Oligosaccharyltransferase Isoforms that Contain Different Catalytic STT3 Subunits Have Distinct Enzymatic Properties

Abstract

Oligosaccharyltransferase (OST) is an integral membrane protein that catalyzes N-linked glycosylation of nascent proteins in the lumen of the endoplasmic reticulum. Although the yeast OST is an octamer assembled from nonhomologous subunits (Ost1p, Ost2p, Ost3p/Ost6p, Ost4p, Ost5p, Wbp1p, Swp1p, and Stt3p), the composition of the vertebrate OST was less well defined. The roles of specific OST subunits remained enigmatic. Here we show that genomes of most multicellular eukaryotes encode two homologs of Stt3p and mammals express two homologs of Ost3p. The Stt3p and Ost3p homologs are assembled together with the previously described mammalian OST subunits (ribophorins I and II, OST48, and DAD1) into complexes that differ significantly in enzymatic activity. Tissue and cell type-specific differences in expression of the Stt3p homologs suggest that the enzymatic properties of oligosaccharyltransferase are regulated in eukaryotes to respond to alterations in glycoprotein flux through the secretory pathway and may contribute to tissue-specific glycan heterogeneity.

The availability of glucose to CHO cells affects the intracellular lipid-linked oligosaccharide distribution, site occupancy and the N-glycosylation profile of a monoclonal antibody

“…The last four mannose and three glucose residues are extremely sensitive to the extracellular glucose level, so that glucose deprivation can limit the production of their sugar donors, Man-PDol and Glc-P-Dol, resulting in truncated LLO precursor (Chambers et al, 1977;Forsee et al, 1977;Jensen and Schutzbach, 1986;Kang et al, 1978a;Spencer and Elbein, 1980). One other very important note is that glucose-starved cells produce truncated LLO intermediates, which are less favorable to OST compared to fully glucosylated donors (Gao et al, 2005;Kelleher et al, 2003;Turco et al, 1977). An accumulation of truncated LLO intermediates can result in the addition of premature N-glycans onto the nascent polypeptides at a low rate or leaving Asn residue completely empty (Jackson et al, 1989;Quellhorst et al, 1999;Sharma et al, 1981).…”

Methylation status of TUSC3 is a prognostic factor in ovarian cancer

“…18,19 Analyses of the Ost3p and its yeast paralogue Ost6p (human MagT1/IAP) demonstrated their function in regulating glycosylation efficiency 36 and recently also uncovered their oxidoreductase activity as well as a possible role in magnesium transport. 37 Aberrant glycosylation of proteins can be observed in essentially all in vitro cancer models and human cancers, and many glycosylated epitopes constitute tumor-associated antigens, 38,39 sustaining a long-standing debate regarding whether and how protein glycosylation is involved in tumorigenesis.…”

“…14 TUSC3 deletions or mutations reportedly are frequently associated with mental retardation. 15,16 TUSC3 shares high sequence homology with Ost3p, a subunit of the oligosaccharyltransferase (OST) complex involved in N-glycosylation of proteins in Saccharomyces cerevisiae, [17][18][19] suggesting an analogous function in mammalian cells. Alterations of protein N-glycosylation have been associated with some carcinogenic traits, such as invasiveness and metastatic potential.…”

Homozygous Truncating Intragenic Duplication in TUSC3 Responsible for Rare Autosomal Recessive Nonsyndromic Intellectual Disability with No Clinical or Biochemical Metabolic Markers

“…Reported patients with TUSC3 mutations have normal results of glycosylation analyses, which could be explained by the compensation, at least partially in a tissue-specific manner, of TUSC3 by IAP, the TUSC3 paralog, coding for an OTase subunit and known to be involved in the N-glycosylation process (Kelleher et al 2003;Garshasbi et al 2008;Molinari et al 2008). Another hypothesis would be that TUSC3 and IAP protein function is essential for the glycosylation of proteins belonging to cellular subgroups in a specific tissue, mainly the central nervous system, which cannot be explored by usual serum glycosylation screening techniques.…”