Cherepanova et al. describe how an oxidoreductase enzyme promotes the glycosylation of newly synthesized proteins in the ER.
Two different oligosaccharyltransferase (OST) complexes glycosylate asparagine-containing acceptor sites in secretory proteins. Complexes containing the catalytic subunit STT3A target nascent polypeptides as they feed into the ER through the protein translocation channel. STT3B-containing complexes subsequently glycosylate acceptor sites ignored by STT3A, but, by this point, the target proteins are beginning to fold into their native conformation and forming disulfide bridges that could limit the complex’s access to the glycosylation acceptor sequence.
Cherepanova et al. were interested in a protein called MagT1, which is mutated in patients with X-linked mental retardation and has been proposed to act as a magnesium transporter at the plasma membrane. The protein is, however, homologous to a budding yeast OST subunit, and the researchers found that it localizes to the ER in human cells. MagT1 associated with STT3B-containing OST complexes, and knocking down the protein inhibited the glycosylation of STT3B-dependent, but not STT3A-dependent, target sites. Many of these sites contained, or were located next to, cysteine residues. MagT1 was no longer required for the glycosylation of these sites when the researchers inhibited disulfide bond formation.
MagT1 contains a domain similar to the oxidoreductase enzyme thioredoxin. Mutating the catalytic cysteine residues in this domain impeded MagT1’s ability to support STT3B-dependent glycosylation, suggesting that the protein forms temporary disulfide bonds with substrate proteins, thereby opening up some of their acceptor sites to STT3B. Author Reid Gilmore now wants to investigate how MagT1 promotes the glycosylation of STT3B-dependent sites that aren’t located near cysteine residues.
Text by Ben Short