Cellular Transport Mechanisms Quiz

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Questions and Answers

What is the function of the nuclear-localization signal (NLS) in proteins?

To facilitate the export of proteins from the nucleus

To enhance protein synthesis in the cytosol

To prevent macromolecules from entering the cell

To direct proteins into the nucleus (correct)

How does importin interact with NLS-containing cargo proteins?

It forms a covalent bond with the nucleoporin

It directly phosphorylates the cargo protein

It tags the cargo protein for degradation

It binds to the NLS of the cargo protein (correct)

What happens to importins once they have transported cargo into the nucleoplasm?

They aggregate and form complex structures

They undergo degradation in the nucleus

They are permanently attached to the cargo

They are recycled after releasing the cargo (correct)

Which of the following processes is similar to the mechanism used for nuclear import?

Export of proteins and ribosomal subunits from the nucleus Signup and view all the answers

What initiates the conformational change in importin that leads to cargo release in the nucleus?

Interaction with Ran-GTP Signup and view all the answers

What is the primary role of exportin in cellular transport?

To transport proteins containing nuclear-export signals (NES) out of the nucleus Signup and view all the answers

How do cargo proteins with NES and NLS behave in a cell?

They shuttle between the nucleus and cytoplasm Signup and view all the answers

What triggers the 'on' state of GTP-binding proteins?

Presence of GEF inducing GDP release Signup and view all the answers

What role do GTPase-activating proteins (GAP) play in the regulation of GTP-binding proteins?

They accelerate the hydrolysis of GTP to GDP, inactivating the proteins Signup and view all the answers

What is a key characteristic of the G protein Ran in the context of protein transport?

It exists in different conformations when bound to GTP or GDP Signup and view all the answers

What type of pathway is associated with proteins synthesized on free ribosomes?

Nonsecretory pathway Signup and view all the answers

What is the role of targeting sequences in protein sorting?

Direct proteins to specific organelles Signup and view all the answers

Which organelles are associated with the nonsecretory pathway?

Peroxisomes and chloroplasts Signup and view all the answers

What typically provides the energy for protein translocation across membranes?

ATP hydrolysis Signup and view all the answers

What distinguishes secretory pathway proteins from those in the nonsecretory pathway?

Location of synthesis Signup and view all the answers

What is the primary function of receptors for signal sequences in protein targeting?

Bind and transfer polypeptides to translocation channels Signup and view all the answers

Which option best describes the fate of water-soluble proteins during targeting?

They are translocated across the membrane into the organelle’s interior. Signup and view all the answers

How many amino acids typically comprise the targeting sequences necessary for protein sorting?

20-50 amino acids Signup and view all the answers

What is the consequence of the mutant AAT not folding properly?

It cannot inhibit elastase, resulting in elastin destruction. Signup and view all the answers

What is the function of the mitochondrial targeting sequence (MTS)?

To direct cytosolic proteins to the mitochondrion. Signup and view all the answers

How do proteins primarily enter mitochondria?

In an unfolded state using the proton-motive force. Signup and view all the answers

What is the primary requirement for importing proteins into peroxisomes?

They must be in a folded state and contain a specific targeting sequence. Signup and view all the answers

What is the role of Hsc70 in mitochondrial import?

To help translocate proteins into the mitochondrial matrix. Signup and view all the answers

What are the primary components associated with the import of mitochondrial proteins?

Tom40 and Tim44. Signup and view all the answers

Which of the following best describes peroxisomes?

They are single-membrane organelles containing oxidases and catalase. Signup and view all the answers

What type of targeting sequence is primarily associated with imports into peroxisomes?

C-terminal PTS1 or N-terminal PTS2. Signup and view all the answers

What role do disulfide bonds play in protein structures?

They help stabilize tertiary and quaternary structures. Signup and view all the answers

Which protein is responsible for catalyzing the addition of disulfide bonds in the ER?

Protein Disulfide Isomerase (PDI) Signup and view all the answers

Which type of proteins typically contain disulfide bridges?

Secretory proteins and exoplasmic domains of membrane proteins Signup and view all the answers

What activates the unfolded protein response in the ER?

Accumulation of unfolded proteins within the lumen Signup and view all the answers

Which of the following proteins assists in the folding of glycosylated proteins?

Calreticulin Signup and view all the answers

What is the consequence of hereditary point mutations in alpha-1-antitrypsin (AAT)?

Loss of elastin, a structural protein in the lungs Signup and view all the answers

Which factor turns on the transcription of genes encoding protein-folding catalysts during the unfolded protein response?

Hac1 Signup and view all the answers

What is the primary function of peptidyl-propyl isomerases in the ER?

Accelerating rotation about peptidyl-prolyl bonds Signup and view all the answers

What is the function of the signal sequence in secretory proteins?

To target the ribosome to the ER membrane Signup and view all the answers

What is the role of the Signal-Recognition Particle (SRP)?

To facilitate the transport of proteins across the ER membrane Signup and view all the answers

What type of proteins typically utilize co-translational translocation?

Membrane-bound and secretory proteins Signup and view all the answers

Which component is necessary for the opening of the translocon?

Binding of the SRP-ribosome complex Signup and view all the answers

In which organisms is post-translational translocation commonly observed?

Some eukaryotes, like yeast Signup and view all the answers

Which statement accurately describes Type I integral membrane proteins?

They contain a cleavable N-terminal signal sequence and stop-transfer anchor Signup and view all the answers

What distinguishes Type IV A and Type IV B proteins in terms of N-terminus positioning?

Type IV A has its N-terminus in the cytosol; Type IV B in the exoplasmic face Signup and view all the answers

What modification occurs in the ER that is crucial for protein function?

Proteolytic cleavages for activation Signup and view all the answers

What is the significance of glycosylation in protein modification?

It assists in cellular signaling and recognition Signup and view all the answers

Which protein is NOT involved in the mechanism of co-translational translocation?

Sec63 complex Signup and view all the answers

What type of protein is classified as GPI-linked?

Proteins anchored by glycosylphosphatidylinositol Signup and view all the answers

In the context of membrane proteins, what does 'topology' refer to?

The orientation and arrangement of membrane-spanning segments Signup and view all the answers

What drives the unidirectional transfer of proteins across the membrane in post-translational translocation?

Potential energy from the protein's hydrophobic regions Signup and view all the answers

Study Notes

Protein Sorting

Proteins are targeted and translocated across membranes to various cellular organelles.
A typical mammalian cell contains thousands of proteins.
Proteins can be localized to the cytosol, within organelles, or embedded in organellar membranes.
Some proteins are meant for export or positioning in the plasma membrane.

Protein Targeting/Sorting Processes

Two processes are involved: signal-based targeting and vesicle-based trafficking.
For membrane proteins, targeting leads to insertion into the lipid bilayer.
For water-soluble proteins, targeting results in translocation across the membrane into the organelle's aqueous interior.

Protein Sorting Pathways

Two general pathways exist for protein sorting: nonsecretory and secretory.
The nonsecretory pathway targets proteins to organellar membranes (e.g., cytosol, peroxisomes, mitochondria, chloroplasts, nucleus).
The secretory pathway targets proteins to the plasma membrane or for export from the cell (e.g., ER, plasma membrane, Golgi apparatus, lysosomes).

Eukaryotic Protein Sorting Pathways

Proteins synthesized on free ribosomes follow the nonsecretory pathway.
Proteins synthesized on bound ribosomes follow the secretory pathway.

Protein Targeting Mechanisms

Information to target proteins to specific organelles is encoded within the protein's amino acid sequence (20-50 amino acids).
These sequences are called targeting sequences, also signal sequences or signal peptides.
Each organelle possesses receptors that bind to specific signal sequences.
The receptor transfers the polypeptide to a translocation channel.
Protein translocation across the lipid bilayer is often coupled to an energetically favorable process (e.g., ATP hydrolysis).

Four Considerations in Protein Targeting Mechanisms

The nature of the signal sequence and what distinguishes it from other signal sequences.
The receptor for the signal sequence.
The structure of the translocation channel and whether folded or unfolded proteins pass through.
The energy source driving unidirectional transfer across the membrane.

Secretory Pathway (Detailed)

Soluble, secreted proteins destined to the plasma membrane, lysosomes, or secretion from the cell utilize the secretory pathway.
Proteins are processed in stages, first in the rough ER, then the Golgi apparatus.

Signal Sequence Targets Nascent Secretory Proteins to the ER

Protein synthesis begins on free ribosomes in the cytosol.
A signal sequence (16-30 amino acids) directs ribosomes to the ER membrane.
The signal sequence is typically found at the N-terminus of the nascent protein.
The signal sequence is typically cleaved before the protein is fully formed.
The signal sequence contains >1 positively charged amino acid next to 6-12 hydrophobic amino acids.
These hydrophobic amino acids are critical for binding to receptors and translocation.

Co-translational Translocation

Translocation of the nascent polypeptide occurs before the protein is fully synthesized.
Fully formed proteins cannot directly enter the ER.
Co-translational translocation requires a signal-recognition particle (SRP) and an SRP receptor on the ER membrane.

Signal-Recognition Particle (SRP)

The SRP consists of RNA and six different polypeptides (P54, P19, P68, P72, P9, P14).
Some components of the SRP are critical for binding to the signal sequence.
Other components are involved in targeting the ribosome to the ER membrane.

SRP Receptor

The SRP receptor is a transmembrane protein composed of two subunits, α and β.
The α subunit interacts with the SRP, and the β subunit is embedded in the ER membrane.
The receptor is vital for moving the ribosome-nascent chain complex to the translocation channel.

Translocon (Translocation Channel)

The translocon, comprised of proteins like the Sec61 complex, enables protein translocation across the ER membrane.
It's closed when not bound to SRP/receptor/ribosome complex.
On binding, it opens, enabling the signal sequence and polypeptide to enter.

Co-translational Translocation Across the ER Membrane

The process of translocation moves the nascent polypeptide across the ER membrane.
The signal peptide is cleaved by signal peptidase.
The protein is folded within the ER lumen.
The polypeptide chain emerges into the ER lumen, as the signal sequence gets cleaved.

Post-translational Translocation

In yeast and some other eukaryotes, some proteins are translocated into the ER after their protein synthesis is completed.
This process utilizes the Sec61 translocon, the Sec63 complex, and chaperones like BiP.
This mechanism does not use the SRP and SRP receptor.

Export of Bacterial Proteins

Bacteria, like Yersinia pestis, utilize type III secretion systems (T3SS) to inject proteins into host cells.
These injected proteins can disable host functions, potentially contributing to infection.

Membrane Proteins: Insertion into ER Membrane

Each membrane protein has a unique orientation within the phospholipid bilayer.
Proteins synthesized in the RER remain embedded in the membrane, preserving the orientation required for their final destination in other organelles and the plasma membrane.

ER Membrane Proteins

Integral membrane proteins are categorized into classes (Types I, II, III, IV) based on their topological features within the lipid bilayer.
GPI-linked proteins are tethered to the membrane via a glycosylphosphatidylinositol (GPI) anchor.

Type I Proteins

Possess an N-terminal signal sequence that's cleaved.
Have a stop-transfer anchor sequence that halts translocation across the ER membrane.

Type II Proteins

Lack a cleavable N-terminus.
The signal-anchor sequence acts as both an ER signal and membrane-anchor sequence.
The N-terminus faces the cytosol.

Type III Proteins

Lack a cleavable N-terminus.
The signal-anchor sequence acts as both an ER signal and membrane-anchor sequence.
The N-terminus faces the exoplasmic space.

Type IV Proteins

Type IV A proteins have their N-terminus in the cytosol and include GLUT transporters and ion channels.
Type IV B proteins have their N-terminus extending into the exoplasmic space, encompassing G-protein-coupled receptors.

Lipid-anchored Proteins

These proteins lack hydrophilic membrane-spanning domains.
They are anchored to the membrane through amphipathic phospholipids, commonly via a GPI anchor.

Protein Modifications

Proteins are often modified within the ER, Golgi, and secretory vesicles.
These modifications include glycosylation, disulfide bond formation, polypeptide folding, and proteolytic cleavages.

Glycosylation

Carbohydrates are appended to proteins, often linked to serine, threonine, or asparagine side chains.
N-linked glycosylation is a common modification, involving the attachment of oligosaccharides to asparagine residues.
O-linked glycosylation also occurs.

Disulfide Bonds

Protein disulfide isomerase (PDI) catalyzes the formation and rearrangement of disulfide bonds.
Disulfide bonds are important for stabilizing tertiary and quaternary structures of proteins within the ER lumen.

Protein Folding

Protein folding is facilitated by chaperones and other ER proteins.
Lectins specifically bind N-linked glycosylated proteins.
Peptidyl-propyl isomerases accelerate the rotation of peptide bonds, facilitating protein folding.

Misfolded Proteins

Accumulation of misfolded proteins within the ER lumen triggers the unfolded protein response (UPR).
The UPR involves upregulating protein-folding machinery and degradation pathways to cope with misfolded proteins.

Hereditary (Familial) Emphysema

Familial emphysema is a genetic disorder resulting in the loss of a lung structural protein, elastin, due to dysfunctional alpha-1-antitrypsin (AAT).
Misfolded AAT proteins cannot be properly transported, leading to a deficiency in the protein's ability to inhibit elastase, causing lung damage.

Degradation of Misfolded Proteins

Misfolded proteins can be exported through the translocon to the cytosol.
The ubiquitin/proteasome pathway plays a role in degrading misfolded proteins.

Sorting of Proteins to Mitochondria

Some mitochondrial proteins are synthesized on mitochondrial ribosomes.
Most mitochondrial proteins are synthesized on cytosolic ribosomes and imported into mitochondria.
Mitochondrially destined proteins contain a targeting sequence (MTS) at their N-terminus.

Mitochondrial Import

Only actively respiring mitochondria can import proteins.
Translocation requires a proton-motive force across the mitochondrial membranes.
Proteins must be unfolded and unassisted to cross these membranes.

Targeting to Sub-Mitochondrial Compartments

Proteins possess different targeting sequences to reach specific compartments within mitochondria (matrix, inner membrane, intermembrane space, outer membrane).
These target sequences include matrix-targeting sequences, intermembrane-space targeting sequences, and outer-membrane localization sequences.

Peroxisomal Proteins

Peroxisomes are single-membrane bound organelles containing oxidases and catalase.
Peroxisomal proteins are synthesized on cytosolic ribosomes and imported in a folded state.
Most require a C-terminal peroxisomal targeting signal (PTS1), often composed of a SKL motif (serine-lysine-leucine).
Some also contain an N-terminal PTS2.

Mechanism of Translocation into Peroxisome

A cytosolic receptor (Pex5) binds the peroxisomal targeting signal (PTS1).
This complex then associates with membrane-bound peroxisome receptors (Pex14 for PTS1 cargo).
Translocation is induced, and the mature protein is directed to the peroxisome compartment.

Zellweger Syndrome

This rare genetic disorder involves defective peroxisomes, impairing transport of proteins to peroxisomes.
Affected individuals often have neurological and liver abnormalities, leading to significant health issues.

Transport Across the Nuclear Membrane

The nucleus is surrounded by a double membrane with nuclear pores.
Passive diffusion and energy-dependent transport mechanisms govern movement through these pores.
Larger molecules and proteins require active transport.

NPC Structure

Nuclear pore complexes (NPCs) are protein assemblies within the nuclear envelope.
NPCs have a complex structure with cytoplasmic filaments, a central transporter, and a nuclear basket.

FG Nucleoporins and Transporters

FG nucleoporins are a critical component of NPCs and contain multiple hydrophobic FG repeats.
FG-repeats interact with the hydrophobic regions of nuclear transport proteins.
These nucleoporins are important structural elements for regulating transport via the NPCs.

Import into the Nucleus

Proteins intended for the nucleus contain a nuclear localization signal (NLS).
Importins and exportins are important protein-transport factors for import into the nucleus.

Mechanism for Nuclear Import

Free cytosolic importins bind to the NLS of cargo proteins.
The complex passes through NPCs.
Conformational change of importin upon entering the nucleus.
Cargo release, followed by return of importins to the cytosol.

Export out of the Nucleus

Proteins and other molecules intended for export from the nucleus possess a nuclear export signal (NES).
Exportins and GTP hydrolysis regulate export.

Summary: Import and Export of Proteins through NPC

Cargo proteins often possess both NLS and NES sequences.
Ran-GTP and Ran-GDP are required for import and export mechanisms, respectively.
Different conformations of Ran regulate import and export.

GTP Switch Proteins

GTP-binding proteins are critical components of cellular signal transduction.
GTP-binding proteins cycle between active (GTP-bound) and inactive (GDP-bound) states.
Activation or deactivation is mediated by GEFs and GAPs, respectively.

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Description

Test your understanding of cellular transport mechanisms, focusing on nuclear-localization signals, importin, and exportin functions. Evaluate how cargo proteins are managed in cellular compartments and the role of GTP-binding proteins. This quiz covers key concepts relevant to cell biology.