The Hyaluronan Synthase Catalyzes the Synthesis and Membrane Translocation of Hyaluronan

Fluorescence Resonance Energy Transfer (FRET) and Proximity Ligation Assays Reveal Functionally Relevant Homo- and Heteromeric Complexes among Hyaluronan Synthases HAS1, HAS2, and HAS3

“…This would solve the dilemma of how a single, relatively small protein with two transferase activities, which binds tightly to hyaluronan, could allow hyaluronan extrusion through a pore made by the enzyme itself. It can be speculated that processive rachet-like shifting of the growing polymer between the UDP-GlcNAc and UDPGlcUA binding sites (15) could involve sites on separate but interacting HAS polypeptides, resembling the model suggested recently for seHAS (17). A larger pore through complex formation can also be envisioned to better accommodate hyaluronan extrusion.…”

“…For example, digitonin-solubilized HAS1 fractionated on a gel filtration column shows enzymatically active HAS1 at a very high molecular mass (ϳ600 kDa) (45). A part of the solubilized seHAS also elutes in a position corresponding to a dimer (17). The Golgi-localized GalT1 and ST6Gal1 transferase activities and proteins elute at the size of enzyme tetramers (34), but large oligomers have also been reported (46).…”

“…Streptococcal HAS itself forms the pore together with phospholipids, (17,22), although this has not been firmly demonstrated for vertebrate HASs. Modulation of multidrug-resistant transporter activity affects hyaluronan synthesis in mammalian cells, and multidrug resistant proteins have been suggested to mediate hyaluronan transport (23).…”

“…Unlike other vertebrate glycoconjugates, hyaluronan is not known to be built on any protein or lipid core. For Streptococcus equisimilis hyaluronan synthase (seHAS) the possibility of lipid or protein carriers that could initiate hyaluronan synthesis, has been excluded (17). Moreover, while HAS polypeptides are co-translationally inserted in the ER membrane and transported to the Golgi apparatus, they become activated only after reaching the plasma membrane (18).…”

Hyaluronan Synthase 1 (HAS1) Requires Higher Cellular UDP-GlcNAc Concentration than HAS2 and HAS3*

“…Because of its physicochemical properties, hyaluronan can influence many cellular functions including migration, proliferation, and adhesion, but it may also act as a signaling molecule. Hyaluronan is produced by three isoforms of hyaluronan synthase (HAS1, 2, and 3), 3 these enzymes act on the inner face of plasma membrane (5) and simultaneously translocate the growing polysaccharide through plasma membrane into extracellular space (6,7).…”