N-Acetylglucosaminyltransferase-V (GnT-V, GnT-V or Mgat5) catalyzes the transfer of GlcNAc from UDP-GlcNAc to the OH-6 position of the α-linked Man residue in GlcNAcβ1-2Manα1-6Manβ1-4GlcNAc. This acceptor sequence is found in N-glycan intermediates at the medial Golgi stage of glycoprotein production (Fig. 1). Maturation of the N-glycans with passage of glycoproteins through the trans-Golgi produces the tri (2,2,6)- and tetra (2,4,2,6)-antennary complex-type N-glycans. A variety of oligosaccharide sequences are added to complete these glycans, comprising various combinations of N-acetyllactosamine and poly-N-acetyllactosamine capped with sialic acid and fucose. GnT-V is a rate-limiting enzyme for the addition of poly-N-acetyllactosamine to N-glycans. In vitro assays indicate that tri (2,2,6)- and tetra (2,4,2,6)-antennary glycans are preferentially elongated by β3GnT(i) and β4GalT to produce poly-N-acetyllactosamine (van den Eijnden et al. 1988). Furthermore, the mutant lymphoma cell lines BW5147-PHAR2.1 (Cummings and Kornfeld 1984), and KBL-1 (Yousefi et al. 1991) are GnT-V-deficient and severely depleted of poly-N- acetyllactosamine on N-glycans, but not the O-glycans. These somatic cell mutants were selected for resistance to the toxic effects of leukoagglutinin (L-PHA) in culture. L-PHA binds preferentially to mature GnT-V products, notably the Galβ1-4GlcNAcβ1- 6 (Galβ1-4GlcNAcβ1-2)Manα1-6 portion of tri- and tetraantennary N-glycans (Cummings and Kornfeld 1982) (Fig. 1). GnT-V-modifled N-glycans detected by L-PHA lectin histochemistry are often increased in human breast and colorectal carcinomas, and correlate with poor prognosis and reduced patient survival time (Fernandes et al. 1991; Seelentag et al. 1998).
KeywordsSialic Acid Rous Sarcoma Virus Baby Hamster Kidney Cell Somatic Cell Mutant Mouse Lymphoma Cell Line
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