CORE Metadata, citation and similar papers at core.ac.uk Provided by RERO DOC Digital Library © Copyright 2003 by Humana Press Inc. All rights of any nature whatsoever reserved. 1085-9195/03/39/223–247/$25.00 REVIEW ARTICLE Calnexin, Calreticulin, and ERp57 Teammates in Glycoprotein Folding Lars Ellgaard* and Eva-Maria Frickel Institute of Biochemistry, ETH Zurich, CH-8093 Zurich, Switzerland Abstract In eukaryotic cells, the endoplasmic reticulum (ER) plays an essential role in the synthesis and maturation of a variety of important secretory and membrane proteins. For glycoproteins, the ER possesses a dedicated maturation system, which assists folding and ensures the quality of final products before ER release. Essential components of this system include the lectin chaperones cal- nexin (CNX) and calreticulin (CRT) and their associated co-chaperone ERp57, a glycoprotein spe- cific thiol-disulfide oxidoreductase. The significance of this system is underscored by the fact that CNX and CRT interact with practically all glycoproteins investigated to date, and by the debilitat- ing phenotypes revealed in knockout mice deficient in either gene. Compared to other important chaperone systems, such as the Hsp70s, Hsp90s and GroEL/GroES, the principles whereby this system works at the molecular level are relatively poorly understood. However, recent structural and biochemical data have provided important new insights into this chaperone system and pre- sent a solid basis for further mechanistic studies. Index Entries: Calnexin; calreticulin; endoplasmic reticulum; ERp57; lectin; molecular chaper- one; oxidoreductase. INTRODUCTION precursor in a reaction catalyzed by the oligosaccharyl transferase complex (1,2). The The addition of N-linked glycans to newly modified asparagine side chain occurs within synthesized polypeptide chains occurs in the the Asn-Xxx-Ser/Thr consensus sequence, endoplasmic reticulum (ER). Here the 14-sac- where Xxx denotes any amino acid except pro- charide core glycan (Fig. 1), Glc3Man9GlcNAc2, line (3). Although only relatively few of the is added to the growing nascent polypeptide soluble ER-resident proteins seem to be glyco- chain from a dolicholpyrophosphate-linked sylated, the majority of extracellular proteins produced in mammalian cells carry N-linked glycans (4). The large number of different gly- coproteins that traffic through the ER are *Author to whom all correspondence and reprint requests should be addressed. E-mail: lars.ellgaard@ involved in many fundamental intra- and inter- bc.biol.ethz.ch cellular processes. Consequently, mutations in Cell Biochemistry and Biophysics 223 Volume 39, 2003 224 Ellgaard and Frickel homologous lectin chaperones, calnexin (CNX) and calreticulin (CRT). Studies in tissue culture cells and ER- derived microsomes have demonstrated that the interaction with CNX and CRT protects glycoproteins from aggregation and prema- ture degradation (16–20). Repeated cycles of binding to and release from CNX and CRT, controlled by enzymes that modify the core oligosaccharide, ensure the retention of sub- strate glycoproteins in the favorable folding environment of the ER until correctly folded. If permanently misfolded, these substrate proteins are degraded. At least for certain gly- coproteins, the removal of a mannose residue in the middle branch of the glycan is a signal for degradation (21). A recently discovered mannose-binding lectin, EDEM, specifically accelerates the degradation of glycoproteins Fig. 1. Schematic representation of the N- and could well be directly involved in target- linked core oligosaccharide attached to an ing Man8GlcNAc2-containing proteins for asparagine side chain in an Asn-Xxx-Ser/Thr degradation (22–28). Overall, the oligosaccha- amino acid consensus sequence. The linkage ride and the different enzymes that modify it for each individual glycosyl residue is indi- play important roles in glycoprotein folding cated along with cleavage sites for various ER and degradation. These aspects have been enzymes that modify the sugar structure. covered in great detail in other reviews (29–33). Cell biological studies have provided most of our current knowledge about the proteins of the N-glycosylation pathway are CNX/CRT chaperone system. However, more known to cause severe disease phenotypes, and recent work has revealed structural informa- a number of congenital disorders of glycosyla- tion at atomic resolution for both CNX and tion are known (5,6). CRT, along with details about their interaction The sugar itself can directly modulate pro- with the co-chaperone ERp57. Moreover, tein structure and function, e.g., by increasing other studies have explored the possibility properties such as stability and solubility that protein–protein interactions, in addition (7–11). In addition, N-linked glycans act as to the glycan-mediated interaction, contribute tags in intracellular trafficking (12), and to the function of CNX and CRT as molecular accordingly different lectins act as sorting chaperones. Taken together, these investiga- receptors in the secretory pathway (13–15). tions have considerably increased our under- Despite the many different and widely recog- standing of the molecular basis for nized functions of N-linked glycans, the role of glycoprotein folding and have also raised the N-linked glycan in protein folding in the interesting new questions. This review ER has only become fully appreciated within describes the function of the CNX/CRT chap- the past 10 yr. Specifically, it has been found erone system. In particular, we emphasize the that modifications of the core glycan to gener- role of molecular properties of CNX, CRT, and ate a monoglucosylated (Glc1Man9GlcNAc2) ERp57 in determining the mechanism of chap- form determine the interaction with two erone function in the living cell. Cell Biochemistry and Biophysics Volume 39, 2003 Calnexin, Calreticulin, and ERp57 225 ER QUALITY CONTROL the nucleus leads to increased transcription of genes encoding ER chaperones to help alleviate The endoplasmic reticulum (ER) plays a fun- the folding problem. In addition, another sig- damental role in the synthesis, folding, and naling pathway involving phosphorylation of assembly of numerous important proteins, such the translation initiation factor eIF2α leads to as cell-surface receptors, membrane channels, attenuation of protein translation to further extracellular matrix components, serum pro- reduce the load on the ER folding machinery. teins, and antibodies. The environment in the ER Lastly, yet other genes involved in degradation is optimal for the correct folding and maturation are upregulated, as recently shown in the case of such proteins. For many proteins the matura- of EDEM (28). Several investigations show that tion process involves co- and posttranslational the processes of ERAD and UPR are closely modifications, such as signal-peptide cleavage, coordinated (44–46). glycophosphatidylinositol (GPI)-anchor addi- tion and N-linked glycosylation. Furthermore, the oxidizing milieu of the ER supports the for- THE CALNEXIN/CALRETICULIN CYCLE mation of disulfide bonds. This stabilization of protein conformation is likely to help proteins The QC system described above applies to all maintain their structure in the extracellular envi- proteins that encounter the lumen of the ER or ronment. Finally, the ER is rich in chaperones are inserted into the ER membrane. CNX, CRT, and enzymes, which are crucial in assisting the and ERp57 are important factors of this general process of correct protein folding (34,35). ER QC system (47). Still, incorrectly folded or incompletely CNX and CRT cooperate with a number of assembled proteins are common side products enzymes in the process of assisting glycopro- during protein synthesis in the ER. Such prod- tein folding. The first step of this so-called ucts, which could be harmful to the cell if CNX/CRT cycle (see Fig. 2) involves binding to allowed to proceed along the secretory pathway either chaperone through the monoglucosy- to the cell surface or another cellular location, are lated glycan, Glc1Man9GlcNAc2, present on subject to a stringent quality control (QC) system nascent chains and on newly synthesized gly- (36,37). A number of general chaperones, includ- coproteins. This form of the sugar appears ing BiP, a member of the Hsp70 family of chap- either as a trimmed intermediate of the triglu- erones, and protein disulfide isomerase (PDI), a cosylated core oligosaccharide or by readdition thiol-disulfide oxidoreductase, recognize and of a glucose residue to the fully deglucosylated retain proteins that expose non-native features. glycan (see below). Sequential trimming of the This system ensures that misfolded and incor- two outermost glucose residues on the core rectly assembled proteins are retained in the ER oligosaccharide is executed by glucosidases I and eventually degraded. Many so-called ER and II. The importance of monoglucosylation storage diseases are known, which arise from the for CNX and CRT binding has been shown in ER retention of mutant alleles of certain proteins. living cells, in microsomes and in vitro for a Such diseases include cystic fibrosis and emphy- number of different proteins (see for instance sema (for reviews, see refs. 38–41). refs. 48–56). Many such studies have used Defective ER-retained proteins are typically inhibitors of glucosidase II, such as cas- degraded by the proteasome after selective tanospermine and deoxynojirimycin, to pre- retrotranslocation to the cytosol and ubiquitina- vent the