ER quality control: towards an understanding at the molecular level

doi:10.1016/S0955-0674(00)00233-7

Current Opinion in Cell Biology

Volume 13, Issue 4, 1 August 2001, Pages 431-437

https://doi.org/10.1016/S0955-0674(00)00233-7 Get rights and content

Abstract

The process of ‘quality control’ in the endoplasmic reticulum (ER) involves a variety of mechanisms that collectively ensure that only correctly folded, assembled and modified proteins are transported along the secretory pathway. In contrast, non-native proteins are retained and eventually targeted for degradation. Recent work provides the first structural insights into the process of glycoprotein folding in the ER involving the lectin chaperones calnexin and calreticulin. Underlying principles governing the choice of chaperone system engaged by different proteins have also been discovered.

Introduction

The endoplasmic reticulum (ER) plays an essential role in the folding and maturation of newly synthesized proteins in the secretory pathway. It provides an environment optimized for folding, oxidation and oligomeric assembly of proteins translocated into the lumen or inserted into the membrane. Folding in the ER is assisted by a large variety of folding enzymes, molecular chaperones and folding sensors [1]. Many of these associate with growing nascent chains and continue to assist folding until a protein has acquired its native structure. To ensure the fidelity of the maturation process, exit from the ER is regulated by a stringent quality control system that inhibits the secretion of incompletely folded or misfolded proteins [2]. In addition to securing extended exposure of proteins to the folding machinery, quality control prevents deployment of potentially malfunctioning proteins that could be detrimental to the cell and the organism. For many proteins in the ER, proper folding and maturation depends on co- and post-translational modifications. Here we primarily focus on the ER quality control system that is in place for proteins containing N-linked glycans.

Section snippets

Quality control mechanisms

ER quality control operates at several levels and by multiple mechanisms [3]. At a general level, all proteins are subject to conformation-based screening by members of major molecular chaperone families. These chaperones have the capacity to recognize properties common to non-native proteins such as exposed hydrophobic areas. They selectively associate with proteins that display such features and in doing so promote folding and assembly. As long as they are engaged in interactions with the

The calnexin/calreticulin cycle

One of the most common modifications of proteins translocated into the ER is the addition of N-linked glycans. For glycoproteins, a particularly well studied ER quality control system is in place, involving two homologous lectins, calnexin (CNX) and calreticulin (CRT) (Fig. 1). The process of N-linked glycosylation occurs through the transfer of a triglucosylated, branched core oligosaccharide (Glc₃Man₉GlcNAc₂) to the nascent polypeptide chain as it enters the ER lumen. Soon after transfer,

Chaperone selection in the endoplasmic reticulum

In addition to CNX and CRT, the ER contains a large collection of other molecular chaperones and folding factors with different properties and functions [27]. Each newly synthesized protein makes use of only a few of the available chaperones. What are the parameters that determine which chaperones a protein will engage and in which order?

Recent work shows that for glycoproteins the choice of chaperone depends, in part, on the position of the glycans in the sequence [28•]. Growing nascent chains

ERp57-catalyzed disulfide bond formation

The oxidizing environment and the presence of several different thiol oxidoreductases allow formation of disulfide bonds in the ER. One of the oxidoreductases, ERp57, functions as a ‘co-chaperone’ with CRT and CNX 25., 38•.. It most probably forms one-to-one complexes with both CRT and CNX [38•] and has been shown to accelerate oxidative refolding of monoglucosylated RNaseB in the presence of CRT or CNX in vitro [39••]. The formation of transient intermolecular disulfide bonds between ERp57 and

Substrate recognition by UDP-glucose: glycoprotein glucosyltransferase

UGGT, the folding sensor in the CNX/CRT cycle, is a large, soluble, lumenal enzyme [26•]. Its catalytic domain displays a conserved 300 amino acid sequence at the carboxyl terminus of the protein with homology to glycosyltransferases of family 8 [41]. UDP-glucose, transported into the ER lumen from the cytosol [42], is the glucose donor, whereas the acceptors are glucose-free high mannose oligosaccharides attached to incompletely folded glycoproteins. UGGT is present throughout the ER including

Three dimensional structures of calreticulin and calnexin

For several years, 3D structure determination of CRT and CNX has been pursued by several groups. Now, as a first step towards a more detailed understanding of their function at the molecular level, the NMR structure of the CRT P-domain has been solved 36••., 50.. In addition, the crystal structure of a CNX ectodomain fragment, for which crystallization conditions have previously been reported [51], has recently been solved but not yet published [23].

The NMR structure of the CRT P-domain

Structural insights into glycoprotein folding

With the oligosaccharide binding function of CRT and CNX mapped to a distinct lectin domain, the functions of the P-domain become all the more intriguing. A priori, the P-domain could be a site for direct interaction with unfolded proteins. Two recent papers describe in vitro experiments that suggest such a function for CRT and CNX 59•., 60.. Both proteins were found to bind to unfolded, non-glycosylated proteins but not to native conformers. In addition, they suppressed thermal denaturation

Conclusions

The quality control mechanisms in the ER ensure the structural integrity of proteins delivered to the organelles of the secretory and endocytic pathways and the extracellular space. Recent progress has provided a better understanding of oxidative folding of glycoproteins through the cooperation of ERp57 and lectin chaperones CRT or CNX, of the basis for recognition of unfolded substrate glycoprotein by the UGGT, as well as of the structure of CRT and CNX. Some of the ‘rules’ underlying

Acknowledgements

We are thankful to Rolf Moser for help with the figures and to Anna Mezzacasa and Christiane Ritter for critical reading of the manuscript. The work was supported by the Swiss National Science Foundation.

References and recommended reading

Papers of particular interest, published within the annual period of review,have been highlighted as:

•of special interest
••of outstanding interest

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ReviewER quality control: towards an understanding at the molecular level

Abstract

Introduction

Section snippets

Quality control mechanisms

The calnexin/calreticulin cycle

Chaperone selection in the endoplasmic reticulum

ERp57-catalyzed disulfide bond formation

Substrate recognition by UDP-glucose: glycoprotein glucosyltransferase

Three dimensional structures of calreticulin and calnexin

Structural insights into glycoprotein folding

Conclusions

Acknowledgements

References and recommended reading

Trends Cell Biol

Cell

Neuron

Cell

Curr Opin Struct Biol

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

FEBS Lett

J Struct Biol

Curr Opin Struct Biol

J Mol Biol

J Biol Chem

FEBS Lett

Cell

Mol Cell

J Biol Chem

Mol Cell

Protein folding in the cell

Nature

Protein oligomerization in the endoplasmic reticulum

Annu Rev Cell Biol

Setting the standards: quality control in the secretory pathway

Science

Integration of endoplasmic reticulum signaling in health and disease

Nat Med

Hsp47: a collagen specific molecular chaperone

Trends Biochem Sci

39kDa receptor-associated protein is an ER resident proteins and molecular chaperone for LDL receptor related proteins

EMBO J

A critical role for tapasin in the assembly and function of multimeric MHC class I-TAP complexes

Science

A di-acidic signal required for selective export from the endoplasmic reticulum

Science

Role of ER export signals in controlling surface potassium channel numbers

Science

Analysis of endoplasmic reticulum trafficking signals by combinatorial screening in mammalian cells

Proc Natl Acad Sci USA

Review
ER quality control: towards an understanding at the molecular level