Glycosyltransferase 4 (GT4) includes both eukaryotic and prokaryotic UDP-D-N-acetylhexosamine:polyprenol phosphate D-N-acetylhexosamine-1-phosphate transferases. They catalyze the transfer of a D-N-acetylhexosamine 1-phosphate to a membrane-bound polyprenol phosphate, which is the initiation step of protein N-glycosylation in eukaryotes and peptidoglycan biosynthesis in bacteria. One member, D-N-acetylhexosamine 1-phosphate transferase (GPT) is a eukaryotic enzyme, which is specific for UDP-GlcNAc as donor substrate and dolichol-phosphate as the membrane bound acceptor. The bacterial members MraY, WecA, and WbpL/WbcO utilize undecaprenol phosphate as the acceptor substrate, but use different UDP-sugar donor substrates. MraY-type transferases are highly specific for UDP-N-acetylmuramate-pentapeptide, whereas WecA proteins are selective for UDP-N-acetylglucosamine (UDP-GlcNAc). The WbcO/WbpL substrate specificity has not yet been determined, but the structure of their biosynthetic endproducts implies that UDP-N-acetyl-D-fucosamine (UDP-FucNAc) and/or UDPN-acetyl-D-quinosamine (UDP-QuiNAc) are used. The eukaryotic reaction is the first step in the assembly of dolichol-linked oligosaccharide intermediates and is essential for N-glycosylation. The prokaryotic reactions lead to the formation of polyprenol-linked oligosaccharides involved in bacterial cell wall and peptidoglycan assembly. Archaeal and eukaryotic enzymes may use the same substrates and are evolutionarily closer than the bacterial enzyme. Archaea possess the same N-glycosylation pathway as eukaryotes. A glycosyl transferase gene Mv1751 in M. voltae encodes for the enzyme that carries out the first step in the pathway, the attachment of GlcNAc to a dolichol lipid carrier in the membrane. A lethal mutation in the alg7 (GPT) gene in Saccharomyces cerevisiae was successfully complemented with Mv1751, the archaea gene.
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