N-glycan trimming in the ER and Calnexin/Calreticulin cycle

N-glycan trimming in the ER and Calnexin/Calreticulin cycle After being synthesized in the ER membrane the 14-sugars lipid-linked oligosaccharide is co-translationally transferred to an unfolded protein, as described in the previous steps. After this point the N-glycan is progressively trimmed of the three glucoses and some of the mannoses before the protein is transported to the cis-Golgi. The role of these trimming reactions is that the N-glycan attached to an unfolded glycoprotein in the ER assume the role of 'tags' that direct the interactions of the glycoprotein with different elements that mediate its folding. The removal of the two outer glucoses leads to an N-glycan with only one glucose, which is a signal for the binding of either one of two chaperone proteins, calnexin (CNX) and calreticulin (CRT). These chaperones provide an environment where the protein can fold more easily. The interaction with these proteins is not transient and is terminated by the trimming of the last remaining glucose, after which the glycoprotein is released from CNX or CRT and directed to the ER Quality Control compartment (ERQC) if it still has folding defects, or transported to the Golgi if the folding is correct. The involvement of N-glycans in the folding quality control of proteins in the ER explains why this form of glycosylation is so important, and why defects in the enzymes involved in these reactions are frequently associated with congenital diseases. However, there are many unknown points in this process, as it is known that even proteins without N-glycosylation sites can be folded properly (Caramelo JJ and Parodi AJ, 2008).