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What is the role of the nuclear-localization signal (NLS) in protein import?
What does the process of importing proteins into the nucleus involve?
Which statement about importins is accurate?
What effect does the interaction with Ran-GTP have on importins?
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Which of the following molecules is NOT typically exported from the nucleus using a similar mechanism to protein import?
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What role does Ran-GTP play in the export of proteins from the nucleus?
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Which of the following statements about NLS and NES in proteins is correct?
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What is the function of GEF in the GTP switch mechanism?
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What is the role of GAP in the transport mechanism of GTP switch proteins?
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How does the exportin-cargo complex exit the nucleus through the nuclear pore complex (NPC)?
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What primarily distinguishes proteins undergoing signal-based targeting from those in vesicle-based trafficking?
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Which of the following components is essential for the nonsecretory pathway of protein sorting?
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What is the role of the signal sequence in protein targeting?
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In which pathway are proteins destined for export from the cell synthesized?
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What mechanism couples the translocation of proteins across membranes?
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Which statement about the secretory pathway is true?
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What occurs after a receptor binds to a signal sequence during protein sorting?
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Which of these is NOT a destination for proteins synthesized on free ribosomes?
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What is the consequence of the mutation in AAT?
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What is the primary difference between the nonsecretory and secretory pathways in protein sorting?
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Which process is primarily responsible for the degradation of misfolded proteins?
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What is the requirement for protein translocation into mitochondria?
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What role do targeting sequences play in protein sorting?
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How are peroxisomal proteins typically imported into peroxisomes?
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Which energy source is commonly involved in the translocation of proteins across membranes?
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What is the role of the targeting sequence called MTS in mitochondrial protein import?
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Which organelles are primarily associated with the nonsecretory pathway of protein sorting?
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What motif is contained in the PTS1 used for peroxisomal targeting?
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What feature characterizes the receptors that bind targeting sequences?
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Which of the following pathways involves proteins localized to the cytosol?
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What type of protein structure is required for mitochondrial import?
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Which chaperone is required to maintain the unfolded state of proteins for mitochondrial import?
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What is the significance of the amino acid sequence of targeting sequences in protein sorting?
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In the context of protein sorting, what does the term 'vesicle-based trafficking' refer to?
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What happens to the exportin-cargo complex after Ran-GTP hydrolysis?
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Which component is necessary for exportin to bind to the cargo protein?
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How does Ran protein contribute to the transport process between the nucleus and cytoplasm?
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What is the primary role of nucleoporins in the export process?
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In the context of GTP switch proteins, what triggers the conversion from inactive to active state?
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What is the primary impact of Zellweger syndrome on peroxisomes?
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Which organ systems are most severely affected by Zellweger syndrome?
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What is a characteristic feature observed at birth in infants with Zellweger syndrome?
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What is the mechanism of transport through the nuclear pore complexes (NPCs)?
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What structures are included in the composition of a nuclear pore complex (NPC)?
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What role do FG nucleoporins play in nuclear transport?
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Which of the following features does the nuclear transporters possess?
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What is a common clinical manifestation of Zellweger syndrome in infants?
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What is the initial role of the ER signal sequence in targeting nascent secretory proteins?
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Which components are essential for the process of co-translational translocation?
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For what type of proteins is post-translational translocation primarily used?
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What classification do Type II and Type III proteins share in terms of their N-terminus?
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Which of the following describes the Sec61 complex in relation to the translocon?
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What is a significant characteristic of Type IV A proteins?
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Which modification is primarily associated with glycosylation in the ER?
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How does the SRP facilitate the attachment of the ribosome to the ER membrane?
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What is the role of the GTP-binding α subunit in the SRP receptor?
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Which statement about chaperones in the ER is accurate?
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Lipid-anchored proteins like GPI-linked proteins primarily differ from other membrane proteins in that they:
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Which of the following is crucial for the translocation of nascent proteins into the ER?
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Which type of protein spans the membrane multiple times and typically includes ion channels?
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Study Notes
Protein Sorting
- Proteins are sorted and targeted to specific locations.
- Two general processes are involved in protein sorting: signal-based targeting and vesicle-based trafficking.
- For membrane proteins, targeting involves inserting the protein into the lipid bilayer.
- For water-soluble proteins, targeting involves translocating the entire protein across the membrane into the aqueous interior of the organelle.
Targeting Mechanisms
- Targeting information, encoded within the protein as a sequence of 20-50 amino acids, determines the destination of the protein.
- These sequences are called targeting sequences, signal sequences, or signal peptides.
- Each organelle has receptors that bind to specific signal sequences.
- Once bound, the receptor then interacts with a translocation channel to move the protein across the membrane.
Key Considerations
- Protein sorting is a complex process that involves understanding the signal sequence, its receptor, and the energy requirements of the process.
- For export, exportins bind to proteins containing nuclear-export signals (NES) and move them out of the nucleus.
- This process requires Ran, a G protein that switches between two conformations depending on whether it is bound to GTP or GDP.
The Nuclear Pore Complex (NPC)
- The NPC acts as a selective gate allowing the passage of water-soluble molecules, but not larger macromolecules.
- The NPC is made of multiple proteins, including nucleoporins.
Nuclear Import
- Proteins destined for the nucleus contain a nuclear-localization signal (NLS), a short amino acid sequence that directs their translocation.
- Importin, a transport protein, binds to the NLS and facilitates movement into the nucleus.
- Once inside the nucleus, importin interacts with Ran-GTP, causing a conformational change that releases the cargo protein.
Nuclear Export
- Similar to import, nuclear export involves specific protein sequences that signal export and utilizes transport proteins.
- This system also relies on Ran-GTP for protein release from the transport complex.
GTP Switch Proteins
- These proteins are involved in signal transduction.
- GTP-bound proteins are "on" and GDP-bound proteins are "off".
- GEF (guanine nucleotide exchange factors) facilitate the conversion from inactive to active state, and GAP (GTPase-activating proteins) accelerate the inactivation of the protein.
Protein Sorting
- Cells use protein sorting to direct proteins to specific cellular compartments.
- Protein sorting is important for maintaining cellular function and organization.
- There are two general processes for protein sorting: signal-based targeting and vesicle-based trafficking.
- Signal-based targeting uses signal sequences to direct proteins to specific locations.
- Vesicle-based trafficking uses membrane-bound vesicles to transport proteins between compartments.
Non-Secretory Pathway
- This pathway sorts proteins to organelles like the cytosol, peroxisomes, mitochondria, chloroplasts, and nucleus.
- Proteins synthesized on free ribosomes are sorted by this pathway.
Secretory Pathway
- This pathway sorts proteins synthesized on bound ribosomes to the ER, plasma membrane, Golgi apparatus, lysosomes, or for export from the cell.
Targeting Sequences
- Targeting sequences, also known as signal sequences or signal peptides, are short sequences of amino acids that direct proteins to specific organelles.
- These sequences bind to receptors on the organelle membrane, initiating protein translocation.
- Different organelles have specific receptors that bind to unique signal sequences.
Co-translational Translocation
- This process involves the translocation of a nascent polypeptide through the ER membrane as the protein is being synthesized.
- It requires the involvement of the Signal Recognition Particle (SRP) and the SRP receptor on the ER membrane.
Signal Recognition Particle (SRP)
- This particle is composed of RNA and six different polypeptides.
- It recognizes and binds to the signal sequence of the nascent protein.
- The SRP then interacts with the SRP receptor on the ER membrane, bringing the ribosome and nascent protein to the ER.
SRP Receptor
- This protein is embedded in the ER membrane and consists of two subunits (α and β).
- The α subunit interacts with the SRP and binds GTP.
- The β subunit is anchored in the membrane.
Translocon
- This is a protein complex that forms a translocation channel through the ER membrane.
- It allows the signal sequence and the polypeptide to enter the ER lumen.
- The signal sequence interacts with amino acids in the Sec61 complex (part of the translocon).
Post-translational Translocation
- This process occurs after protein synthesis is complete.
- It is used for proteins that enter the ER lumen after translation.
- It involves the Sec61 translocon, Sec63 complex, and BiP (a chaperone protein).
Type I Proteins
- These are single-pass transmembrane proteins with their N-terminus facing the exoplasmic face and C-terminus facing the cytosol.
- They have a stop-transfer anchor sequence that stops translocation and inserts the protein into the membrane.
Type II Proteins
- These are single-pass transmembrane proteins with their N-terminus facing the cytosol.
- They have a signal-anchor sequence that acts as both an ER signal sequence and a membrane-anchor sequence.
Type III Proteins
- These are single-pass transmembrane proteins with their N-terminus facing the exoplasmic face.
- They have a signal-anchor sequence that acts as both an ER signal sequence and a membrane-anchor sequence.
Type IV Proteins
- These are multi-pass transmembrane proteins.
- They have multiple transmembrane domains.
- Type IVA have their N-terminus facing the cytosol, while Type IVB have their N-terminus facing the exoplasmic face.
GPI-linked Proteins
- These proteins are anchored to the membrane through a glycosylphosphatidylinositol (GPI) molecule.
- They lack hydrophobic transmembrane domains.
Glycosylation
- This is a major protein modification that involves the addition of carbohydrates to proteins.
- There are two types of glycosylation: N-linked (addition to asparagine) and O-linked (addition to serine or threonine).
- Glycosylation plays a role in protein folding, stability, and function.
Degradation of Misfolded Proteins
- Misfolded proteins are exported through the translocon to the cytosol.
- They are degraded by the ubiquitin/proteasome system.
Mitochondrial Import
- Most mitochondrial proteins are synthesized on free ribosomes and imported from the cytosol.
- They have a mitochondrial targeting sequence (MTS) at their N-terminus, which is cleaved in the matrix.
- Import into the mitochondria requires ATP and a proton motive force.
Peroxisomal Proteins
- These proteins are synthesized on free ribosomes and imported into peroxisomes.
- They possess a peroxisomal targeting sequence (PTS) that can be either PTS1 (C-terminal) or PTS2 (N-terminal).
- PTS1 typically contains an SKL (serine-lysine-leucine) motif.
- Import into peroxisomes requires ATP and involves receptor proteins (Pex5 and Pex7).
Nuclear Pore Complex (NPC)
- This complex allows molecules to enter and exit the nucleus.
- It consists of nucleoporin proteins arranged in a structure with a central transporter.
- Small molecules can passively diffuse through the NPC, while larger molecules require specific transporters.
Nuclear Localization Signal (NLS)
- Proteins destined for the nucleus have a NLS that interacts with importins (nuclear import receptors).
- Importins bind to the NLS and guide the protein through the NPC.
Nuclear Export Signal (NES)
- Proteins destined for export from the nucleus contain an NES that interacts with exportins (nuclear export receptors).
- Exportins bind to the NES and guide the protein through the NPC.
Ran
- Ran is a GTP-binding protein that plays a role in both nuclear import and export.
- It exists in different conformations depending on whether GTP or GDP is bound.
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Description
This quiz covers the processes involved in protein sorting, including signal-based targeting and vesicle-based trafficking. It examines how targeting sequences dictate protein destination and the interaction with organelle receptors. Test your understanding of these complex biological mechanisms.