Module 15 - ER-associated Degradation & Quality Control - PDF

MODULE 15 ER-associated Degradation (ERAD): Quality Control Mechanisms misfolded proteins do not proceed their path beyond the ER (into the Golgi, etc.) glycoproteins: the polypeptide chain emerging in the ER lumen is glycosylated; the glycan has a branch ending in three glucose residues u...

MODULE 15 ER-associated Degradation (ERAD): Quality Control Mechanisms misfolded proteins do not proceed their path beyond the ER (into the Golgi, etc.) glycoproteins: the polypeptide chain emerging in the ER lumen is glycosylated; the glycan has a branch ending in three glucose residues usually, glucosidases I and II trim the glucose residues of the core glycan, and misfolded proteins with a single glucose residue left, bind foldases calnexin (CNX), calreticulin (CRT) a glucosyltransferase (GT) recognizes misfolded proteins and re-glucosylates them to be re-engaged by CNX/CRT correctly folded protein is released from the CNX/CRT cycle and transported to its destination, whereas misfolded protein ends up in the cytosol for degradation by proteasomes Unfolded Protein Response (UPR) if the ERAD cannot clear the misfolded proteins, UPR is triggered by BiP, an ER-resident chaperone BiP binds/inhibits three transmembrane proteins (stress sensors): kinase PERK, kinase/ endonuclease IRE1, transcription factor ATF6 upon accumulation of unfolded proteins in the ER, BiP binds these and releases the stress sensors, leading to their activation active stress sensors lead to the expression of only specific proteins, whereas the inhibition of eukaryotic initiation factor 2 (eIF2α) blocks the global protein synthesis prolonged eIF2α inhibition leads to apoptosis Protein Misfolding Diseases: Cystic Fibrosis cystic fibrosis transmembrane conductance (CFTR) is an ABC transporter with ATP-driven conformational changes that allow anion flow down the electrochemical gradient (unlike other ABC proteins that drive uphill transport) CFTR mutations dysregulate epithelial fluid transport in many organs, resulting in cystic fibrosis with symptoms of difficult breathing, lung and sinus infections, poor growth, fatty stool, infertility in some males 2012: FDA approved ivacaftor for cystic fibrosis in individuals with a specific mutation some misfolded CFTR mutants retain function and yet, are degraded by ERAD; therefore, suppression of ERAD might be therapeutic Protein Misfolding Diseases: Amyloid Diseases if protein folding is disrupted, proteins may display sticky (hydrophobic) surfaces and aggregate into fibers non-functional protein aggregates can be toxic many of the deposits (aggregates) were originally identified by their histochemical staining property, hence their designation as amyloid (starch-like) Membrane Biosynthesis in the ER membranes arise through the insertion of newly synthesized proteins and lipids in the existing ER membrane there is asymmetry: components at the luminal surface of the ER membrane end up at the external surface of the plasma membrane all lipids are made in the sER, except for sphingomyelin and glycolipids (their synthesis starts in the ER and ends in the Golgi) there are enzymes that remove phospholipids from one membrane and insert them into another The Golgi Network cisternal maturation model: new cisternae form at the cis face and migrate through the stacks as they mature carbohydrate synthesis and sorting/dispatching of proteins proteolytic cleavage of some membrane and secreted proteins sulfation of oligosaccharide chains modifications of N-linked oligosaccharides formation of O-linked oligosaccharides by addition of sugar residues to Ser and Thr phosphorylation of N-linked oligosaccharides, forming mannose-6-phosphate for lysosomes MODULE 15 Polarity of the Golgi Stack divided into compartments from the cis (entry) face closest to the ER to the trans (exit) face at the opposite end of stacks the cis-Golgi network (CGN) sorts proteins that can proceed through the Golgi from those that are to be returned to the ER the trans-Golgi network (TGN) sorts proteins into different types of vesicles with different destinations Coated Vesicles the coat: (1) concentrates specific proteins in a patch giving rise to a vesicle, (2) assembles proteins into lattices clathrin-coated, COPI-coated, and COPII-coated clathrin-coated: transport between the Golgi, plasma membrane, lysosomes, endosomes COPI: retrograde transport from ERGIC and the Golgi to ER, from trans- to cis-Golgi; COPII: anterograde transport from the ER to the Golgi resident proteins are maintained by (1) 'retrieval' signals, (2) the return of escaped proteins to their proper compartments Glycosylation in the Golgi O-linked oligosaccharides are attached at Ser or Thr; a process initiated in the ER with the removal of three glucose residues and a mannose residue the Golgi mannosidase I removes more mannose residues, and an N-acetylglucosamine is added; mannosidase II removes two mannose residues three types of glycosyltransferases act sequentially with substrates activated by linkage with nucleotides the Golgi has nucleotide sugar transporters for nucleotide sugars from the cytoplasm Targeting of Lysosomal Proteins soluble acid hydrolases are targeted to lysosomes by the tag mannose 6-phosphate (M6P), allowing recognition by M6P receptors in the Golgi and transport to the endo-lysosomal system for the label, the three mannose residues are kept; N-acetylglucosamine phosphates are added, and then the N- acetylglucosamine groups are removed other soluble proteins are transported to lysosomes in an M6P-independent manner by receptors Putting It Together misfolded proteins are retained in the ER by quality control mechanisms; thus, such proteins are not processed in the secretory pathway beyond the ER ER is involved in protein unfolding response; defects in this response result in diseases due to the accumulation of misfolded proteins during cell division, organelles are distributed intact to each daughter cell; organelles are not made 'from scratch' clathrin-coated vesicles mediate transport between the plasma membrane and TGN COPI- and COPII-coated vesicles mediate transport between the ER and the Golgi apparatus and between the Golgi cisternae the Golgi apparatus is where O-linked glycosylation of proteins takes place, and targeting of lysosomal proteins is ensured by the formation of mannose-6-phosphate residues on proteins