Glycosyltransferase required for the biosynthesis of heparan-sulfate and responsible for the alternating addition of beta-1-4-linked glucuronic acid (GlcA) and alpha-1-4-linked N-acetylglucosamine (GlcNAc) units to nascent heparan sulfate chains. Botv is the trigger of heparan sulfate chain initiation and polymerization takes place by a complex of ttv and sotv. Plays a central role in the diffusion of morphogens hedgehog (hh), wingless (wg) and decapentaplegic (dpp) via its role in heparan sulfate proteoglycans (HSPGs) biosynthesis which are required for movement of hh, dpp and wg morphogens. 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 Protein modification; protein glycosylation. Glycan metabolism; heparan sulfate biosynthesis. Glycan metabolism; heparin biosynthesis. Interacts with sau (PubMed:23720043). Localization to the Golgi may be regulated by sau (PubMed:23720043). Ubiquitously expressed in early embryos. Later (in stage 10 embryos), it is expressed at higher level in the nervous system. Ubiquitously expressed in wing imaginal disk. Expressed both maternally and zygotically. According to some authors (PubMed:10549295) ttv mutants have no effect on wg signaling, while according to others (PubMed:14645127, PubMed:14729575 and PubMed:15056609) wg signaling is affected. Such discrepancy may be explained by the fact that the absence of ttv could be partially compensated by the intact sotv protein. 'Tout velu' means 'very hairy' in French. Belongs to the glycosyltransferase 47 family.