Podcast
Questions and Answers
What is the primary role of signal sequences in protein targeting?
What is the primary role of signal sequences in protein targeting?
- To modify the protein structure for enhanced stability.
- To initiate protein degradation within the cytosol.
- To direct proteins to their correct subcellular location. (correct)
- To prevent protein aggregation during translation.
If a protein lacks a signal sequence, where will it likely end up?
If a protein lacks a signal sequence, where will it likely end up?
- The endoplasmic reticulum.
- The cytosol. (correct)
- The Golgi apparatus.
- The nucleus.
What is the first step in co-translational translocation of a soluble protein?
What is the first step in co-translational translocation of a soluble protein?
- The signal sequence binds to the SRP receptor.
- The signal peptidase cleaves the signal sequence.
- The SRP binds to the signal sequence and pauses translation. (correct)
- The ribosome binds to the translocator channel.
Which of the following is characteristic of constitutive secretion?
Which of the following is characteristic of constitutive secretion?
What is the role of Mannose-6-Phosphate (M6P) in targeting lysosomal enzymes?
What is the role of Mannose-6-Phosphate (M6P) in targeting lysosomal enzymes?
Which of the following statements accurately describes the orientation of transmembrane proteins after insertion into the ER membrane?
Which of the following statements accurately describes the orientation of transmembrane proteins after insertion into the ER membrane?
How does a hydropathy plot help predict transmembrane domains in a protein?
How does a hydropathy plot help predict transmembrane domains in a protein?
A protein is synthesized without a signal sequence. What is its most likely final destination?
A protein is synthesized without a signal sequence. What is its most likely final destination?
During co-translational translocation, what component initially recognizes the signal sequence on a nascent polypeptide?
During co-translational translocation, what component initially recognizes the signal sequence on a nascent polypeptide?
In regulated secretion, how are proteins managed before they are released?
In regulated secretion, how are proteins managed before they are released?
What modification signals lysosomal hydrolases to be transported from the Golgi to the lysosome?
What modification signals lysosomal hydrolases to be transported from the Golgi to the lysosome?
If a transmembrane protein is inserted into the ER membrane with its N-terminus in the lumen, what will be the location of its N-terminus after transport to the plasma membrane?
If a transmembrane protein is inserted into the ER membrane with its N-terminus in the lumen, what will be the location of its N-terminus after transport to the plasma membrane?
In a hydropathy plot, what does a positive hydropathy index typically indicate?
In a hydropathy plot, what does a positive hydropathy index typically indicate?
Which of the following is NOT a destination within the endomembrane system that proteins can be targeted to?
Which of the following is NOT a destination within the endomembrane system that proteins can be targeted to?
What distinguishes co-translational translocation from post-translational translocation?
What distinguishes co-translational translocation from post-translational translocation?
What is the role of chaperones in the context of protein targeting to the mitochondria?
What is the role of chaperones in the context of protein targeting to the mitochondria?
What is the primary function of the TIM and TOM complexes in protein import?
What is the primary function of the TIM and TOM complexes in protein import?
Which of the following is MOST likely to occur if a protein destined for the ER lumen lacks a signal peptidase cleavage site?
Which of the following is MOST likely to occur if a protein destined for the ER lumen lacks a signal peptidase cleavage site?
A mutation in the gene encoding COPII protein disrupts its function. What cellular process will be MOST directly affected?
A mutation in the gene encoding COPII protein disrupts its function. What cellular process will be MOST directly affected?
How does the cell ensure that resident ER proteins are not inadvertently shipped to the Golgi?
How does the cell ensure that resident ER proteins are not inadvertently shipped to the Golgi?
Which of the statements is the MOST accurate description of how proteins are targeted to the thylakoid lumen?
Which of the statements is the MOST accurate description of how proteins are targeted to the thylakoid lumen?
A researcher is studying a newly discovered protein and observes that it has several stretches of hydrophobic amino acids. Based on this information, where is the protein MOST likely to be located?
A researcher is studying a newly discovered protein and observes that it has several stretches of hydrophobic amino acids. Based on this information, where is the protein MOST likely to be located?
What is the role of signal peptidase?
What is the role of signal peptidase?
What is the primary role of motor proteins like kinesin in the endomembrane system?
What is the primary role of motor proteins like kinesin in the endomembrane system?
A protein destined for secretion contains a mutation that prevents proper folding in the ER. What is the MOST likely outcome for this protein?
A protein destined for secretion contains a mutation that prevents proper folding in the ER. What is the MOST likely outcome for this protein?
Which of these transport mechanisms moves proteins DIRECTLY across a membrane?
Which of these transport mechanisms moves proteins DIRECTLY across a membrane?
The signal sequences that direct proteins to particular locations in the cell are both ________ and ________.
The signal sequences that direct proteins to particular locations in the cell are both ________ and ________.
What would a hydropathy plot look like for a seven membrane spanning transmembrane protein?
What would a hydropathy plot look like for a seven membrane spanning transmembrane protein?
What is the purpose of a 'stop-transfer' sequence in protein translocation?
What is the purpose of a 'stop-transfer' sequence in protein translocation?
What type of signal sequence is required to get a double pass transmembrane protein with the N-terminus and C-terminus in the Er lumen?
What type of signal sequence is required to get a double pass transmembrane protein with the N-terminus and C-terminus in the Er lumen?
What is the key difference between the roles of COPI and COPII coat proteins in vesicular transport?
What is the key difference between the roles of COPI and COPII coat proteins in vesicular transport?
A cell has a mutation that impairs the function of the SRP receptor in the ER membrane. What is the MOST likely consequence of this mutation?
A cell has a mutation that impairs the function of the SRP receptor in the ER membrane. What is the MOST likely consequence of this mutation?
A protein contains an ER signal sequence at its N-terminus, a transmembrane domain in the middle, and a KDEL sequence at its C-terminus. Where is this protein MOST likely to reside?
A protein contains an ER signal sequence at its N-terminus, a transmembrane domain in the middle, and a KDEL sequence at its C-terminus. Where is this protein MOST likely to reside?
A researcher is studying a protein that localizes to the mitochondrial intermembrane space (IMS). Which of the following targeting signals and mechanisms is MOST likely involved in its localization?
A researcher is studying a protein that localizes to the mitochondrial intermembrane space (IMS). Which of the following targeting signals and mechanisms is MOST likely involved in its localization?
A mutation results in a protein being produced without a cleavable signal sequence, but which still contains a stop-transfer sequence. How is this protein MOST likely to be positioned in the membrane?
A mutation results in a protein being produced without a cleavable signal sequence, but which still contains a stop-transfer sequence. How is this protein MOST likely to be positioned in the membrane?
What is the general mechanism for post-translational translocation?
What is the general mechanism for post-translational translocation?
Flashcards
Signal Sequences
Signal Sequences
Short amino acid sequences that direct proteins to specific subcellular locations.
Co-translational Translocation
Co-translational Translocation
Proteins are moved across membranes during translation.
Post-translational Translocation
Post-translational Translocation
Proteins are moved after translation is complete.
Constitutive vs. Regulated Secretion
Constitutive vs. Regulated Secretion
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Start and Stop Signal Sequences
Start and Stop Signal Sequences
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N-terminal Signal Sequence
N-terminal Signal Sequence
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Internal Signal Sequences
Internal Signal Sequences
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Orientation of Transmembrane Proteins
Orientation of Transmembrane Proteins
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Hydropathy Plot
Hydropathy Plot
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Positive Hydropathy Index
Positive Hydropathy Index
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Negative Hydropathy Index
Negative Hydropathy Index
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Presequence
Presequence
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Transit Peptide
Transit Peptide
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Study Notes
- Study Path part 2 and part 3's due date has been extended until tonight at 9 PM
- Part 2 has the same grading policy as homework
- Part 3 is graded for completion
- Homework is due Saturday at 9 PM
Today's Agenda
- Include a short lecture on the endomembrane system.
- Include a group worksheet.
- Review of the group worksheet.
Protein Variety in Cells
- Cells produce soluble (secreted) proteins.
- Cells produce membrane-bound proteins.
- Degraded proteins are replaced.
- Cell division is necessary.
Protein Localization Mechanisms
- Almost all proteins start translation in the cytoplasm.
- Cells use three mechanisms to localize proteins correctly.
- Signal sequences are short amino acid sequences that guide proteins to their subcellular locations.
Signal Sequences
- Signal sequences are necessary and sufficient to direct proteins to their correct locations.
- Each location is targeted with a specific signal sequence.
Protein Translocation
- Proteins can be translocated after translation or during translation.
Endomembrane System Overview
- The endoplasmic reticulum is responsible for protein folding and modifications.
- The Golgi apparatus is responsible for further modifications like glycosylation, and sorting materials.
- Secretory vesicles use motor proteins (kinesin) to reach their destination in an ATP-dependent manner and release soluble and integral proteins into the plasma membrane.
- Endosomes are the destination for newly synthesized proteins and materials brought in by endocytosis, often targeted to lysosomes, and release materials by exocytosis.
- Lysosomes break down newly synthesized digestive enzymes at a degradation site.
Co-translational Translocation Steps for Soluble Proteins
- Signal sequences can be N-terminal or internal.
- Signal recognition particles (SRP) pause translation.
- SRP binds to the receptor, and translation resumes.
- Ribosome binds to the translocator, the plug moves, and the signal sequence binds to the translocator.
- Translation continues as the protein translocates into the ER lumen.
- The signal sequence is then cleaved and degraded, and the plug closes the translocator.
Secretion Pathways
- Constitutive secretion occurs continuously.
- In regulated secretion, proteins packaged into vesicles are stored and secreted only in response to a specific signal.
Targeting Lysosomal Enzymes
- Enzymes are targeted to the lysosome via the ER, Golgi apparatus, M6P receptor, clathrin coat, retromer coat, endosome, and lysosome.
Integral Membrane Proteins Insertion
- Start and stop signal sequences facilitate various transmembrane passes.
- "Start signal" initiates protein movement into the ER lumen.
- "Stop signal" halts protein movement, resulting in a cytosolic location.
- Initial signal sequences can be N-terminal or internal.
- N-terminal sequences are always cleaved and result in the N-terminus being in the ER lumen.
- Internal sequences places the N-terminus on the cytosolic side.
- C-terminus location depends on the number and type of start-stop sequences in the protein.
Single Pass Transmembrane Proteins
- Internal START sequences produce proteins with one transmembrane domain and an N-terminus on the cytosolic side.
- N-terminal START transfer sequences, followed by a STOP transfer, result in single-pass proteins with an N-terminus in the lumen and a C-terminus in the cytosol.
Double Pass Transmembrane Proteins
- Internal START sequences, followed by a STOP, create double-pass membrane proteins with both N and C-termini in the cytosol.
Transmembrane Protein Orientation
- Once transmembrane proteins are inserted into the ER membrane, their orientation remains topologically equivalent.
- Cytosolic sides always face the cytosol.
- Lumen sides remain in the lumen of transport vesicles and the extracellular space.
Hydropathy Plots
- Sliding windows calculate the hydropathy of amino acids utilizing the protein's primary sequence.
- Hydropathy measures free energy required to move amino acid sequences from a nonpolar solvent to water.
- Positive values indicates hydrophobic sequences
- Negative values indicate hydrophilic sequences
- 20-30 amino acids with a positive hydropathy index may indicate an alpha helix that spans the lipid bilayer.
Protein Targeting to the Mitochondrial Matrix
- The presequence is the mitochondrial targeting signal.
- The protein is fully translated in the cytosol and chaperones prevent folding.
- Presequences bind to receptors, and proteins move across the double membrane through TOM and TIM translocators.
- Chaperones in the matrix help in protein folding.
- Mitochondrial Processing Peptidase (MPP) removes the presequence.
Protein Targeting to the Chloroplast
- A transit peptide is the chloroplast targeting signal.
- Proteins are fully translated in the cytosol, where chaperones prevent folding.
- Transit peptides bind to receptors, and proteins move across the double membrane through TOC and TIC translocators.
- Stromal Processing Peptidase (SPP) cleaves the transit peptide.
- A second target sequence directs movement into the thylakoid.
- Chaperones in the stroma/thylakoid assist in protein folding.
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