What Is The Function Of Glycoproteins In The Plasma Membrane – Selection of differentiation media strongly influences cell lineage and sensitivity to CFTR modulators in fully differentiated primary cultures of cystic fibrosis human airway epithelial cells.

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What Is The Function Of Glycoproteins In The Plasma Membrane

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Glycoproteins And Glycolipids

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Sugars On Cell Surface Are Key To Flu Infections

Shmunis School of Biomedicine and Cancer Research, Department of Cell Biology, George Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel

Received: 31 August 2020 / Revised: 17 September 2020 / Accepted: 18 September 2020 / Published: 22 September 2020

N-linked glycosylation and sugar chain processing as well as disulfide bond formation are among the most common post-translational protein modifications in the endoplasmic reticulum (ER). They are essential modifications required for membrane and secreted proteins to achieve their correct folding and native structure. Several oxidoreductases responsible for disulfide bond formation, isomerization and reduction have been shown to form stable functional complexes with enzymes and chaperones involved in the initial addition of N-glycans and in the folding and quality control of glycoproteins. Some of these oxidoreductases are selenoproteins. Recent studies have also focused on the glycan machinery–oxidoreductase complexes in recognizing and processing misfolded glycoproteins and reducing and targeting AR-associated degradation. This review focuses on the fascinating collaboration between glycoprotein-specific cell machineries and ER oxidoreductases, and highlights open questions regarding the functions of many members of this large family.

Secretory and membrane proteins are folded by various molecular chaperones and oxidoreductases in the endoplasmic reticulum (ER) to oxidize and reduce their cysteine ​​residues. Most secretory proteins are glycosylated, usually with one or more N-linked oligosaccharides attached to asparagine (Asn) sidechains. Nascent N-linked glycoproteins have a large oligosaccharide precursor, Glc.

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, which was later fixed in order. Protein folding and obtaining the correct native structure depends on the addition and correct processing of these sugar chains. Defects in N-linked glycosylation can cause a number of diseases known as glycosylation disorders. The ability of mammalian cells to correctly identify and degrade misfolded secreted proteins is critical for their proper function and survival. An ineffective elimination mechanism leads to misfolded proteins and consequently ER stress.

Disulfide bond formation by the disulfide isomerase (PDI) family of proteins is one of the most important modifications affecting secretory protein folding. Most glycoproteins have disulfides, which are present in 78% of secretory proteins. The oxidative environment in the ER allows ER-resident oxidoreductases to facilitate disulfide bond formation, which stabilizes protein structures. Proper redox conditions in the ER, along with the regulation of calcium homeostasis, are essential for the function of many light pathways and the maintenance of homeostasis.

Oxidoreductases are enzymes involved in protein folding, which are responsible for converting exposed cysteine ​​thiol groups through oxidation, reduction and isomerization. Oxidoreductants have so-called thioredoxin-like domains that are similar to cytosolic thioredoxins. The disulfide bond formation reaction is facilitated by the exchange of two electrons between the substrate protein cysteine ​​and the cysteines in the catalytic Cys-X-X-Cys motif of the thioredoxin-like oxidoreductase domain (reviewed in this special issue by Robinson and Bulleid). [2]). More than 20 oxidoreductants have been identified in the ER, of which PDI, also known as P4HB/PDIA1, is the best known. Some of these enzymes contain more than one thioredoxin-like domain, including non-catalytic ones (eg, PDI contains two active and two non-catalytic thioredoxin-like domains). The enzymes are soluble ER luminal proteins, with the exception of five members of the TMX subfamily, which are transmembrane (reviewed in this special issue by Guerra and Molinari). Oxidoreductases cleave cysteine ​​residues to form disulfide bonds. The reduced oxidoreductase molecules are reoxidized by several oxidases—Ero1a, Ero1β, Prx4, GPX7, and GPX8 [ 4 , 5 , 6 , 7 , 8 ]. Aero1α is the primary oxidant and donates a pair of electrons to molecular oxygen in a flavin-mediated reaction to produce hydrogen peroxide. A number of oxidoreactors are capable of, and in some cases have a preference for, reducing disulfide bonds. The stability of active-site disulfides determines the reducing or oxidizing activity of a PDI family member. General reviews about oxidoreductases and their mechanism of action can be found in [9, 10, 11].

There are also a series of ER-resident oxidoreductases that are selenoproteins. Selenoproteins are proteins that contain a selenium atom instead of sulfur in one or more cysteines, called selenocysteines. Incorporation of Sec into selenoproteins is the result of a selenocysteine-specific tRNA that decodes the UGA codon in the mRNA. Selenoproteins intervene in several physiological pathways such as immune responses, antioxidant defense, redox signaling, and thyroid hormone metabolism. Some of the selenoproteins act as oxidoreductases, thioredoxin-like domains with one or more selenocysteines instead of cysteines in the catalytic Cys-X-X-Sec or similar motif [12, 13, 14].

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In addition to their catalytic activity in disulfide bonding, many oxidoreductases have been shown to recognize hydrophobic pockets in their substrates, and thus they are folding sensors. They are involved in recognition of misfolded glycoprotein domains during sugar chain cleavage and formation of the so-called glycan code to target ER-associated degradation (ERAD). Oxidoreductases interact with glycoproteins, from their translocation to the ER and glycosylation to their final productive folding or targeting to the ER. Several oxidoreductants have been discovered in recent years to functionally link to the glycan recognition and processing machinery in the ER, which is the focus of this review.

Most secreted proteins are N-glycosylated, a reaction that occurs mostly during co-translation in higher eukaryotes. In the ER lumen, the transforming proteins are exposed to various enzymes and chaperones that promote protein maturation through both folding and post-translational modifications. Upon illumination of the Sec61 transduction channel, proteins encounter the membrane-bound enzyme complex oligosaccharide transferase (OST), which cleaves the high-mannose oligosaccharide precursor GCC.

From a dolichol diphosphate-linked oligosaccharide donor to a side chain of asparagine (ASN) residues [16]. The Asnu residue appears as part of the consensus sequence Asn-X-Ser/Thr (X = any amino acid but proline). After the transfer of the sugar chain by OST, the N-linked carbohydrate molecule undergoes a series of processes by several ER-localized enzymes, resulting in the appearance of different structures, which serve as a link for ER protein folding and quality control machinery. A subject that will be discussed later and reviewed elsewhere [15, 17, 18].

OST consists of one catalytic subunit (either STT3A or STT3B) and at least 6 of the following subunits: Ribophorin 1 and 2 (RPN1, RPN2), DAD1, OST48, TMEM258, OST4, KCP2, DC2/OSTC, and either TUSC3/N33 or IAP/MAGT1 [19]. Each subunit covers the membrane and has a significant light domain. Complexes of OST and Sec61 were found to be tightly bound to each other, presumably to ensure consensus N-glycosylation as the polypeptide exits the translocon into the ER lumen. The immediate action of OST is critical for the fate of glycoproteins. It contributes to the solubilization of nascent glycoproteins and directs them to maturation and quality control machinery by creating recognition tags [22]. Recent atomic-resolution cryo-electron microscopy structures of OST reveal its association with the Sec61 translocon and its receptor-recognizing subunits.

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