Cell-free expression and biochemical characterization of polysaccharide-synthesizing glycosyltransferases

Abstract

Polysaccharides like cellulose and hyaluronan are synthesized by membrane-bound family-2 glycosyltransferases (GTs) that play critical structural, metabolic, or functional roles in cells. Though GT-2 family has the maximum number of deposited gene sequences, the biochemistry is poorly understood due to enzyme production challenges. Here, we developed a cell-free expression (CFE) protocol to produce two GT-2 family representative cellulose synthase (PttCesA8 from Populus tremula x tremuloides) and hyaluronan synthase (SeHAS from Streptococcus equisimilis). The CFE products were obtained as reconstituted proteoliposomes directly at high yields and short processing times compared to other approaches. Enzyme expression was confirmed using SDS-PAGE and immunoblotting, while integration of GTs into lipid layers was observed using cryogenic electron microscopy. Both GTs were catalytically active with Michalis-Menten kinetic constants, K_m for PttCesA8, was 295.8 µM, and SeHAS was 321.51 µM (toward UDP N-Acetyl Glucosamine) and 207.88 µM (toward UDP Glucuronic Acid), respectively, and with UDP inhibiting both GTs. Mutation of conserved residues in SeHAS also confirmed the importance of lysine-139, glutamine-248, and threonine-283 residues in hyaluronan biosynthesis. In summary, CFE methods enable expression of polysaccharide-synthesizing enzymes as proteoliposomes at high yields with relative ease for rapid biochemical and structural characterization studies.