Identification and characterization of a novel starch branching enzyme from the picoalgae Ostreococcus tauri

Starch branching enzyme is a highly conserved protein from plants to algae. This enzyme participates in starch granule assembly by the addition of α-1,6-glucan branches to the α-1,4-polyglucans. This modification determines the structure of amylopectin thus arranging the final composition of the starch granule. Herein, we describe the function of the Ot01g03030 gene from the picoalgae Ostreococcus tauri. Although in silico analysis suggested that this gene codes for a starch debranching enzyme, our biochemical studies support that this gene encodes a branching enzyme (BE). The resulting 1058 amino acids protein has two in tandem carbohydrate binding domains (CBMs, from the CBM41 and CBM48 families) at the N-terminal (residues 64-403) followed by the C-terminal catalytic domain (residues 426-1058). Analysis of the BE truncated isoforms show that the CBMs bind differentially to whole starch, amylose or amylopectin. Furthermore, both CBMs seem to be essential for BE activity, as no catalytic activity was detected in the truncated enzyme comprising only by the catalytic domain. Our results suggest that the Ot01g03030 gene codifies for a functional BE containing two CBMs from CBM41 and CBM48 families which are critical for enzyme function and regulation.