Starch synthesis in Ostreococcus tauri: Role of the starchbinding domains on the activity of Starch Synthase III-B and its interaction with other starch metabolism proteins

VELAZQUEZ, M.B.; BUSI, M.V.; HEDIN, N.; BARCHIESI, J.

San Martin

Simposio; GlycoAR2019 - 3er Simposio Argentino de Glicobiología; 2019

GlycoAR - UNSAM

Starch is the major energy storage carbohydrate in photosynthetic eukaryotes. Several enzymes are involved in the biosynthesis of the highly organized semi-crystalline starch granules, including starch-synthase III (SSIII), which is widely conserved in photosynthetic organisms. This enzyme catalyzes the extension of the α-1,4 glucan chain and plays a regulatory role in the synthesis of starch. Interestingly, unlike most plants, the unicellular green alga Ostreococcus tauri has three SSIII isoforms. In the present study, we describe the structural and functional characterization of OsttaSSIII-B (OT_ostta13g01200), which has a modular organization that resembles that of SSIII present in higher plants, comprising three starch-binding domains (SBDs) at the N-terminal region and a C-terminal catalytic domain (CD). Purified recombinant OsttaSSIII-B displayed a high affinity toward branchedpolysaccharides such as glycogen and amylopectin, and also to ADPGlc. To complete the functional characterization of the OsttaSSIII-B we studied the possible interactions with other proteins related to starch metabolism (OsttaSBE (Ot01g03030) and OsttaAMY (Ot06g02060) using fluorescence techniques (thermal stability and conformational changes) and scanning electron microscopy of starch granules before and after the incubation with such proteins. Our results suggest that OsttaSSIIIB interacts with OsttaAMY and not with OsttaSBE. This suggest that, as occurs in higher plants, the formation of a multienzymatic complex is relevant for the synthesis of the starch granule in algae.