Biochemical characterization of starch synthase III from Arabidopsis thaliana

BUSI, M. V.; PALOPOLI, N.; VALDEZ, H.; FORNASARI, M. S.; DIEGO FABIAN GOMEZ CASATI; PARISI, G.; UGALDE, R.

Pinamar

Congreso; XLI Reunión Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2005

Glycogen and starch are the major energy storage compounds in all living organisms. The metabolic pathways leading to their synthesis involve the action of several enzymes, among which glycogen (GS) or starch synthases (SS) catalyze the elongation of the alpha-1,4-glucans in each polymer. At least the existence of five isoforms was described in Arabidopsis thaliana; it has been reported that one of that isoforms (SSIII) has a regulatory function in the synthesis of transient starch. We carried out the cloning, expression in E. coli and characterization of the recombinant full length SSIII and a truncated form (named SSIII catalytic domain, SSIIICD) from A. thaliana. Kinetic analysis in vitro showed that both proteins were active and suggested a possible regulatory role of the N-terminal region. Fold class assignment methods and homology modeling resulted in a strong global similarity with A. tumefaciens GS, with ADP-binding residues fully conserved; remarkably, some ligand binding residues showed a significant evolutionary divergence. Indeed, we found that SSIIICD could complement an A. tumefaciens null mutant lacking GS, suggesting that the truncated isoform, like bacterial GS, could initiate glycogen synthesis in vivo.