Structure of the human heparan sulfate polymerase complex EXT1-EXT2

Heparan sulfates are complex polysaccharides that mediate the interaction with a broad range of protein ligands at the cell surface. A key step in heparan sulfate biosynthesis is catalyzed by the bi-functional glycosyltransferases EXT1 and EXT2, which generate the glycan backbone consisting of repeating N -acetylglucosamine and glucuronic acid units. The molecular mechanism of heparan sulfate chain polymerization remains, however, unknown. Here, we present the cryo-electron microscopy structure of human EXT1-EXT2, which reveals the formation of a tightly packed hetero-dimeric complex harboring four glycosyltransferase domains with their catalytic sites facing in opposite directions. Along with in vitro activity assays using fluorescently labeled and chemically defined substrates, these findings provide a molecular insight into donor substrate recognition and demonstrate that the glycosyltransferase reactions are highly specific. A combination of in vitro and in cellulo mutational studies was used to dissect the functional role of the four catalytic sites. While EXT1 is able to catalyze both glycosyltransferase reactions, EXT2 harbors only N -acetylglucosamine transferase activity. Our results provide mechanistic insight into heparan sulfate chain elongation as a non processive process and lay the cornerstone for future studies on EXT1-EXT2 function in health and disease. ### Competing Interest Statement The authors have declared no competing interest.