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Questions and Answers
What role do calnexin and calreticulin play in the endoplasmic reticulum (ER)?
What role do calnexin and calreticulin play in the endoplasmic reticulum (ER)?
What triggers the release of a protein from calnexin or calreticulin?
What triggers the release of a protein from calnexin or calreticulin?
What activity does a glucosyl transferase perform regarding incompletely folded proteins?
What activity does a glucosyl transferase perform regarding incompletely folded proteins?
What condition is characterized by mutations in the CFTR gene leading to protein misfolding?
What condition is characterized by mutations in the CFTR gene leading to protein misfolding?
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What is the function of ERp57 in the ER?
What is the function of ERp57 in the ER?
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What happens to misfolded proteins that are recognized in the ER lumen?
What happens to misfolded proteins that are recognized in the ER lumen?
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What is one consequence of the CFTR delta 508 mutation?
What is one consequence of the CFTR delta 508 mutation?
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What common process facilitates the degradation of misfolded proteins in the cytosol?
What common process facilitates the degradation of misfolded proteins in the cytosol?
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What role does mannose 6-phosphate play in lysosomal protein transport?
What role does mannose 6-phosphate play in lysosomal protein transport?
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What is one function of the H+ ATPase in lysosomal membranes?
What is one function of the H+ ATPase in lysosomal membranes?
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How do mannose 6-phosphate-modified proteins segregate from other proteins in the trans Golgi network?
How do mannose 6-phosphate-modified proteins segregate from other proteins in the trans Golgi network?
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What happens to M6P receptors after lysosomal hydrolases dissociate from them in the endosome?
What happens to M6P receptors after lysosomal hydrolases dissociate from them in the endosome?
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What modification occurs to lysosomal proteins at the 6 position of Mannose?
What modification occurs to lysosomal proteins at the 6 position of Mannose?
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What is the role of KDEL receptors in the Endoplasmic Reticulum?
What is the role of KDEL receptors in the Endoplasmic Reticulum?
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Which process occurs during protein glycosylation in the ER?
Which process occurs during protein glycosylation in the ER?
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What is one important function of protein glycosylation?
What is one important function of protein glycosylation?
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Which of the following is NOT a function performed in the Endoplasmic Reticulum?
Which of the following is NOT a function performed in the Endoplasmic Reticulum?
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Which proteins are classified as ER resident proteins?
Which proteins are classified as ER resident proteins?
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What is the primary characteristic of N-glycosylation on asparagine?
What is the primary characteristic of N-glycosylation on asparagine?
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What is a consequence of extensive trimming of oligosaccharides in the Golgi apparatus?
What is a consequence of extensive trimming of oligosaccharides in the Golgi apparatus?
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Which amino acid is primarily targeted during O-glycosylation?
Which amino acid is primarily targeted during O-glycosylation?
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What is the primary cause of Gaucher disease?
What is the primary cause of Gaucher disease?
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Which mutation specifically leads to lower catalytic activity in Gaucher disease?
Which mutation specifically leads to lower catalytic activity in Gaucher disease?
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What role does isofagomine play in treating Gaucher disease?
What role does isofagomine play in treating Gaucher disease?
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What is the first step in the processing of N-linked carbohydrates?
What is the first step in the processing of N-linked carbohydrates?
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What enzyme is specifically used to distinguish complex from high-mannose oligosaccharides?
What enzyme is specifically used to distinguish complex from high-mannose oligosaccharides?
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What does the processing pathway of N-linked carbohydrates depend on?
What does the processing pathway of N-linked carbohydrates depend on?
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Which step immediately follows the removal of glucoses in the N-linked carbohydrate processing?
Which step immediately follows the removal of glucoses in the N-linked carbohydrate processing?
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What is the ultimate result of the N-linked carbohydrate processing in terms of protein structure?
What is the ultimate result of the N-linked carbohydrate processing in terms of protein structure?
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What is the primary role of the Golgi apparatus in the synthesis of complex oligosaccharides?
What is the primary role of the Golgi apparatus in the synthesis of complex oligosaccharides?
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Which statement correctly differentiates N-linked and O-linked glycosylation?
Which statement correctly differentiates N-linked and O-linked glycosylation?
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Which function is NOT associated with glycosylation of proteins?
Which function is NOT associated with glycosylation of proteins?
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What distinguishes glycosyl transferase enzymes in the Golgi apparatus from those in the ER?
What distinguishes glycosyl transferase enzymes in the Golgi apparatus from those in the ER?
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What component is part of the GPI anchor?
What component is part of the GPI anchor?
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What is a characteristic feature of proteoglycans?
What is a characteristic feature of proteoglycans?
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Which of the following is NOT a role of proteoglycans?
Which of the following is NOT a role of proteoglycans?
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In the process of protein glycosylation, where are the sugars assembled in N-linked glycosylation?
In the process of protein glycosylation, where are the sugars assembled in N-linked glycosylation?
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What is the role of mannose 6-phosphate in lysosomal protein transport?
What is the role of mannose 6-phosphate in lysosomal protein transport?
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How do lysosomal hydrolases dissociate from M6P receptors in endosomes?
How do lysosomal hydrolases dissociate from M6P receptors in endosomes?
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What modification occurs at the 6 position of mannose for lysosomal enzymes?
What modification occurs at the 6 position of mannose for lysosomal enzymes?
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What happens to the M6P receptors after they have delivered hydrolases to the lysosome?
What happens to the M6P receptors after they have delivered hydrolases to the lysosome?
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What role does the H+ ATPase play in lysosomes?
What role does the H+ ATPase play in lysosomes?
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What is the primary role of N-glycosylation in proteins?
What is the primary role of N-glycosylation in proteins?
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Which amino acids are primarily targeted in N-glycosylation?
Which amino acids are primarily targeted in N-glycosylation?
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What modification occurs frequently in proteins processed in the Golgi apparatus?
What modification occurs frequently in proteins processed in the Golgi apparatus?
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What is a key function of glycosylation in proteins?
What is a key function of glycosylation in proteins?
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Which process occurs through the KDEL receptor's function?
Which process occurs through the KDEL receptor's function?
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Which of the following statements is true regarding protein folding in the ER?
Which of the following statements is true regarding protein folding in the ER?
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What is the significance of O-glycosylation on Serine and Threonine?
What is the significance of O-glycosylation on Serine and Threonine?
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What is a primary function of the ER in relation to lipid synthesis?
What is a primary function of the ER in relation to lipid synthesis?
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What is the primary role of glycosylation in proteins?
What is the primary role of glycosylation in proteins?
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Where are the final steps of complex oligosaccharide synthesis conducted?
Where are the final steps of complex oligosaccharide synthesis conducted?
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What distinguishes O-linked glycosylation from N-linked glycosylation?
What distinguishes O-linked glycosylation from N-linked glycosylation?
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What component is essential for the GPI anchor structure?
What component is essential for the GPI anchor structure?
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What feature is typical of proteoglycans?
What feature is typical of proteoglycans?
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What functional roles do proteoglycans play in the extracellular matrix?
What functional roles do proteoglycans play in the extracellular matrix?
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Which enzyme is involved in the attachment of the GPI anchor to a precursor protein?
Which enzyme is involved in the attachment of the GPI anchor to a precursor protein?
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What is a unique characteristic of O-linked carbohydrates in proteoglycans compared to N-linked carbohydrates?
What is a unique characteristic of O-linked carbohydrates in proteoglycans compared to N-linked carbohydrates?
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What function does the transmembrane oligosaccharyl transferase enzyme serve in the endoplasmic reticulum?
What function does the transmembrane oligosaccharyl transferase enzyme serve in the endoplasmic reticulum?
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What is the main role of glucosyl transferase in the endoplasmic reticulum?
What is the main role of glucosyl transferase in the endoplasmic reticulum?
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How do calnexin and calreticulin interact with newly synthesized proteins?
How do calnexin and calreticulin interact with newly synthesized proteins?
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Which of the following accurately describes the Unfolded Protein Response (UPR)?
Which of the following accurately describes the Unfolded Protein Response (UPR)?
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What is the role of ERp57 in the endoplasmic reticulum?
What is the role of ERp57 in the endoplasmic reticulum?
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What process occurs to misfolded proteins after they are recognized in the ER lumen?
What process occurs to misfolded proteins after they are recognized in the ER lumen?
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Which mutation in the CFTR protein is associated with ineffective protein folding?
Which mutation in the CFTR protein is associated with ineffective protein folding?
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What is the fate of proteins in the cytosol that have been misfolded and ubiquitylated?
What is the fate of proteins in the cytosol that have been misfolded and ubiquitylated?
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What is a consequence of defects in protein processing and transport?
What is a consequence of defects in protein processing and transport?
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Why is glycosylation important for proteins?
Why is glycosylation important for proteins?
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What role do carbohydrate receptors play in neutrophil localization?
What role do carbohydrate receptors play in neutrophil localization?
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Which cellular component is primarily responsible for glycosylation?
Which cellular component is primarily responsible for glycosylation?
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What happens to lysosomal enzymes when they are improperly glycosylated?
What happens to lysosomal enzymes when they are improperly glycosylated?
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What is a key feature of lysosomal storage diseases like I-cell disease?
What is a key feature of lysosomal storage diseases like I-cell disease?
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In what way does glycosylation affect protein stability?
In what way does glycosylation affect protein stability?
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Which of the following is NOT a common consequence of defects in protein transport?
Which of the following is NOT a common consequence of defects in protein transport?
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Study Notes
ER: Protein Processing and Sorting
- ER Resident Proteins: contain a KDEL sequence, facilitate protein folding and lipid synthesis
- ER-Resident Protein Receptors (KDEL Receptors): ensure the recycling of ER resident proteins back to the ER
- Proteins Processed in ER: ER resident proteins, secreted proteins, membrane proteins, organelle proteins (Golgi, lysosomes, endosomes), glycosylated proteins
- Processes Occurring in ER: Protein oligomerization, disulfide bond formation, N-linked oligosaccharide addition, lipid synthesis
- Importance of Protein Glycosylation: aids in protein folding and maturation, contributes to protein stability, facilitates protein sorting, alters biophysical properties, enables protein-protein interactions
- N-Glycosylation: occurs on asparagine residues, important for protein sorting and maturation, precursor oligosaccharide attached to asparagine in the sequences Asn-X-Ser and Asn-X-Thr (where X is any amino acid except proline) - O-Glycosylation: occurs on serine or threonine residues, modifies biophysical properties of proteins
Quality Control in the ER
- Chaperone proteins (Calnexin and Calreticulin): bind carbohydrates of newly synthesized proteins, retain incompletely folded proteins in the ER, recruit enzymes to aid in folding, release proteins upon removal of terminal glucose
- Glucosyl Transferase: crucial enzyme that determines protein folding status - if incompletely folded, it re-adds glucose to N-linked oligosaccharide, renewing affinity for calnexin/calreticulin and retaining the protein in the ER
- ERp57: collaborates with calnexin and calreticulin, recognizes free sulfhydryl groups (indicative of incomplete disulfide bond formation)
- ER Associated Protein Degradation (ERAD): misfolded proteins are recognized and targeted for degradation, interact with chaperones, disulfide isomerases, and lectins in the ER lumen, exported to the cytosol, ubiquitylated, deglycosylated, and degraded in proteasomes
- Unfolded Protein Response (UPR) Signaling: ER stress signaling
Diseases Associated with Misfolded Proteins
- Cystic Fibrosis: mutations in CFTR (chloride channel) lead to misfolded proteins and protein degradation, some mutants (e.g., CFTR delta 508) result in a slowly folding protein that is rapidly degraded, cells grown at low temperatures can produce functional channels, helping the mutated protein escape UPR detection
- Gaucher Disease: results from a mutation in lysosomal acid beta-glucosidase (breaks down glucosylceramide), mutations in N370S lead to lower catalytic activity and impeded ER exit, if the protein exits the ER, it can function in the lysosome, an inhibitor (iminosugar isofagomine) binds to the active site, facilitating folding and ER exit (chemical chaperone), increasing lysosomal availability, washout of the drug restores activity over time
Protein Transport
- Golgi: comprised of distinct cisternae, each with a specific role in protein modification, enzymatic activity of each stack determines its function
- Golgi Modifications: site of final carbohydrate side chain modifications, other protein modifications occur sequentially
- N-Linked Carbohydrate Processing: removal and addition of sugars on branched chain N-linked carbohydrates leads to diversity and variety in mature protein structure, diversity enables selective recognition by receptors, processing pathway is highly ordered
- O-Linked Glycosylation: added to OH of threonine, typically long, unbranched polymers (very large)
Proteoglycans
- Structure: have O-linked carbohydrates and contain numerous glucosaminoglycans, part of the extracellular matrix, long linear hydrophilic structures
- Functions: play roles in cell-cell contact, serve as a reservoir of soluble growth factors, act as a mechanical cushion (e.g., cartilage), and line mucous membranes (component of mucins)
GPI Anchor
- Mechanism: Immediately after protein synthesis, the precursor protein is anchored in the ER membrane by a hydrophobic C-terminal sequence, an enzyme in the ER cuts the protein free and attaches the new C-terminus to a GPI intermediate, the sugar chain contains inositol, glucosamine, and mannoses linked to a phosphoethanolamine, the signal is contained within the hydrophobic C-terminal sequence
- Function: anchors the protein to the membrane, with amino acids exposed initially on the lumenal side of the ER and eventually on the exterior of the plasma membrane
Lysosomes
- Function: contain acid hydrolases (hydrolytic enzymes active under acidic conditions), an H+ ATPase pumps H+ into the lysosome, maintaining its lumen at an acidic pH
- Mannose 6-Phosphate Modification: lysosomal proteins are modified by addition of N-acetylglucosamine to the 6 position of mannose, glucoseamine is trimmed off, resulting in a mannose 6-phosphate residue, these proteins are soluble and targeted to lysosomes, mannose 6-phosphate serves as a recognition signal
- M6P Receptor: binds M6P-tagged hydrolases in the TGN, clathrin-coated vesicles bud off from the TGN, shed their coat, and fuse with early endosomes, at a lower pH, hydrolases dissociate from the M6P receptors, empty receptors are retrieved to the TGN, phosphate is removed from the M6P attached to the hydrolases in the endosomes, preventing return to the TGN with the receptor
ER and Protein Processing
- ER contains chaperones which facilitate protein folding and enzymes involved in protein/lipid synthesis.
- ER resident proteins contain a KDEL retention signal at the C-terminus.
- ER-resident protein receptors (KDEL receptors) ensure recycling of ER-resident proteins back to ER.
Proteins Processed in ER
- ER resident proteins
- Secreted proteins
- Membrane proteins
- Organelle proteins (Golgi, lysosomes, endosomes)
- Glycosylated proteins
ER Processes
- Protein oligomerization
- Disulfide bond formation in polypeptides
- Addition of N-linked oligosaccharides to peptides
- Lipid synthesis
Importance of Glycosylation
- Helps in protein folding and maturation
- Important for protein stability
- Helps protein sorting to its destination
- Changes the biophysical properties of the proteins
- Important in protein-protein interaction
N-Glycosylation
- Polypeptide chain is glycosylated on target asparagine amino acids.
- The precursor oligosaccharide is attached to asparagine in the sequences Asn-X-Ser and Asn-X-Thr (where X is any amino acid except proline).
- The five sugars form the core region of this oligosaccharide.
- The precursor oligosaccharide undergoes extensive trimming in the Golgi apparatus.
- The precursor oligosaccharide is transferred from a dolichol lipid anchor to the asparagine as an intact unit in a reaction catalyzed by a transmembrane oligosaccharyl transferase enzyme.
- One copy of this enzyme is associated with each protein translocator in the ER membrane.
Quality Control in ER
- ER chaperone proteins calnexin and calreticulin bind carbohydrate (lectin) of newly synthesized protein in ER.
- They bind to incompletely folded proteins containing one terminal glucose on N-linked oligosaccharides.
- They trap these proteins in the ER and recruit other ER enzymes to help fold the protein.
- Removal of the terminal glucose by a glucosidase releases the protein from calnexin/calreticulin
- A glucosyl transferase determines whether the protein is folded properly or not.
- If the protein is still incompletely folded, the glucosyl transferase transfers a new glucose from UDP-glucose to the N-linked oligosaccharide, renewing the protein’s affinity for calnexin/calreticulin and retaining it in the ER.
- The cycle repeats until the protein has folded completely.
- ERp57 collaborates with calnexin and calreticulin in retaining an incompletely folded protein in the ER.
ER Associated Protein Degradation (ERAD)
- Misfolded proteins in the ER lumen are recognized and targeted to a translocator complex in the ER membrane.
- They first interact in the ER lumen with chaperones, disulfide isomerases, and lectins.
- They are then exported into the cytosol through the translocator.
- In the cytosol, they are ubiquitylated, deglycosylated, and degraded in proteasomes.
Unfolded Protein Response (UPR) Signaling
- ER Stress signaling
Examples of Diseases Due to Misfolded Proteins
- Cystic Fibrosis
- Mutations in CFTR (Chloride channel) leads to misfolded proteins and protein degradation.
- Some mutants (e.g.CFTR delta 508) results in a slowly folding protein which is not functional due to rapid protein degradation.
- Cells with this mutation grown at low temperature can produce functional channel by helping the mutated protein escape detection by UPR.
O-Linked Glycosylation
- O-linked glycosylation added to OH of Threonine.
- They are typically long, unbranched polymers (very large)
Proteoglycans
- Proteoglycans have O-linked carbohydrates.
- They contain a large number of glucosaminoglycans.
- They are long linear hydrophilic structures.
- Play a role in cell-cell contact, as a reservoir of soluble growth factors, act as a mechanical cushion (e.g.cartilage), and lining mucous membranes (component of muscins).
GPI Anchor
- Immediately after the completion of protein synthesis, the precursor protein remains anchored in the ER membrane by a hydrophobic C-terminal sequence of 15–20 amino acids.
- An enzyme in the ER cuts the protein free from its membrane-bound C-terminus and simultaneously attaches the new C-terminus to an amino group on a preassembled GPI intermediate.
- The sugar chain contains an inositol attached to the lipid from which the GPI anchor derives its name.
- It is followed by a glucosamine and three mannoses.
- The terminal mannose links to a phosphoethanolamine that provides the amino group to attach the protein.
- The signal that specifies this modification is contained within the hydrophobic C-terminal sequence and a few amino acids adjacent to it on the lumenal side of the ER membrane.
- Because of the covalently linked lipid anchor, the protein remains membrane-bound, with all of its amino acids exposed initially on the lumenal side of the ER and eventually on the exterior of the plasma membrane.
Lysosomes
- Contain acid hydrolases which are hydrolytic enzymes that are active under acidic conditions.
- An H+ ATPase in the membrane pumps H+ into the lysosome, maintaining its lumen at an acidic pH.
Mannose 6-Phosphate Modification
- Lysosomal proteins are modified by addition of N-acetyl glucosamine to the 6 position of the Mannose.
- The glucoseamine is then trimmed off to result in a protein with a mannose 6-phosphate residue.
- These proteins are soluble proteins and targeted to lysosomes.
- Mannose 6-phosphate acts as a recognition signal to send the protein to lysosome.
- The sequential action of two enzymes in the cis and trans Golgi network adds mannose 6-phosphate (M6P) groups to the precursors of lysosomal enzymes.
- The M6P-tagged hydrolases then segregate from all other types of proteins in the TGN because the M6P-modified lysosomal hydrolases bind M6P receptor in TGN.
- The clathrin-coated vesicles bud off from the TGN, shed their coat, and fuse with early endosomes.
- At the lower pH (acid) of the endosome, the hydrolases dissociate from the M6P receptors, and the empty receptors are retrieved in vesicles to the TGN for further rounds of transport.
- In the endosomes, the phosphate is removed from the M6P attached to the hydrolases, which may further ensure that the hydrolases do not return to the TGN with the receptor.
I-cell Disease: Lysosomal Storage Disease
- Characterized by a deficiency in the enzyme phosphotransferase.
- The phosphotransferase adds mannose 6-phosphate (M6P) to lysosomal enzymes, which is required for their targeting to lysosomes.
- Without this modification, lysosomal enzymes are secreted from the cell and cannot perform their function.
- This results in the accumulation of undigested substrates in lysosomes, leading to a range of clinical symptoms.
Traffic Signals for Neutrophil Localization in the Vasculature
- Carbohydrate, G protein coupled receptors, and Integrins are involved in the process.
- Selectins, chemoattractants, and Ig family members mediate different aspects of neutrophil localization.
- These molecules interact with ligands on the surface of neutrophils to regulate their rolling, activation, and adhesion to the vascular endothelium.
Diseases Resulted from Defects in Protein Processing and Transport
- Cystic Fibrosis
- Atherosclerosis
- Diabetes (type II, adult onset)
- Viral Diseases (e.g., influenza)
- Alzheimer's disease
- Prion diseases (e.g., Mad Cow)
- Lysosomal storage diseases (e.g.Niemann-Pick, I-Cell disease)
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Description
Explore the essential processes occurring in the Endoplasmic Reticulum (ER) related to protein processing and sorting. This quiz covers ER resident proteins, their receptors, protein glycosylation, and the types of proteins that are processed within the ER. Test your knowledge on the significance of these processes for cellular function.
