Structure of the human heparan sulfate polymerase complex EXT1-EXT2

Area of Science:

• Biochemistry
• Glycobiology
• Structural Biology

Background:

• Heparan sulfates are crucial cell surface polysaccharides involved in protein ligand interactions.
• The enzymes EXT1 and EXT2 are key for heparan sulfate biosynthesis, forming the glycan backbone.
• The molecular mechanism of heparan sulfate chain polymerization is not well understood.

Purpose of the Study:

• To elucidate the molecular mechanism of heparan sulfate chain polymerization.
• To determine the structure of the human EXT1-EXT2 complex.
• To investigate the functional roles of the catalytic sites within EXT1-EXT2.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the human EXT1-EXT2 complex.
• In vitro and in cellulo mutational studies were performed to analyze enzyme function.
• Biochemical assays were employed to assess glycosyltransferase activity.

Main Results:

• The cryo-EM structure revealed a tightly packed hetero-dimeric EXT1-EXT2 complex with four glycosyltransferase domains.
• EXT1 demonstrated the ability to catalyze both N-acetylglucosamine and glucuronic acid transfer reactions.
• EXT2 appeared to possess only N-acetylglucosamine transferase activity.
• Heparan sulfate chain elongation was characterized as a nonprocessive mechanism.

Conclusions:

• The study provides the first structural insights into the human EXT1-EXT2 complex, a key enzyme in heparan sulfate biosynthesis.
• The findings suggest a nonprocessive mechanism for heparan sulfate chain elongation, with distinct roles for EXT1 and EXT2 catalytic sites.
• This work lays the groundwork for understanding EXT1-EXT2 function in health and disease contexts.