Structural basis for heparan sulfate co-polymerase action by the EXT1-2 complex

Heparan sulfate (HS) proteoglycans are extended (-GlcA beta 1,4GlcNAc alpha 1,4-)(n) co-polymers containing decorations of sulfation and epimerization that are linked to cell surface and extracellular matrix proteins. In mammals, HS repeat units are extended by an obligate heterocomplex of two exostosin family members, EXT1 and EXT2, where each protein monomer contains distinct GT47 (GT-B fold) and GT64 (GT-A fold) glycosyltransferase domains. In this study, we generated human EXT1-EXT2 (EXT1-2) as a functional heterocomplex and determined its structure in the presence of bound donor and acceptor substrates. Structural data and enzyme activity of catalytic site mutants demonstrate that only two of the four glycosyltransferase domains are major contributors to co-polymer syntheses: the EXT1 GT-B fold beta 1,4GlcA transferase domain and the EXT2 GT-A fold alpha 1,4GlcNAc transferase domain. The two catalytic sites are over 90 & ANGS; apart, indicating that HS is synthesized by a dissociative process that involves a single catalytic site on each monomer.

Automated summary

In this study, the structure of the human EXT1-EXT2 heterocomplex was determined in the presence of donor and acceptor substrates. The results showed that only two out of the four glycosyltransferase domains were major contributors to co-polymer syntheses, namely the EXT1 GT-B fold beta 1,4GlcA transferase domain and the EXT2 GT-A fold alpha 1,4GlcNAc transferase domain.