As newly synthesized MHC class I α-chains enter the endoplasmic reticulum, calnexin binds on to them retaining them in a partly folded state. Ĭalnexin associates with the protein folding enzyme ERp57 to catalyze glycoprotein specific disulfide bond formation and also functions as a chaperone for the folding of MHC class I α-chain in the membrane of the ER. Yos-9 recognizes mannose residues exposed after α-mannosidase removal of an outer mannose of misfolded glycoproteins. The mannose lectin Yos-9 (OS-9 in humans) marks and sorts misfolded glycoproteins for degradation. The improperly-folded glycoprotein chain thus loiters in the ER and the expression of EDEM/Htm1p which eventually sentences the underperforming glycoprotein to degradation by removing one of the nine mannose residues. If the glycoprotein is not properly folded, an enzyme called UGGT (for UDP-glucose:glycoprotein glucosyltransferase) will add the glucose residue back onto the oligosaccharide thus regenerating the glycoprotein's ability to bind to calnexin. Glucosidase II can also remove the third and last glucose residue. These monoglucosylated oligosaccharides result from the trimming of two glucose residues by the sequential action of two glucosidases, I and II. Ĭalnexin binds only those N- glycoproteins that have GlcNAc2Man9Glc1 oligosaccharides. It specifically acts to retain unfolded or unassembled N-linked glycoproteins in the ER. Function Ĭalnexin is a chaperone, characterized by assisting protein folding and quality control, ensuring that only properly folded and assembled proteins proceed further along the secretory pathway. It consists of a large (50 kDa) N-terminal calcium- binding lumenal domain, a single transmembrane helix and a short (90 residues), acidic cytoplasmic tail.
0 Comments
Leave a Reply. |