Role of EXT Family Enzymes in Heparan Sulfate Biosynthesis

Heparan sulfate (HS) proteoglycans are essential regulators of cell signaling, development, and extracellular matrix organization. Central to the HS biosynthesis pathway is the exostosin family of enzymes that control commitment to HS assembly and catalyze formation of the polymer backbone in the Golgi apparatus. Recent structural studies have substantially revised our understanding by demonstrating that the HS co-polymerase is a tightly organized EXT1-EXT2 heterodimer, wherein the GT-B domain of EXT1 and the GT-A domain of EXT2 provide the β1,4-GlcA and α1,4-GlcNAc transferase activities, respectively. Parallel work on EXTL3 clarified how selective recognition of the linker-region glycopeptide commits to HS extension, outcompeting the default chondroitin sulfate pathway. In contrast, EXTL2 is best viewed as a regulatory GlcNAc transferase that can cap or divert linker intermediates rather than as a polymerase, whereas EXTL1 remains the least defined family member despite evidence for GlcNAc transferase activity. Genetic studies continue to reveal their importance in human disease, including hereditary multiple exostoses and EXTL3-associated immunodeficiency. Despite this progress, major questions remain regarding chain length control, enzyme coordination, and therapeutic targeting. This short review integrates recent structural, biochemical, and genetic advances to provide an updated narrative of how mammalian EXT proteins govern HS biosynthesis.