In Vitro Assay for Hyaluronan Synthase

Abstract

Hyaluronan (HA) is synthesized at the plasma membrane as a free linear polymer of composition [β1→4GlcAβ1→3GlcNAc]ⁿ (1–3). All models suggest that polymerization occurs at the inner face of the plasma membrane while the polymer is coordinately translocated or extruded across the membrane to the extracellular face of the cell [for review, see (2–5)]. In mammals (6), and all vertebrates (Spicer, unpublished data), HA is synthesized by any one of three HA synthases (HAS). The three HAS proteins are encoded by three related yet distinct genes (6,7). All HAS proteins are predicted integral plasma membrane proteins with N-terminal and C-terminal transmembrane domains separating a large cytoplasmic domain [for review, see (4)]. Any one HAS protein is capable of catalyzing the de novo synthesis of HA (8), suggesting that each is capable of specifically binding both UDP-sugar substrates and creating the alternate β1→3 and the β1→4 glycosidic bonds. Indeed, the related prokaryotic HA synthase, spHAS, from the Gram-positive bacterium, Streptococcus pyogenes, has been purified to apparent homogeneity and is capable of synthesizing high molecular mass HA chains in vitro, when provided with a source of UDP-GlcA, UDP-GlcNAc, and Mg²⁺ ions (9).