Identification of a novel starch synthase III from the picoalgae Ostreococcus tauri

• Published in: Biochimie 2017-02, Vol.133, p.37-44
• Main Authors: Barchiesi, Julieta, Hedin, Nicolás, Iglesias, Alberto A., Gomez-Casati, Diego F., Ballicora, Miguel A., Busi, María V.
• Format: Article
• Language: English
• Subjects: Amylose - chemistry; Animals; Arabidopsis - enzymology; Catalysis; Catalytic Domain; Chlorophyta - enzymology; Escherichia coli - enzymology; Glucose - metabolism; Glycogen - chemistry; Glycogen - metabolism; Glycogen Synthase - chemistry; Glycogen Synthase - genetics; Glycogen Synthase - metabolism; O. tauri; Polysaccharides - metabolism; Protein activity; Protein Isoforms - chemistry; Protein Isoforms - genetics; Protein Isoforms - metabolism; Rabbits; Sequence Analysis, Protein; Starch synthase; Starch Synthase - chemistry; Starch Synthase - genetics; Starch Synthase - metabolism
• ISSN: 0300-9084; 1638-6183
• DOI: 10.1016/j.biochi.2016.12.003

Hydrosoluble glycogen is the major energy storage compound in bacteria, archaea, fungi, and animal cells. In contrast, photosynthetic eukaryotes have evolved to build a highly organized semicrystalline granule of starch. Several enzymes are involved in polysaccharide synthesis, among which glycogen or starch synthase catalyze the elongation of the α-1,4-glucan chain. Ostreococcus tauri, accumulates a single starch granule and contains three starch synthase III (SSIII) isoforms, known as OsttaSSIII-A, OsttaSSIII-B and OsttaSSIII-C. After amino acids sequence analysis we found that OsttaSSIII-C lacks starch-binding domains, being 49% identical to the catalytic region of the SSIII from Arabidopsis thaliana and 32% identical to the entire Escherichia coli glycogen synthase. The recombinant, highly purified OsttaSSIII-C exhibited preference to use as a primer branched glycans (such as rabbit muscle glycogen and amylopectin), rather than amylose. Also, the enzyme displayed a high affinity toward ADP-glucose. We found a marked conservation of the amino acids located in the catalytic site, and specifically determined the role of residues R270, K275 and E352 by site-directed mutagenesis. Results show that these residues are important for OsttaSSIII-C activity, suggesting a strong similarity between the active site of the O. tauri SSIII-C isoform and other bacterial glycogen synthases. •We characterized a starch synthase III (SSIII) isoform from O. tauri which lacks starch binding domains (SBDs).•Recombinant OsttaSSIII-C exhibited preference to branched glycans such as glycogen and amylopectin, rather than amylose.•Results suggest a strong similarity between the active site of the O. tauri SSIII-C and other bacterial glycogen synthases.•Mutations in residues R270, K275 and E352 affect OsttaSSIII-C catalytic activity.