results of the present study point to a role for TKL phosphorylation in the regulation of carbon allocation.
authors: *Department of Biology I, LMU Munich, Groβhaderner Str. 2-4, D-82152 Planegg-Martinsried, Germany.
authors: Agostinho G
authors: AG
authors: Rocha
authors: *Department of Biology I, LMU Munich, Groβhaderner Str. 2-4, D-82152 Planegg-Martinsried, Germany.
authors: Norbert
authors: N
authors: Mehlmer
authors: †Department of Biochemistry, MFPL, University of Vienna, Dr. Bohr Gasse 9/5, A-1030 Vienna, Austria.
authors: Simon
authors: S
authors: Stael
authors: ‡Department of Molecular Systems Biology (MoSys), University of Vienna, Althanstr. 14, A-1090 Vienna, Austria.
authors: Andrea
authors: A
authors: Mair
authors: *Department of Biology I, LMU Munich, Groβhaderner Str. 2-4, D-82152 Planegg-Martinsried, Germany.
authors: Nargis
authors: N
authors: Parvin
authors: §Center for Integrated Protein Science (Munich), Department of Biology, LMU Munich, D-82152 Martinsried, Germany.
authors: Fatima
authors: F
authors: Chigri
authors: Markus
authors: M
authors: Teige
authors: Ute C
authors: UC
authors: Vothknecht
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002244:Carbon / Q000378:metabolism*; D002736:Chloroplasts / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D010766:Phosphorylation / Q000502:physiology; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012694:Serine / Q000235:genetics / Q000378:metabolism*; D014174:Transketolase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D020033:Protein Isoforms; D012694:Serine; D002244:Carbon; D014174:Transketolase