Functional and structural characterization of starch synthase III from Arabidopsis thaliana

BUSI, MV, PALOPOLI, N, VALDEZ, H, FORNASARI, MS, GOMEZ-CASATI, D, PARISI, G AND UGALDE, R

PROTEINS: STRUCTURE, FUNCTION AND GENETICS

WILEY-LISS, DIV JOHN WILEY & SONS INC

Año: 2008 vol. 70 p. 31 - 40

0887-3585

Glycogen and starch are the majorenergy storage compounds in most livingorganisms. The metabolic pathwaysleading to their synthesis involve theaction of several enzymes, among whichglycogen synthase (GS) or starch synthase(SS) catalyze the elongation of thea-1,4-glucan backbone. At least five SSisoforms were described in Arabidopsisthaliana; it has been reported that theisoform III (SSIII) has a regulatoryfunction on the synthesis of transientplant starch. The catalytic C-terminaldomain of A. thaliana SSIII (SSIII-CD)was cloned and expressed. SSIII-CDfully complements the production ofglycogen by an Agrobacterium tumefaciensglycogen synthase null mutant,suggesting that this truncated isoformrestores in vivo the novo synthesis ofbacterial glycogen. In vitro studiesrevealed that recombinant SSIII-CDuses with more efficiency rabbit muscleglycogen than amylopectin as primerand display a high apparent affinity forADP-Glc. Fold class assignment methodsfollowed by homology modelingpredict a high global similarity toA. tumefaciens GS showing a fully conservationof the ADP-binding residues.On the other hand, this comparisonrevealed important divergences of thepolysaccharide binding domain betweenAtGS and SSIII-CD.