Getting In and Out from Calnexin/Calreticulin Cycles

Abstract

The N-glycan-dependent quality control mechanism of glycoprotein folding was proposed initially by Helenius and coworkers several years ago; with a few minor modifications, it is still valid today ( Fig. 1) (1-3).2 Glycan processing starts immediately after its transfer from a dolichol-P-P derivative to Asn residues in nascent polypeptide chains entering the lumen of the ER. 3 Removal of the outermost and following glucoses by the successive action of GI and GII exposes the Glc 1 Man 9 GlcNAc 2 epitope (Fig. 2). This structure is then recognized by two ER resident lectins (CNX and CRT) that specifically bind monoglucosylated polymannose glycans. This is followed by removal of the innermost glucose by GII, thus liberating the glycoprotein from the lectin anchor. The proteinlinked glycan is then reglucosylated by the soluble ER enzyme GT only if the protein moiety displays non-native three-dimensional structures, as this enzyme behaves as a conformational sensor. Cycles of CNX/CRT-glycoprotein binding and liberation, catalyzed by the opposing activities of GT and GII, are terminated once glycoproteins attain their native structures. Glucose-free glycoproteins then continue their transit through the secretory pathway. Alternatively, permanently misfolded glycoproteins may be then transported to the cytosol for proteasomal degradation. Lectin-glycoprotein association not only thwarts Golgi exit of folding intermediates and irreparably misfolded glycoproteins but also enhances folding efficiency by preventing aggregation and promoting proper disulfide bonding. The latter is catalyzed by an oxidoreductase of the proteindisulfide isomerase family (ERp57) that acts exclusively on glycoproteins, as it is loosely associated with CNX/CRT.GT is the only component of the quality control mechanism that senses protein conformations, as it recognizes hydrophobic amino acid patches exposed in molten globule-like conformers (4, 5). GT may also glucosylate glycoproteins in not fully assembled oligomeric complexes because it also recognizes hydrophobic surfaces exposed as a consequence of the absence of subunit components (6). The aim of this review is to give an overview of recent reports dealing with the entrance and exit of glycoproteins from CNX/CRT cycles. Getting In: GII Is Not What It Was Thought to BeThe first step in the pathway leading to the entrance of glycoproteins into CNX/CRT cycles is the removal of the outermost glucose unit from the glycan by the membrane enzyme GI. This reaction occurs almost simultaneously with glycan transfer. The rapid GI-mediated deglucosylation of the protein-linked glycan, as well as the apparent inability of the enzyme to remove in vivo (but not in vitro) the glucose from the dolichol-P-P-linked glycan, strongly suggests the existence of a supercomplex formed by the oligosaccharyltransferase, GI, and the dolichol derivative, with a very precise orientation of the components.It was assumed that the sole role of GII was that of removing glucose residues l and n (Fig. 2). Recent work has sugg...

CALR mutations in myeloproliferative neoplasms: Hidden behind the reticulum

“…Evidence that ER retention may also be determined by the acidic region of CALR (residues 351-359) comes from studies of a calreticulin construct lacking this region where the protein is secreted despite the presence of KDEL. This region may also be involved in the translocation of CALR to the cytosol in a nondegrading process [6].…”

“…CALR has structural homology with calnexin and both function in concert in the so-called "calnexin/calreticulin cycle", a N-glycan-dependent quality control process that ensures correct glycoprotein folding and/or degradation and prevents protein aggregation [6]. Amongst other roles, CALR is implicated in the assembly and cell surface expression of major histocompatibility complex (MHC) class I molecules.…”

Protein Folding in the Endoplasmic Reticulum

“…UGT1 modifies glycans based on the structural integrity of the glycoprotein substrate (Caramelo and Parodi 2008). Studies using purified UGT1 and engineered substrates have showed that UGT1 recognizes near-native molten globule substrates through surface-exposed hydrophobic patches (Sousa and Parodi 1995;Caramelo et al 2003Caramelo et al , 2004.…”

Ero1α requires oxidizing and normoxic conditions to localize to the mitochondria-associated membrane (MAM)

“…At the expense of ATP, Bip/GRP78 promotes the folding of these hydrophobic amino acids to the interior of the protein structure (Gething 1999;Hammond and Helenius 1994;Kim et al 1992). The lectin chaperones calreticulin and calnexin also mediate the folding of newly synthesized proteins by cycling on and off their folding substrates, depending on the presence of a glucose linked to the high mannose sugar core (Caramelo and Parodi 2008;Michalak et al 2009). Another important group of ER protein-folding enzymes is oxidoreductases that catalyze the formation of disulfide bonds.…”