Pathway: N-glycan trimming in the ER and Calnexin/Calreticulin cycle
Reactions in pathway: N-glycan trimming in the ER and Calnexin/Calreticulin cycle :
N-glycan trimming in the ER and Calnexin/Calreticulin cycle
After being synthesized in the ER membrane the 14-sugars lipid-linked oligosaccharide is co-translationally transferred to an unfolded protein, as described in the previous steps. After this point the N-glycan is progressively trimmed of the three glucoses and some of the mannoses before the protein is transported to the cis-Golgi. The role of these trimming reactions is that the N-glycan attached to an unfolded glycoprotein in the ER assume the role of 'tags' that direct the interactions of the glycoprotein with different elements that mediate its folding. The removal of the two outer glucoses leads to an N-glycan with only one glucose, which is a signal for the binding of either one of two chaperone proteins, calnexin (CNX) and calreticulin (CRT). These chaperones provide an environment where the protein can fold more easily. The interaction with these proteins is not transient and is terminated by the trimming of the last remaining glucose, after which the glycoprotein is released from CNX or CRT and directed to the ER Quality Control compartment (ERQC) if it still has folding defects, or transported to the Golgi if the folding is correct. The involvement of N-glycans in the folding quality control of proteins in the ER explains why this form of glycosylation is so important, and why defects in the enzymes involved in these reactions are frequently associated with congenital diseases. However, there are many unknown points in this process, as it is known that even proteins without N-glycosylation sites can be folded properly (Caramelo JJ and Parodi AJ, 2008).
After translation, many newly formed proteins undergo further covalent modifications that alter their functional properties. Modifications associated with protein localization include the attachment of oligosaccharide moieties to membrane-bound and secreted proteins (N-linked and O-linked glycosylation), the attachment of lipid (RAB geranylgeranylation) or glycolipid moieties (GPI-anchored proteins) that anchor proteins to cellular membranes, and the vitamin K-dependent attachment of carboxyl groups to glutamate residues. Modifications associated with functions of specific proteins include gamma carboxylation of clotting factors, hypusine formation on eukaryotic translation initiation factor 5A, conversion of a cysteine residue to formylglycine (arylsulfatase activation), methylation of lysine and arginine residues on non-histone proteins (protein methylation), protein phosphorylation by secretory pathway kinases, and carboxyterminal modifications of tubulin involving the addition of polyglutamate chains.
Protein ubiquitination and deubiquitination play a major role in regulating protein stability and, together with SUMOylation and neddylation, can modulate protein function as well.
Metabolism of proteins, as annotated here, covers the full life cycle of a protein from its synthesis to its posttranslational modification and degradation, at various levels of specificity. Protein synthesis is accomplished through the process of Translation of an mRNA sequence into a polypeptide chain. Protein folding is achieved through the function of molecular chaperones which recognize and associate with proteins in their non-native state and facilitate their folding by stabilizing the conformation of productive folding intermediates (Young et al. 2004). Following translation, many newly formed proteins undergo Post-translational protein modification, essentially irreversible covalent modifications critical for their mature locations and functions (Knorre et al. 2009), including gamma carboxylation, synthesis of GPI-anchored proteins, asparagine N-linked glycosylation, O-glycosylation, SUMOylation, ubiquitination, deubiquitination, RAB geranylgeranylation, methylation, carboxyterminal post-translational modifications, neddylation, and phosphorylation. Peptide hormones are synthesized as parts of larger precursor proteins whose cleavage in the secretory system (endoplasmic reticulum, Golgi apparatus, secretory granules) is annotated in Peptide hormone metabolism. After secretion, peptide hormones are modified and degraded by extracellular proteases (Chertow, 1981 PMID:6117463). Protein repair enables the reversal of damage to some amino acid side chains caused by reactive oxygen species. Pulmonary surfactants are lipids and proteins that are secreted by the alveolar cells of the lung that decrease surface tension at the air/liquid interface within the alveoli to maintain the stability of pulmonary tissue (Agassandian and Mallampalli 2013). Nuclear regulation, transport, metabolism, reutilization, and degradation of surfactant are described in the Surfactant metabolism pathway. Amyloid fiber formation, the accumulation of mostly extracellular deposits of fibrillar proteins, is associated with tissue damage observed in numerous diseases including late phase heart failure (cardiomyopathy) and neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's.