39 Questions
What is the function of the rough endoplasmic reticulum (RER)?
Protein biosynthesis
How are the sacs and tubules in the endoplasmic reticulum interconnected?
By a single continuous membrane
What does ER stand for in the context of the endoplasmic reticulum?
Endoplasmic Reticulum
How can the quantity of rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) in a cell change?
Depending on changing metabolic needs
What is the role of oligosaccharyl transferases in the endoplasmic reticulum (ER)?
They catalyze protein glycosylation in the ER
What determines the unique folding pathways of different polypeptides in the ER?
Their sequences
Which proteins assist in protein folding in the ER?
BiP, Calnexin, and Calreticulin
What role do disulfide bonds (DBs) play in the ER?
Stabilizing the structure of secretory and membrane proteins
What happens to properly folded proteins in the ER?
They are transported from the ER to the Golgi
In what conditions does ER stress lead to proteotoxicity and activate the unfolded protein response (UPR)?
Various pathophysiological conditions like ischemia, diabetes, and neurodegenerative diseases
What is the consequence of ER stress in severe cases?
Apoptosis
What is the role of ER resident proteins containing KDEL sequences?
They are selectively transported back to the ER from the Golgi
What is the unfolded protein response (UPR) aimed at during ER stress?
Restoring ER function and preventing cell death
What happens to misfolded proteins in the ER?
They are degraded in the cytosol
What is the consequence of ER stress in mild cases?
Transcriptional induction of chaperones
Which type of targeting occurs for proteins destined for the nucleus, mitochondria, and peroxisomes?
Post-translational targeting
What enables the cellular transport machinery to correctly position proteins inside or outside the cell?
Sorting signals
Where are proteins with signal sequences synthesized?
Endoplasmic reticulum (ER) membrane
What is the role of signal recognition particles (SRP) in protein targeting?
Recognize signal peptides
What is believed to open a channel for polypeptide transport into the ER lumen?
Interaction between the signal sequence and the ER membrane
Which organelle carries out specific proteolytic cleavages for newly synthesized polypeptides in the ER?
Endoplasmic reticulum (ER)
Where are carbohydrates added and processed for many secreted and membrane proteins?
Endoplasmic reticulum (ER)
What is the main function of the signal peptidase in the ER?
Carry out specific proteolytic cleavages
Where are proteins with signal peptides recognized by signal recognition particles (SRP)?
Cytosol
What is the destination of proteins related to the GER?
Endoplasmic reticulum (ER)
Where are proteins with signal sequences synthesized?
Endoplasmic reticulum (ER) membrane
What type of targeting occurs for proteins destined for the endoplasmic reticulum (ER), Golgi, lysosomes, plasma membrane, and secreted proteins?
Co-translational targeting
Where does co-translational targeting occur for proteins?
Endoplasmic reticulum (ER), Golgi, lysosomes, plasma membrane
What enables the cellular transport machinery to correctly position proteins inside or outside the cell?
Targeting signals
Where are proteins with signal sequences synthesized?
Ribosomes attached to the ER membrane
What recognizes signal peptides in the cytosol?
Signal recognition particles (SRP)
What is believed to open a channel for polypeptide transport into the ER lumen?
Interaction between the signal sequence and the ER membrane
What kind of proteins undergo specific proteolytic cleavages in the ER?
Newly synthesized polypeptides
What are added and processed in the ER and Golgi Complex for many secreted and membrane proteins?
Covalently attached carbohydrates
Where does post-translational targeting occur for proteins?
Nucleus, mitochondria, peroxisomes
What is responsible for specific proteolytic cleavages in the ER?
Signal peptidase
Where are signal peptides recognized by signal recognition particles (SRP)?
In the cytosol
Where are proteins processed and assembled into multimeric proteins in the ER?
Endoplasmic reticulum (ER)
What occurs for proteins destined for the nucleus, mitochondria, and peroxisomes?
Post-translational targeting
Study Notes
Protein Targeting and Signal Hypothesis
- Protein targeting is essential for correct activity of newly synthesized proteins in both prokaryotes and eukaryotes
- Sorting or translocation of proteins can occur through co-translational or post-translational translocation processes
- Co-translational targeting occurs for proteins destined for the endoplasmic reticulum (ER), Golgi, lysosomes, plasma membrane, and secreted proteins
- Post-translational targeting occurs for proteins destined for the nucleus, mitochondria, and peroxisomes
- Proteins related to the GER include those that are secreted from the cell, lysosomal enzymes, and plasma membrane glycoproteins
- Targeting signals, such as signal sequences or signal peptides, enable the cellular transport machinery to correctly position proteins inside or outside the cell
- Proteins with signal sequences are synthesized on ribosomes attached to the ER membrane
- Signal peptides are recognized by signal recognition particles (SRP) in the cytosol, which then take the ribosome to the ER
- The interaction between the signal sequence and the ER membrane is believed to open a channel for polypeptide transport into the ER lumen
- Newly synthesized polypeptides in the ER undergo specific proteolytic cleavages, addition and processing of carbohydrates, proper folding, and assembly into multimeric proteins
- Specific proteolytic cleavages are carried out by signal peptidase
- Many secreted and membrane proteins contain covalently attached carbohydrates, which are added and processed in the ER and Golgi Complex
Test your knowledge of protein folding, quality control in the endoplasmic reticulum (ER), protein targeting, and the signal hypothesis with this comprehensive quiz. Explore topics such as protein glycosylation, chaperones, disulfide bonds, ER stress, protein translocation, targeting signals, and the cellular transport machinery.
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