Structural and mechanistic characterization of bifunctional heparan sulfate N-deacetylase-N-sulfotransferase 1
Heparan sulfate (HS) polysaccharides are major constituents of the extracellular matrix, involved in myriad structural and signaling processes. Mature HS polysaccharides contain complex, non-templated patterns of sulfation and epimerization, which mediate interactions with diverse protein partners. Complex HS modifications form around initial clusters of glucosamine-N-sulfate (GlcNS) on nascent polysaccharide chains, but the mechanistic basis underpinning incorporation of the GlcNS modification itself into HS remains unclear. We have determined cryo-electron microscopy structures of human N-deacetylase-N-sulfotransferase (NDST)1, the bifunctional enzyme responsible for initial GlcNS modification of HS. Our structures reveal the architecture of both NDST1 deacetylase and sulfotransferase catalytic domains, alongside a previously unreported non-catalytic N-terminal domain. Surprisingly, the two catalytic domains of NDST1 adopt an unusual back-to-back topology that limits direct cooperativity. Binding analyses, aided by novel activity modulating nanobodies, suggest that sulfotransferase domain substrate anchoring initiates the NDST1 catalytic cycle, providing a plausible mechanism for cooperativity despite spatial domain separation. Our data shed light on key determinants of NDST1 activity, and describe tools to probe NDST1 function in vitro and in vivo.