Cotranslational and posttranslocational N-glycosylation of proteins in the endoplasmic reticulum

Abstract

Asparagine linked glycosylation of proteins is an essential protein modification reaction in most eukaryotic organisms. N-linked oligosaccharides are important for protein folding and stability, biosynthetic quality control, intracellular traffic and the physiological function of many N-glycosylated proteins. In metazoan organisms, the oligosaccharyltransferase is composed of a catalytic subunit (STT3A or STT3B) and a set of accessory subunits. Duplication of the catalytic subunit gene allowed cells to evolve OST complexes that act sequentially to maximize the glycosylation efficiency of the large number of proteins that are glycosylated in metazoan organisms. We will summarize recent progress in understanding the mechanism of (a) cotranslational glycosylation by the translocation channel associated STT3A complex, (b) the role of the STT3B complex in mediating cotranslational or posttranslocational glycosylation of acceptor sites that have been skipped by the STT3A complex, and (c) the role of the oxidoreductase MagT1 in STT3B-dependent glycosylation of cysteine-proximal acceptor sites.

Keywords: Asparagine linked glycosylation; Congenital disorders of glycosylation; Endoplasmic reticulum; Oligosaccharyltransferase.

Fig. 1

Cotranslational glycosylation of proteins by the STT3A complex. (A) The STT3A complex is associated with the protein translocation channel and scans the growing polypeptide for glycosylation acceptor sites. The STT3B complex occupies a more distal position, but is able to modify skipped glycosylation sites by a cotranslational or posttranslocational mechanism. (B) Cotranslational scanning of the nascent polypeptide allows efficient glycosylation of acceptor sites in cysteine-rich proteins before disulfide bond formation stabilizes protein tertiary structure. (C) Examples of N-terminal, internal and extreme C-terminal acceptor sites (red asterisks) that have been skipped by the STT3A complex.

Fig. 2

Posttranslocational glycosylation of skipped acceptor sites by the STT3B complex. (A) The STT3B complex mediates posttranslocational glycosylation of acceptor sites that are skipped by the STT3A complex. The MagT1 subunit of the STT3B complex in necessary for glycosylation of all tested STT3B dependent sites including those adjacent to cysteine residues. (B) The oxidized form of MagT1 is proposed to form a mixed disulfide with an acceptor site proximal free cysteine residue thereby delaying disulfide bond formation and promoting acceptor site recognition by the STT3B active site. Disulfide chemistry releases the nascent glycoprotein to regenerate the oxidized form of MagT1.