More Than One Glycan Is Needed for ER Glucosidase II to Allow Entry of Glycoproteins into the Calnexin/Calreticulin Cycle (original) (raw)

from the Cnx/Crt cycle, a chaperone system that supports glycoprotein maturation and quality control in the Institute of Biochemistry Swiss Federal Institute of Technology (ETH) Zurich ER. Cnx and Crt are lectins that bind to monoglucosylated intermediates of the core glycan. Zurich 8093 Switzerland The key trimming enzymes in the ER are GI, GII, and ER mannosidase I. GI removes the outermost of the three glucoses, and GII is responsible for removing the Summary middle glucose (cleavage 1). The monoglucosylated glycans thus generated allow newly synthesized glyco-Nascent and newly synthesized glycoproteins enter proteins to bind to Cnx and Crt. The release of the glythe calnexin (Cnx)/calreticulin (Crt) cycle when two coprotein from these chaperones involves a second GII out of three glucoses in the core N-linked glycans cleavage (cleavage 2), which removes the remaining have been trimmed sequentially by endoplasmic reglucose. If a glycoprotein is properly folded after reticulum (ER) glucosidases I (GI) and II (GII). By analyzlease from the cycle, it is free to leave the ER. If not, ing arrested glycopeptides in microsomes, we found it is reglucosylated by UDP-Glc: glycoprotein glucosyl that GI removed the outermost glucose immediately transferase, which acts as a folding sensor driving the after glycan addition. However, although GII associimproperly folded protein back into association with ated with singly glycosylated nascent chains, trim-Cnx and Crt (Sousa et al., 1992). In this way, glycoproming of the second glucose only occurred efficiently teins can go through multiple rounds of interaction with when a second glycan was present in the chain. Conthe chaperones. If they still fail to fold properly, the sistent with a requirement for multiple glycans to actislow-acting ER mannosidase I will cleave the terminal vate GII, pancreatic RNase in live cells needed more mannose in the B branch, thereby targeting the glycothan one glycan to enter the Cnx/Crt cycle. Thus, protein for degradation. whereas GI trimming occurs as an automatic exten-To characterize the glucose-trimming events and sion of glycosylation, trimming by GII is a regulated their role in determining substrate flux into the Cnx/Crt process. By adjusting the number and location of glycycle, we addressed the activities of GI and GII in cans, glycoproteins can instruct the cell to engage microsomes and live cells. GI is a type II membrane them in an individually determined folding and quality glycoprotein with a lumenal catalytic domain (Shailubcontrol pathway. hai et al., 1991), and GII is a soluble heterodimer. In addition to the catalytic α chain (GIIα), GII contains a