Starch Synthesis in Ostreococcus tauri: The Starch-Binding Domains of Starch Synthase III-B Are Essential for Catalytic Activity

BARCHIESI, JULIETA; VELAZQUEZ, MARIA BELEN; PALOPOLI, NICOLAS; IGLESIAS, ALBERTO A.; GOMEZ-CASATI, DIEGO F.; BALLICORA, MIGUEL ANGEL; BUSI, MARIA VICTORIA

Frontiers in Plant Science

Frontiers

Lugar: Lussane; Año: 2018 vol. 9

Starch is the major energy storage carbohydrate in photosynthetic eukaryotes. Severalenzymes are involved in building highly organized semi-crystalline starch granules,including starch-synthase III (SSIII), which is widely conserved in photosyntheticorganisms. This enzyme catalyzes the extension of the a-1,4 glucan chain and playsa regulatory role in the synthesis of starch. Interestingly, unlike most plants, theunicellular green alga Ostreococcus tauri has three SSIII isoforms. In the presentstudy, we describe the structure and function of OsttaSSIII-B, which has a similarmodular organization to SSIII in higher plants, comprising three putative starch-bindingdomains (SBDs) at the N-terminal region and a C-terminal catalytic domain (CD). Purifiedrecombinant OsttaSSIII-B displayed a high affinity toward branched polysaccharidessuch as glycogen and amylopectin, and to ADP-glucose. Lower catalytic activity wasdetected for the CD lacking the associated SBDs, suggesting that they are necessaryfor enzyme function. Moreover, analysis of enzyme kinetic and polysaccharide-bindingparameters of site-directed mutants with modified conserved aromatic amino acidresidues W122, Y124, F138, Y147, W279, and W304, belonging to the SBDs, revealedtheir importance for polysaccharide binding and SS activity. Our results suggest thatOT_ostta13g01200 encodes a functional SSIII comprising three SBD domains that arecritical for enzyme function.