Processing by Endoplasmic Reticulum Mannosidases Partitions a Secretion-impaired Glycoprotein into Distinct Disposal Pathways

Abstract

In the early secretory pathway, a distinct set of processing enzymes and family of lectins facilitate the folding and quality control of newly synthesized glycoproteins. In this regard, we recently identified a mechanism in which processing by endoplasmic reticulum mannosidase I, which attenuates the removal of glucose from asparagine-linked oligosaccharides, sorts terminally misfolded ␣ 1 -antitrypsin for proteasome-mediated degradation in response to its abrogated physical dissociation from calnexin (Liu, Y., Choudhury, P., Cabral, C., and Sifers, R. N. (1999) J. Biol. Chem. 274, 5861-5867). In the present study, we examined the quality control of genetic variant PI Z, which undergoes inappropriate polymerization following biosynthesis. Here we show that in stably transfected hepatoma cells the additional processing of asparagine-linked oligosaccharides by endoplasmic reticulum mannosidase II partitions variant PI Z away from the conventional disposal mechanism in response to an arrested posttranslational interaction with calnexin. Intracellular disposal is accomplished by a nonproteasomal system that functions independently of cytosolic components but is sensitive to tyrosine phosphatase inhibition. The functional role of ER mannosidase II in glycoprotein quality control is discussed.In the endoplasmic reticulum (ER), 1 an assortment of molecular chaperones and folding enzymes facilitate the conformational maturation of newly synthesized polypeptides destined for deployment to the cell surface as biologically active proteins (1). In recent years, a picture has emerged that describes how asparagine-linked glycosylation, in combination with several independently acting enzymes, facilitates glycoprotein folding (for reviews, see Refs. 2 and 3). In a widely accepted model (4), the partial deglucosylation of asparagine-linked Glc 3 Man 9 -GlcNAc 2 induces cotranslational physical interaction between glycoproteins and members of a small family of lectins, each of which recognizes the monoglucosylated glycan as ligand (2). Dissociation of the complex coincides with the removal of glucose by glucosidase II (5). In the absence of conformational maturation, UDP-glucose:glycoprotein glucosyltransferase (UGTR) functions as a folding sensor (6) that recognizes structural determinants common to nonnative glycoprotein structure (7,8). Reglucosylation of asparagine-linked oligosaccharides induces the reassembly of folding intermediates with calnexin (4). As such, reversible glucosylation hinders premature exit from the ER (2, 4, 9) until correctly folded molecules that are no longer substrates for UGTR are released from the lectin-mediated retention cycle (4).As a rule, failure to attain conformational maturation following biosynthesis results in the selective elimination of misfolded polypeptides and unassembled protein complexes by a relatively stringent mechanism of conformation-based quality control (10, 11). Molecular characterization of primary and secondary disposal systems, plus the identification of the full reperto...

Mutants of Neuroserpin That Cause Dementia Accumulate as Polymers within the Endoplasmic Reticulum

“…The mechanism of degradation of ER-retained Z ␣ 1 -antitrypsin is controversial, with evidence both in favor and against proteasomal degradation (41,(43)(44)(45). In addition, a nonproteasomal degradation pathway and autophagy have been proposed as disposal pathways for Z ␣ 1 -antitrypsin (29,41,45).…”

Autophagic disposal of the aggregation-prone protein that causes liver inflammation and carcinogenesis in α-1-antitrypsin deficiency

“…Although retrograde translocation through the microsomal import channel has been proposed for some ER proteins, there is growing evidence for a mechanism in which the proteasome, as part of a multiprotein complex that forms a platform on the cytosolic side of the ER membrane, directly mediates extraction of substrates from the ER membrane. 19,20 Nevertheless, these early studies in yeast, mammalian cell lines and cell-free systems all indicated that the proteasomal pathway could not fully account for disposal of ATZ [16][17][18][21][22][23] -that is in the absence of proteasomal activity there was clear-cut residual and time-dependent degradation of ATZ.…”

Alpha-1-Antitrypsin Deficiency: A New Paradigm for Hepatocellular Carcinoma in Genetic Liver Disease *

“…22 Moreover, there is evidence for non-proteosomal mechanisms for degradation of ␣1ATZ. 19 Cabral et al 23 have provided evidence for a nonproteosomal degradation pathway that is sensitive to tyrosine phosphatase inhibitors. We have shown that macroautophagy contributes to disposal of retained ␣1ATZ, using chemical inhibitors 24 and cell lines genetically engineered for deficient autophagy (unpublished observations).…”