Plays two roles in the biosynthesis of the exopolysaccharide alginate: protects alginate from degradation as the polymer traverses the periplasm, and also plays a role in its O-acetylation. Probably has acetyltransferase activity in vivo (By similarity). Glycan biosynthesis; alginate biosynthesis. Monomer. Consists of two domains, with an N-terminal domain with structural homology to members of the SGNH (GDSL) hydrolase superfamily and a C-terminal carbohydrate-binding module (CBM) that may bind alginate. Belongs to the AlgX family.