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Tannous A, Pisoni GB, Hebert DN, Molinari M. N-Linked Sugar

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Tannous A, Pisoni GB, Hebert DN, Molinari M. N-Linked Sugar Seminars in Cell & Developmental Biology 41 (2015) 79–89 Contents lists available at ScienceDirect Seminars in Cell & Developmental Biology j ournal homepage: www.elsevier.com/locate/semcdb Review N-linked sugar-regulated protein folding and quality control in the ER a,1 b,1 a,∗ b,c,d,∗∗ Abla Tannous , Giorgia Brambilla Pisoni , Daniel N. Hebert , Maurizio Molinari a Department of Biochemistry and Molecular Biology, Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, MA 01003, USA b Università della Svizzera italiana, CH-6900 Lugano, Switzerland c Institute for Research in Biomedicine, Protein Folding and Quality Control, CH-6500 Bellinzona, Switzerland d Ecole Polytechnique Fédérale de Lausanne, School of Life Sciences, CH-1015 Lausanne, Switzerland a r t a b i s c l e i n f o t r a c t Article history: Asparagine-linked glycans (N-glycans) are displayed on the majority of proteins synthesized in the endo- Available online 19 December 2014 plasmic reticulum (ER). Removal of the outermost glucose residue recruits the lectin chaperone malectin possibly involved in a first triage of defective polypeptides. Removal of a second glucose promotes engage- Keywords: ment of folding and quality control machineries built around the ER lectin chaperones calnexin (CNX) Endoplasmic reticulum and calreticulin (CRT) and including oxidoreductases and peptidyl–prolyl isomerases. Deprivation of the N-glycosylation last glucose residue dictates the release of N-glycosylated polypeptides from the lectin chaperones. Cor- Protein folding and quality control rectly folded proteins are authorized to leave the ER. Non-native polypeptides are recognized by the ER Calnexin Calreticulin quality control key player UDP-glucose glycoprotein glucosyltransferase 1 (UGT1), re-glucosylated and re-addressed to the CNX/CRT chaperone binding cycle to provide additional opportunity for the protein to UDP-glucose glycoprotein glucosyltransferase 1 fold in the ER. Failure to attain the native structure determines the selection of the misfolded polypeptides for proteasome-mediated degradation. © 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 80 2. Protein folding in the ER . 80 2.1. N-glycosylation plays a key role in protein folding . 80 2.2. N-glycan processing dictates the binding to the ER-resident lectin chaperones . 80 2.2.1. ␣-Glucosidase I generates di-glucosylated glycans. 80 2.2.2. ␣-Glucosidase II induces the entry of glycoproteins into the CNX/CRT cycle . 80 2.2.3. The roles of CNX and CRT . 82 2.3. CNX and CRT engage enzymes required for proper protein folding . 82 2.3.1. The PDI family: formation of native disulfide bonds . 82 2.3.2. The PPI family: peptidyl–prolyl bonds isomerization . 84 2.4. Regulation of the chaperone function and sub-cellular localization of CNX and CRT . 84 3. Role of UGT1 as a folding sensor . 85 3.1. Proposed mechanisms of how substrates exit the CNX and CRT cycle. 85 4. Export from the ER: proposed models and roles of ERGIC-53, VIP36 and other lectins in glycoprotein sorting . 85 5. Concluding remarks . 86 Acknowledgments. 86 References . 86 ∗ Corresponding author. Tel.: +1 4135450079; fax: +1 4135453291. ∗∗ Corresponding author at: Institute for Research in Biomedicine, Protein Folding and Quality Control, CH-6500 Bellinzona, Switzerland. Tel.: +41 918200319; fax: +41 918200302. E-mail addresses: [email protected] (D.N. Hebert), [email protected] (M. Molinari). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.semcdb.2014.12.001 1084-9521/© 2014 Elsevier Ltd. All rights reserved. 80 A. Tannous et al. / Seminars in Cell & Developmental Biology 41 (2015) 79–89 1. Introduction 2.2.1.2. ˛-Glucosidase I generates the ligand for malectin. For a long time, the di-glucosylated form of protein-bound oligosaccha- The ER is the site for maturation of secretory and membrane rides generated by the action of ␣-glucosidase I was considered proteins in eukaryotic cells, which represent about one third of an extremely short-lived trimming intermediate lacking biolog- the total proteome of the cell [1–3]. Although several proteins ical significance. The discovery of malectin, a membrane-bound have been reported to acquire spontaneously their folded struc- ER-resident lectin specifically binding di-glucosylated glycans, ture in vitro, a large number of newly synthesized polypeptides changed this view [23]. Malectin is induced under conditions of in the cell requires assistance to reach the final and biologically ER stress [24] and is proposed to preferentially associate with active conformations [4–6]. The lumen of the ER contains resident off-pathway non-native conformers of well-studied glycoproteins molecular chaperones and folding factors to optimize the folding like influenza hemagglutinin (HA) and null Hong Kong (NHK), a efficiency [7]. The folding state of a protein is evaluated by qual- folding-defective variant of the secretory protein ␣-1-antitrypsin ␣ ity control mechanisms: properly folded proteins are secreted or ( 1AT). The putative capacity of malectin to detect terminally mis- targeted to their final intra- or extra-cellular destination, whereas folded proteins so early after their expression in the ER lumen misfolded proteins are recognized as aberrant products and tar- and to distinguish them from non-native intermediates of folding geted for the ER-associated degradation ((ERAD) [8,9] and reviewed programs is a peculiar property that merits further investigation by Benyair et al., in this issue). Most of the proteins designated for and may require the formation of a functional complex with the the secretory pathway are subjected to co- or post-translational oligosaccharyl transferase complex subunit ribophorin I [24–26]. glycosylation of asparagine residues side chains ((N-glycosylation) This conclusion is reinforced by the observation that the over- [10,11] and reviewed by Shrimal et al., in this issue). The rapid expression of ribophorin I enhances malectin association with removal of terminal glucose and mannose residues from protein- misfolded NHK [26]. bound oligosaccharides and the regulated addition of a specific ˛ glucose residue dictate the sequential schedule of events occurring 2.2.2. -Glucosidase II induces the entry of glycoproteins into the during maturation and selection for degradation. Here we present CNX/CRT cycle the body of knowledge concerning the sugar-regulated mecha- 2.2.2.1. Organization of ˛-glucosidase II. ␣-Glucosidase II is a het- nisms that govern protein folding and quality control in the ER erodimeric protein, with two non-covalently bound subunits. GII␣ lumen. is the catalytic subunit and GII␤ contains an ER retention sequence in mammals and S. pombe but not in S. cerevisiae [27]. GII␣ activity requires the presence of GII␤ as shown in S. pombe and mam- 2. Protein folding in the ER malian cells, as well as in cell free assays [28,29]. GII␤ contains a mannose-6-phosphate receptor homology (MRH) domain, which 2.1. N-glycosylation plays a key role in protein folding was proposed to regulate GII␣ activity by interacting with the man- noses on the B and C branches of the nascent protein glycan [29]. The majority of the client proteins entering the ER lumen exhibit -N-X-S/T- (asparagine-any amino acid but proline–serine/ 2.2.2.2. Regulation of ˛-glucosidase II trimming for entry in the threonine) sequences within the polypeptide chain [11,12]. The CNX/CRT cycle. ␣-Glucosidase II trims the second and the third glu- asparagine residue of this consensus motif is rapidly modified cose of the N-linked glycan (Fig. 2, steps 3 and 5). The two trimming through the covalent attachment of a pre-formed oligosaccha- events play opposing roles. After the first trimming step, maturing ride composed of three residues of glucose, nine mannoses substrates associate with the lectin chaperones CNX and CRT, which and two N-acetylglucosamines (Glc3Man9GlcNAc2) (Fig. 1). The recognize mono-glucosylated glycans. The second trimming step transfer of the 14-subunits oligosaccharide is catalyzed by the releases the protein from the lectin chaperones due to the reduced oligosaccharyltransferase complex (OST) (Fig. 2, step 1) [13] and affinity of these chaperones to glycans lacking terminal glucose thoroughly described in the article by Shrimal and Gilmore in residues (Fig. 2, step 5) [30–32]. A series of de-glucosylations by this issue. N-glycosylation increases the solubility of the newly ␣-glucosidase II (step 5) and re-glucosylations by a glucosyltrans- synthesized polypeptides and processing of the protein-bound ferase, the UGT1 (step 6) can occur [32,33]. Re-glucosylation by oligosaccharides creates the signal required for the recruitment UGT1, which will be detailed in Section 3, directs the rebinding of of ER-resident lectin chaperones that regulate glycopolypeptide CNX or CRT to the mono-glucosylated substrate, hence the name folding [14–17]. CNX/CRT binding cycle. Despite having one catalytic site, the rates of the first and the 2.2. N-glycan processing dictates the binding to the ER-resident second trimming events by the ␣-glucosidase II are different as the lectin chaperones first cleavage occurs faster than the second cleavage [34,35]. Unlike the rapid trimming by ␣-glucosidase I, trimming by ␣-glucosidase 2.2.1. ˛-Glucosidase I generates di-glucosylated glycans II appears to be more regulated. Work in ER-derived microsomes 2.2.1.1. Organization of ˛-glucosidase I. The modification of and mammalian cells showed that more than one glycan on client protein-bound oligosaccharides by ER-resident glycanases dictates proteins is required for recognition by ␣-glucosidase II [20]. the fate of newly synthesized polypeptides. As soon as the gly- The activity of ␣-glucosidase II appears to be influenced by can is added to nascent chains, the first glucose is trimmed by the number of mannoses on a glycan because reduced activity of ␣ -glucosidase I [18–20] (Fig. 2, step 2), a single pass transmem- ␣-glucosidase II in rat liver microsomes was observed towards sub- brane ER protein with the catalytic domain facing the ER lumen strates possessing a reduced number of mannose residues [36].
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