Characterization of a high-Mr plasma-membrane-bound protein and assessment of its role as a constituent of hyaluronate synthase complex

Abstract

A high-Mr phosphoprotein (Mr 442,000) was purified from Nonidet-P-40-solubilized plasma membranes of cultured human skin fibroblasts. The protein comprised one 200,000-Mr subunit consisting of 116,000- and 84,000-Mr polypeptides and two identical 121,000-Mr subunits each consisting of 66,000- and 55,000-Mr polypeptides. The 200,000-Mr subunit and its polypeptides contained phosphotyrosine residues and were also [32P]phosphorylated at these residues from [gamma-32P]ATP in vitro by an intrinsic tyrosine kinase activity of the protein molecule in response to the presence of hyaluronate precursors, UDP-glucuronic acid and UDP-N-acetylglucosamine. The 121,000-Mr subunits and their polypeptides contained phosphoserine residues that could not be [32P]phosphorylated during autophosphorylation of the protein in vitro. The protein molecules separated from exponential- and stationary-growth-phase cells were identical in their quaternary structure, but appeared to exist in different proportions with respect to the state of phosphorylation of their 121,000-Mr subunits during different growth phases of the cell. Phosphorylation of polypeptides appeared to predispose in favour of their UDP-glucuronic acid- and UDP-N-acetylglucosamine-binding activities. The phosphorylated 116,000- and 84,000-Mr polypeptides of 200,000-Mr subunits possessed a single binding site for UDP-glucuronic acid and UDP-N-acetylglucosamine respectively. The phosphorylated 200,000-Mr subunit could also cleave the UDP moiety from UDP-glucuronic acid and UDP-N-acetylglucosamine precursors. The phosphorylated 121,000-Mr subunit possessed two binding sites with equal affinity towards UDP-glucuronic acid and UDP-N-acetylglucosamine but did not possess UDP-moiety-cleavage activity. The phosphorylation of 200,000-Mr subunit by an intrinsic kinase activity of the protein molecule appeared to elicit its oligosaccharide-synthesizing activity, whereas phosphorylation of 121,000-Mr subunits, presumably carried out in vivo, abolished this activity of the protein molecule. The oligosaccharides synthesized by the protein were about Mr 5000 and about 12 disaccharide units in length. Neither nucleotide sugars nor glycosyl residues nor newly synthesized oligosaccharides were bound covalently to the protein molecule. The UDP moiety of nucleotide sugar precursors did not constitute a link between protein molecule and oligosaccharide during its synthesis. Although isolated 442,000-Mr protein did not synthesize high-Mr hyaluronate in vitro, this protein molecule can be considered as a constituent of membrane-bound hyaluronate synthase complex because of its observed properties.