UDP-Glucose: Glycoprotein Glucosyltransferase 1,2 (UGGT1,2)

Abstract

Almost one-third of proteins synthesized by eukaryotic cells belong to the secretory pathway, entering the endoplasmic reticulum (ER) either co- or posttranslationally. In the ER, proteins acquire their native tertiary fold, disulfide bonds are formed, and in some cases, they assemble into oligomeric structures. Numerous folding-assisting enzymes and chaperones are in place to ensure the efficiency of these processes. Additionally, almost 70 % of the secretory pathway proteins are N-glycosylated by the translocon-associated oligosaccharyltransferase complex in the consensus sequence Asn-X-Ser/Thr, in which X can be any amino acid except for Pro (Apweiler et al. 1999). The consensus sequences are N-glycosylated as they emerge into the ER lumen when there are about 12–13 amino acids between the Asn residue and the inner ER membrane surface. In some cases, the same modification may occur posttranslationally (Ruiz-Canada et al. 2009). N-glycosylation is one of the most abundant and relevant protein modifications as N-glycans are central players in molecular recognition events, a function particularly suitable for them given their diverse composition. Additionally, N-glycans may modulate the biophysical behavior of their protein moieties. N-glycans may inhibit protein aggregation, may increase resistance to proteolytic degradation, and may promote acquisition of elements of secondary structure such as turns (Chen et al. 2010). Of particular relevance is the involvement of N-glycans in glycoprotein folding in the ER (Caramelo and Parodi 2007; D’Alessio et al. 2010). In this case, N-glycans act as an epigenetic information platform indicating the folding status of glycoproteins. This information is generated by glycosyltransferases and glycosidases that translate the conformational status of glycoproteins into particular N-glycan structures. Upon their recognition, ER-resident lectins retain the immature species in the ER, thus promoting their proper folding and hindering their Golgi exit. N-glycosylation starts in most eukaryotic cells with the transfer of the entire glycan Glc3Man9GlcNAc2 from a dolichol-P-P derivative.