38 Questions
What is the function of transitional ER in eukaryotic cells?
Bud off transport vesicles carrying newly synthesized proteins and lipids to the Golgi apparatus
What is the purpose of signal sequences in protein sorting?
To direct secreted proteins to the ER membrane
What is the role of signal peptidase in the ER membrane?
To cleave off signal sequences from polypeptide chains
What is the purpose of the SRP receptor in the ER membrane?
To guide the ER signal sequence to a specific receptor
What is the function of the ER in most eukaryotic cells?
To sequester Ca2+ from the cytosol
What is the result of the release of Ca2+ into the cytosol from the ER?
Rapid responses to extracellular signals
What is the role of organelle contact sites in the ER?
To facilitate communication or transport of key metabolites between juxtaposed membranes
What is the function of the signal-recognition particle (SRP) in protein sorting?
To guide the ER signal sequence to a specific receptor at the ER membrane
What is the primary function of the endoplasmic reticulum (ER) in a eukaryotic cell?
Performing intermediary metabolism
What is the main component of the cytoplasm?
Cytosol
Which organelle is responsible for degrading defunct intracellular organelles?
Lysosomes
What is the primary function of the Golgi apparatus?
Modifying and dispatching proteins and lipids
Which organelles are the primary site of ATP generation in a cell?
Mitochondria and chloroplasts
What determines the final location of a protein in a cell?
The amino acid sequence of the protein
What is the role of sorting receptors in protein transport?
To recognize and mediate protein movement
What happens to proteins that lack sorting signals?
They remain in the cytosol as permanent residents
What type of ribosomes synthesize proteins encoded by the nuclear genome?
Free ribosomes unattached to any membrane
What is the purpose of the ER signal sequence in a protein?
To direct the protein to the ER membrane
What is the function of the Sec61 complex?
To recognize and open for proteins containing a signal sequence
What is the difference between membrane-bound and free ribosomes?
They differ only in the proteins they are making at any given time
What happens to the mRNA molecule during protein synthesis?
It remains attached to the ER membrane by a changing population of ribosomes
What is the purpose of chaperones in post-translational translocation?
To keep proteins unfolded in the cytosol
What type of translocation occurs when ribosomes bind to the ER membrane during protein synthesis?
Co-translational translocation
What is the purpose of the proofreading step provided by the Sec61 translocator?
To ensure that only proteins intended for the ER are allowed to enter
What is the primary mechanism by which protein translocation is driven through the Sec61 translocator?
The energy-driven cycle of binding and release of molecules
In co-translational translocation, what is the role of the SRP and SRP receptor?
To bring the ribosome to the membrane and engage with the Sec61 translocator
What is the main difference between post-translational and co-translational translocation?
The timing of protein synthesis and translocation.
In post-translational translocation in eukaryotic cells, what is the function of the Sec62-Sec63 complex?
To position BiP molecules where they can bind to the translocating chain
What is the role of general chaperone proteins in post-translational translocation?
They prevent the folding or aggregation of precursors before translocation.
What is the bacterial homolog of the Sec61 translocator?
SecY
What is the purpose of BiP in co-translational translocation?
It binds tightly to the imported protein chain to prevent it from sliding backwards.
What is required for the release of the polypeptide chain from BiP?
ATP hydrolysis by BiP.
In post-translational translocation in bacteria, what is the role of the SecA ATPase?
To feed the completed polypeptide chain into the SecY translocator
What is the function of the Sec61 translocator in post-translational translocation?
It forms a narrow channel for the translocation of unfolded polypeptides.
Which complex is found exclusively in eukaryotic cells?
Sec62-Sec63 complex
What is the role of Sec62 and Sec63 in co-translational translocation?
They associate with the Sec61 translocator and position BiP adjacent to the lumenal opening.
Why can only unfolded polypeptides be translocated through the Sec61 translocator?
Because the narrow channel of the translocator can only accommodate unfolded polypeptides.
What is the result of the binding of BiP to the imported protein chain?
The protein chain is prevented from sliding backwards.
Study Notes
Intracellular Organization
- The nucleus contains the genome
- Cytoplasm consists of cytosol and cytoplasmic organelles suspended in it
- Cytosol is the main site of protein synthesis and degradation, making up over half of the cell's total volume
- Endoplasmic reticulum (ER) performs most of the cell's intermediary metabolism
- ER consists of rough ER and smooth ER, with the latter found in all cells and specialized in regions that make intimate contacts with other organelles
- Golgi apparatus receives lipids and proteins from the ER and dispatches them to various destinations, often modifying them en route
- Lysosomes contain digestive enzymes that degrade defunct intracellular organelles
- Mitochondria and chloroplasts generate most of the ATP used by cells to drive reactions requiring an input of free energy
- Peroxisomes are small vesicular compartments containing enzymes used in various oxidative reactions
Protein Sorting
- Proteins can move between compartments in different ways
- Nearly all proteins begin their synthesis on ribosomes in the cytosol
- The final location of each protein depends on its amino acid sequence, which can contain one or more sorting signals
- Sorting signals in transported proteins are recognized by complementary sorting receptors that mediate movement between compartments
- Proteins without sorting signals remain in the cytosol as permanent residents
ER Signal Sequences
- Signal sequences were discovered in secreted water-soluble proteins that are first translocated across the ER membrane
- The signal hypothesis proposes that the mRNA for secretory proteins codes for a protein that is bigger than the protein that is eventually secreted
- The extra polypeptide is a signal sequence that directs the secreted protein to the ER membrane
- After the signal sequence has served its function, it is cleaved off by a signal peptidase in the ER membrane before the polypeptide chain has been completed
Signal-Recognition Particle (SRP)
- SRP binds to the signal sequence and guides it to the ER membrane
- SRP receptor in the ER membrane receives the signal sequence
- Free ribosomes synthesize all other proteins encoded by the nuclear genome
- Polyribosomes can form on a single mRNA molecule, and if the mRNA encodes a protein with an ER signal sequence, the polyribosome becomes attached to the ER membrane
Translocation
- Co-translational translocation: ribosomes bind to the ER membrane during translation, and the growing polypeptide chain is threaded across the membrane as it is made
- Post-translational translocation: proteins are completely synthesized in the cytosol as precursors before they are imported into the ER
- Sec61 translocator is used in both co-translational and post-translational translocation, and only opens for proteins containing a signal sequence
- BiP (binding protein) interacts with the Sec61 translocator and provides the driving force for protein import in post-translational translocation through ATP-driven cycles of binding and release
Three Ways of Protein Translocation
- Co-translational translocation: growing polypeptide chain is threaded across the membrane as it is made, requiring no additional energy
- Post-translational translocation in eukaryotic cells: requires Sec62 and Sec63 proteins, which position BiP molecules to bind to the translocating chain and pull it into the lumen
- Post-translational translocation in bacteria: completed polypeptide chain is fed into the SecY translocator in the plasma membrane by the SecA ATPase
Learn about the intracellular organization and protein sorting in eukaryotic cells, including the nucleus, cytoplasm, cytosol, and endoplasmic reticulum.
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