Glycosyltransferase required for the biosynthesis of heparan-sulfate. The EXT1/EXT2 complex possesses substantially higher glycosyltransferase activity than EXT1 or EXT2 alone. Required for the exosomal release of SDCBP, CD63 and syndecan (By similarity). 3-O-{[(1->4)-beta-D-GlcA-(1->4)-alpha-D-GlcNAc](n)-(1->4)-beta-D-GlcA-(1->3)-beta-D-Gal-(1->3)-beta-D-Gal-(1->4)-beta-D-Xyl}-L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-{alpha-D-GlcNAc-[(1->4)-beta-D-GlcA-(1->4)-alpha-D-GlcNAc](n)-(1->4)-beta-D-GlcA-(1->3)-beta-D-Gal-(1->3)-beta-D-Gal-(1->4)-beta-D-Xyl}-L-seryl-[protein] + H(+) + UDP 3-O-{alpha-D-GlcNAc-[(1->4)-beta-D-GlcA-(1->4)-alpha-D-GlcNAc](n)-(1->4)-beta-D-GlcA-(1->3)-beta-D-Gal-(1->3)-beta-D-Gal-(1->4)-beta-D-Xyl}-L-seryl-[protein] + UDP-alpha-D-glucuronate = 3-O-{[(1->4)-beta-D-GlcA-(1->4)-alpha-D-GlcNAc](n+1)-(1->4)-beta-D-GlcA-(1->3)-beta-D-Gal-(1->3)-beta-D-Gal-(1->4)-beta-D-Xyl}-L-seryl-[protein] + H(+) + UDP Manganese. Divalent cations. Protein modification; protein glycosylation. Forms a homo/heterooligomeric complex with EXT2. Interacts with NDST1. The EXT1/EXT2 complex is localized in the Golgi apparatus. Belongs to the glycosyltransferase 47 family.