Protein Transport and Targeting to the Nucleus

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Questions and Answers

What is the primary distinction between signal-based and vesicle-based protein targeting?

Signal-based targeting is exclusive to the nucleus, while vesicle-based targeting handles all other organelles.
Signal-based targeting relies on lipid modifications, while vesicle-based targeting uses protein modifications.
Signal-based targeting only occurs in prokaryotes, while vesicle-based targeting is unique to eukaryotes.
Signal-based targeting directs proteins to specific organelles using amino-acid sequences, whereas vesicle-based targeting involves transport via vesicles to the Golgi or plasma membrane. (correct)

Which of the following cellular locations is NOT a typical destination for proteins targeted via the signal-based targeting mechanism?

Endoplasmic Reticulum (ER)
Nucleus
Mitochondria
Lysosome (correct)

How does the nuclear envelope structure influence protein targeting to the nucleus?

It is directly connected to the endoplasmic reticulum, simplifying protein entry.
It consists of two membranes, which require specific targeting mechanisms for proteins to cross. (correct)
It contains a single membrane that freely allows all proteins to enter.
It dissolves during protein import, allowing proteins direct access to the nucleoplasm.

What is the role of Importin in nuclear protein targeting?

It binds to proteins with a nuclear localization sequence (NLS) and transports them through the nuclear pore complex. (B) Signup and view all the answers

How does Ran-GTP contribute to releasing a protein with a nuclear localization sequence (NLS) inside the nucleus?

Ran-GTP binds to Importin, causing it to release the NLS-carrying protein. (B) Signup and view all the answers

Following the release of a nuclear protein, how is Importin recycled back to the cytoplasm?

The Ran-GTP/Importin complex exits, and GTP is hydrolyzed to GDP by GAP, releasing Importin. (A) Signup and view all the answers

What role does the Tom complex play in protein targeting to the mitochondria?

It recognizes the N-terminal signal sequence of proteins destined for the outer mitochondrial membrane and allows the proteins to pass through the outer membrane. (C) Signup and view all the answers

During protein targeting to the ER lumen, what initially binds to the ER signal sequence on a nascent protein?

Signal Recognition Particle (SRP) (C) Signup and view all the answers

What is the function of the translocon during ER protein targeting?

It forms a channel through the ER membrane, allowing the nascent protein to enter the ER lumen. (C) Signup and view all the answers

How is the energy provided to open the translocon channel for protein entry into the ER lumen?

Hydrolysis of GTP bound to the SRP and SRP receptor. (B) Signup and view all the answers

What is the initial step in vesicle-based trafficking?

Proteins are synthesized by ribosomes and translocated into the ER lumen. (C) Signup and view all the answers

Following the ER, what is the next destination for proteins in vesicle-based trafficking?

Cis-Golgi (A) Signup and view all the answers

What is the role of retrograde transport in the Golgi apparatus?

To transport proteins from the cis-Golgi to the ER or retain immature proteins within the Golgi. (B) Signup and view all the answers

How does the Golgi apparatus mature during vesicle-based trafficking?

Through the transport of Golgi enzymes and other proteins from Golgi cisternae from the cis to the trans face. (D) Signup and view all the answers

What typically occurs to proteins in the trans-Golgi network before they reach their final destination?

They are sorted and packaged into vesicles for delivery to their final destination. (D) Signup and view all the answers

What post-translational modification occurs to Proalbumin as it moves to its target?

Proalbumin is cleaved by a furin-type endopeptidase into Albumin. (D) Signup and view all the answers

Which class of proteins utilizes vesicle-based trafficking?

Lysosomal proteins (B) Signup and view all the answers

What is the primary role of GTP hydrolysis in the context of SRP and SRP receptor interaction during protein targeting to the ER?

It provides the energy necessary to open the translocon channel. (C) Signup and view all the answers

During nuclear protein import, what would be the consequence of a non-functional Guanine Exchange Factor (GEF) in the nucleus?

Ran-GTP would be unable to displace the NLS-carrying protein from Importin. (C) Signup and view all the answers

In vesicle-based trafficking, what is the most direct consequence if a cell lacks the ability to form functional COPII-coated vesicles at the ER?

Proteins normally secreted would accumulate in the ER. (B) Signup and view all the answers

If a protein is found to be glycosylated and sorted into vesicles for secretion, which of the cellular compartments would it have passed through during its trafficking?

ER → Golgi → Plasma Membrane (C) Signup and view all the answers

A mutation in a mitochondrial protein's N-terminal signal sequence prevents it from binding to the Tom complex. What is the most likely consequence?

The protein will remain in the cytosol. (A) Signup and view all the answers

A researcher discovers a new protein that is synthesized as an inactive precursor. It becomes activated in the trans-Golgi network via cleavage by a furin-type endopeptidase. If the furin cleavage site is mutated such that the enzyme can no longer recognize and cleave the precursor protein, what is the most likely outcome?

The precursor protein will be secreted in its inactive form. (D) Signup and view all the answers

A cell line is engineered to express a mutant SRP receptor that cannot bind GTP. What is the most likely consequence for proteins that are normally targeted to the ER?

The proteins will be synthesized, will associate with ribosomes, but will not be able to dock at the ER membrane. (B) Signup and view all the answers

Which of the following features distinguishes vesicle-based targeting from signal-based targeting?

The use of transport vesicles to move cargo between cellular compartments (C) Signup and view all the answers

What is the impact on nuclear protein import if the Ran-GAP protein in the cytoplasm is non-functional?

Importin will remain bound to Ran-GTP and will not release cargo proteins in the cytoplasm for subsequent rounds of import (D) Signup and view all the answers

Which of the following protein sorting pathways relies primarily on post-translational modifications, such as glycosylation and sorting signals, to direct proteins to their final destination?

Vesicle based targeting in the Golgi apparatus (B) Signup and view all the answers

How does signal-based targeting ensure that proteins reach the correct destination within an organelle containing multiple compartments, such as the mitochondria?

The initial signal sequence is cleaved off and a new signal sequence is added for the next compartment (D) Signup and view all the answers

If the SRP is unable to bind to the signal sequence of a protein destined for the ER, what is the most likely outcome?

The protein will be synthesized in the cytoplasm, and the ribosome will not be targeted to the ER. (A) Signup and view all the answers

A cell has a mutation that prevents cisternal maturation in the Golgi. What is the most direct consequence of this mutation?

Proteins will be unable to move from the cis-Golgi network to the trans-Golgi network (A) Signup and view all the answers

What is the primary purpose of synthesizing some proteins as inactive pro-proteins that are cleaved in the Golgi?

To control the timing and location of protein activity. (A) Signup and view all the answers

Which proteins rely on the Sec61 complex (translocon) for their integration into the ER membrane?

Proteins with a signal sequence (A) Signup and view all the answers

How is protein targeting to the nucleus affected if a cell lacks functional nuclear localization signal (NLS) receptors?

Proteins that require NLS for entry will remain in the cytoplasm (B) Signup and view all the answers

How does depletion of GTP affect protein targeting to the ER?

Proteins wouldn't be translocated into the ER (B) Signup and view all the answers

Which of the processes listed here relies on GTP hydrolysis?

Opening a channel called a translocon in the SRP receptor (C) Signup and view all the answers

How does altering the pH of the Golgi apparatus affect protein targeting?

It alters the glycoslyation proteins resulting in missorting. (D) Signup and view all the answers

Which of the mechanisms is in vesicle-based trafficking?

Proteins are synthesized by ribosomes and translocated into the ER lumen. (D) Signup and view all the answers

What is a function of a cisternal?

All of the above (D) Signup and view all the answers

Flashcards

Protein Targeting Overview

mRNA is made in the nucleus and then moved to the cytoplasm. Ribosomes then use the mRNA to make specific proteins.

Protein Targeting Mechanisms

Proteins are delivered to the right parts of the cell using signal-based targeting or vesicle-based targeting.

Signal-based targeting definition

Proteins go to specific spots in the cytoplasm, ER, mitochondria, nucleus, and peroxisomes. These proteins have special sequences which guide them.

Vesicle-based targeting

Proteins are secreted outside the cell, put into the plasma membrane, or sent to the Golgi or lysosomes via vesicles.