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Metazoan organisms assemble two isoforms of the oligosaccharyltransferase (OST) that have different catalytic subunits (STT3A or STT3B) and partially nonoverlapping roles in asparagine-linked glycosylation. The STT3A isoform of the OST is primarily responsible for co-translational glycosylation of the nascent polypeptide as it enters the lumen of the endoplasmic reticulum. The C-terminal 65-75 residues of a glycoprotein will not contact the translocation channel-associated STT3A isoform of the OST complex before chain termination. Biosynthetic pulse labeling of five human glycoproteins showed that extreme C-terminal glycosylation sites were modified by an STT3B-dependent posttranslocational mechanism. The boundary for STT3B-dependent glycosylation of C-terminal sites was determined to fall between 50 and 55 residues from the C terminus of a protein. C-terminal NXT sites were glycosylated more rapidly and efficiently than C-terminal NXS sites. Bioinformatics analysis of glycopeptide databases from metazoan organisms revealed a lower density of C-terminal acceptor sites in glycoproteins because of reduced positive selection of NXT sites and negative selection of NXS sites.

Citation

Shiteshu Shrimal, Steven F Trueman, Reid Gilmore. Extreme C-terminal sites are posttranslocationally glycosylated by the STT3B isoform of the OST. The Journal of cell biology. 2013 Apr 1;201(1):81-95

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PMID: 23530066

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