unfolded protein:(Glc)3 (GlcNAc)2 (Man)9 (Asn)1 unfolded protein Transfer of N-glycan to the protein Authored: Dall'Olio, GM, 2009-11-10 EC Number: 2.4.1.119 Edited: Jassal, B, 2009-11-10 Pubmed11470266 Pubmed14514716 Pubmed16317064 Pubmed19835842 Reactome Database ID Release 43446209 Reactome, http://www.reactome.org ReactomeREACT_22208 Reviewed: Gagneux, P, 2010-04-16 The 14-sugar N-glycan precursor, synthesized in the previous reactions, is attached in a single step to a nascent protein, releasing the dolichyl phosphate anchor and the as yet unfolded glycoprotein. The reaction occurs cotranslationally as the growing peptide chain leaves a ribosome associated with the ER membrane and enters the ER lumen. This reaction is catalyzed by the OST complex, composed of at least seven proteins; DAD1, DDOST (OST48 in yeast), RPN1 (ribophorin 1), RPN2 (ribophorin 2), OST4, TUSC3 (N33), and either STT3A or STT3B, which contain the catalytic domain (Kelleher DJ and Gilmore R, 2006). A mutation in RPN2 is associated with CDG-Ix (Vleugels W et al, 2009).<br>The signal for glycosylation is the consensus sequence Asn - X - Thr/Ser, where the first amino acid is always Asn, the second can be any amino acid except for Pro, and the third position may be Thr, Ser or Cys, with a preference for the first (Breuer W et al, 2001). Not all Asn - X - Thr/Ser sites are modified in vivo (Petrescu AJ et al, 2004).<br>