Cell Biology: Intracellular Organization and Protein Sorting

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Questions and Answers

What is the main site of protein synthesis and degradation in a eukaryotic cell?

Endoplasmic reticulum

Cytosol (correct)

Golgi apparatus

Lysosomes

What is the primary function of the Golgi apparatus?

Cell signaling

Lipid synthesis

Modification and dispatch of proteins and lipids (correct)

Protein synthesis

What is the function of lysosomes?

Degradation of defunct intracellular organelles (correct)

Cell signaling

ATP generation

Protein synthesis

Where do nearly all proteins begin their synthesis?

Cytosol Signup and view all the answers

What determines the final location of a protein in a cell?

Amino acid sequence of the protein Signup and view all the answers

What is the function of sorting receptors in protein traffic?

To recognize sorting signals in proteins Signup and view all the answers

What is the function of peroxisomes in a cell?

Oxidative reactions Signup and view all the answers

What type of transport involves the movement of proteins from the cytosol into a topologically distinct space?

Protein translocation Signup and view all the answers

What happens to proteins that do not have any sorting signals?

They remain in the cytosol as permanent residents Signup and view all the answers

Which type of transport involves the movement of proteins and RNA molecules between the cytosol and the nucleus?

Gated transport Signup and view all the answers

What is the primary function of transmembrane protein translocators?

To unfold protein molecules for transport Signup and view all the answers

What is the result of vesicular transport?

Proteins are ferried from one compartment to another Signup and view all the answers

What is the purpose of nuclear pore complexes?

To support the active transport of specific macromolecules Signup and view all the answers

What is the result of engulfment, such as autophagy?

Double membrane sheets wrap around portions of the cytoplasm Signup and view all the answers

What type of transport is involved in the re-formation of the nuclear envelope after mitosis?

Engulfment Signup and view all the answers

What is the common feature of protein transport mechanisms?

All proteins are ferried between topologically equivalent compartments Signup and view all the answers

What is the ER lumen continuous with?

The space between the inner and outer nuclear membranes Signup and view all the answers

What percentage of the total cell volume does the ER often occupy?

More than 10% Signup and view all the answers

What is the ER membrane the site of production for?

Many of the transmembrane proteins and lipids of the cell's organelles Signup and view all the answers

What is the function of the smooth ER?

Biosynthesis and metabolism of lipids Signup and view all the answers

What is the rough ER's main function?

Synthesizing proteins for secretion Signup and view all the answers

What type of cells are packed with an abundance of rough ER?

Cells specialized to secrete vast amounts of protein Signup and view all the answers

What is the role of the ER in the biosynthesis of lipids?

The ER produces lipids for the cell's organelles, including the mitochondria and plastid membranes Signup and view all the answers

What is the ER's role in protein synthesis?

The ER synthesizes almost all of the proteins that will be secreted to the cell exterior Signup and view all the answers

What is the primary function of the transitional ER?

To bud off transport vesicles carrying newly synthesized proteins and lipids Signup and view all the answers

What is the structure of the signal-recognition particle (SRP)?

A hinged rodlike structure that can wrap along the large ribosomal subunit Signup and view all the answers

What is the main purpose of ER in most eukaryotic cells?

To sequester Ca2+ from the cytosol Signup and view all the answers

What happens to the signal sequence after it has served its function?

It is cleaved off by a signal peptidase Signup and view all the answers

What is the function of SRP in protein synthesis?

To decrease the rate of protein synthesis and allow for targeting to the ER membrane Signup and view all the answers

What is responsible for guiding the ER signal sequence to the ER membrane?

SRP and SRP receptor Signup and view all the answers

Where is the signal sequence–binding pocket of SRP positioned?

Near the ribosomal tunnel through which newly made polypeptides emerge Signup and view all the answers

What is the function of the signal-recognition particle (SRP)?

To bind to the signal sequence Signup and view all the answers

What happens to the ribosome after SRP binds to its receptor?

It is brought to an unoccupied protein translocator in the ER membrane Signup and view all the answers

What is the function of the SRP receptor?

To bind to the SRP and bring it to an unoccupied protein translocator in the ER membrane Signup and view all the answers

What is the result of the release of Ca2+ into the cytosol from the ER?

Rapid responses to extracellular signals Signup and view all the answers

What is the function of the organelle contact sites?

To facilitate communication or transport of key metabolites Signup and view all the answers

What type of ribosomes are engaged in the synthesis of proteins that are being concurrently translocated across the ER membrane?

Membrane-bound ribosomes Signup and view all the answers

What happens to the growing polypeptide chain after the translocator occupies the position near the ribosomal tunnel?

It is transferred across the ER membrane Signup and view all the answers

What is the result of the contact between the ER and the plasma membrane?

Modulation of plasma membrane phosphoinositides Signup and view all the answers

Why does SRP bind to the interface between the large and small ribosomal subunits?

To prevent the release of the polypeptide chain into the cytosol Signup and view all the answers

Study Notes

Intracellular Organization and Protein Sorting

Eukaryotic cells have distinct intracellular compartments, including the nucleus, cytoplasm, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, mitochondria, and peroxisomes.
The nucleus contains the genome, while the cytoplasm consists of cytosol and cytoplasmic organelles suspended in it.
The cytosol makes up over half the cell's volume and is the main site of protein synthesis and degradation.

Protein Traffic and Sorting

Proteins can move between compartments in different ways, depending on their amino acid sequence and sorting signals.
Proteins without sorting signals remain in the cytosol as permanent residents.
There are four fundamentally different ways a protein is moved from one compartment to another:
- Protein translocation: transmembrane protein translocators transport specific proteins from the cytosol into a topologically distinct space.
- Gated transport: proteins and RNA molecules move between the cytosol and the nucleus through nuclear pore complexes.
- Vesicular transport: membrane-enclosed transport intermediates ferry proteins from one compartment to another.
- Engulfment: double membrane sheets wrap around portions of the cytoplasm, including fragments of organelles, to enclose a separate compartment.

Endoplasmic Reticulum (ER)

The ER has a central role in the biosynthesis of lipids and proteins, and stores intracellular Ca2+ that is mobilized in many cell signaling responses.
The ER membrane is the site of production of many transmembrane proteins and lipids of the cell's organelles.
Almost all proteins that will be secreted to the cell exterior or destined for the lumen of the ER, Golgi apparatus, or lysosomes are initially delivered to the ER lumen.
The ER is structurally and functionally diverse, with functional specialization leading to dramatic changes in the proportional abundance of different parts of the ER.

ER Specialization

Rough ER has a characteristic rough appearance due to the abundance of ribosomes bound to its surface, and is dedicated to the biosynthesis of proteins.
Smooth ER lacks ribosomes and is dedicated to other ER functions such as the biosynthesis and metabolism of lipids.
Transitional ER is a type of smooth ER found in all cells, from which transport vesicles carrying newly synthesized proteins and lipids bud off for transport to the Golgi apparatus.
ER can be specialized in regions that make intimate contacts with other organelles, such as mitochondria, plastids, endosomes, and the plasma membrane.

Signal Sequences and Protein Sorting

Signal sequences are discovered in secreted water-soluble proteins that are first translocated across the ER membrane.
The signal hypothesis proposes that the mRNA for the secretory protein 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, where it is cleaved off by a signal peptidase before the polypeptide chain has been completed.

Signal-Recognition Particle (SRP) and ER Signal Sequence

The ER signal sequence is guided to the ER membrane by the signal-recognition particle (SRP), which binds to the signal sequence, and an SRP receptor in the ER membrane.
SRP is a large complex that consists of six different polypeptide chains bound to a single RNA molecule.
SRP wraps along the large ribosomal subunit, positioning the signal sequence–binding pocket near the ribosomal tunnel through which newly made polypeptides emerge.
SRP engages a signal sequence as it emerges from the ribosome, slowing down translation and allowing the ribosome to bind to the ER membrane before completion of the polypeptide chain.
SRP exposes a binding site for an SRP receptor, bringing the SRP–ribosome complex to an unoccupied protein translocator in the ER membrane.

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Description

Explore the structure and function of eukaryotic cells, including intracellular compartments and protein synthesis and degradation. Learn about protein traffic and sorting mechanisms.