Cell Biology: Vesicular Transport and Membrane Topology

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Questions and Answers

What drives the budding process of vesicles?

Fusion with the target membrane

Energy released from ATP hydrolysis

Assembly of a protein coat (correct)

Active transport mechanisms

Which GTPase is responsible for COPII coat assembly at the ER membrane?

ARF

Rab

Rho

Sar1 (correct)

What is the primary role of cargo within vesicles?

To facilitate membrane fusion

To be transported from donor to target compartments (correct)

To provide structural support for vesicle integrity

To promote ATP synthesis

What is the function of Sar1-GEF in the vesicle transport process?

It regulates the spatial activity of Sar1 Signup and view all the answers

Which pathway involves retrieving vesicles back to the ER?

Retrieval pathway Signup and view all the answers

What does COP stand for in the context of vesicle transport?

Coat Protein Signup and view all the answers

Which of the following is NOT a component found in the vesicles?

DNA Signup and view all the answers

What type of process is involved in the targeting of vesicles to their respective compartments?

Signal-dependent sorting Signup and view all the answers

What role do specific Rab proteins play in the targeting process of vesicles?

They are recognized by tethering proteins on the target membrane. Signup and view all the answers

What is the primary function of SNARE proteins in vesicular transport?

To facilitate fusion between the vesicle and the target membrane. Signup and view all the answers

How close must membranes be to allow for fusion during vesicular transport?

1.5 nm Signup and view all the answers

Which of the following pairs of proteins is unique to each transport vesicle-target membrane interaction?

Both Rab-tether and v-SNARE-t-SNARE pairs Signup and view all the answers

What happens to the cargo once a vesicle fuses with the target membrane?

It is released from the cargo receptor into the target compartment. Signup and view all the answers

What is the primary function of COPII adaptor proteins in vesicle formation?

They select cargo proteins to be packaged. Signup and view all the answers

Which step follows the recruitment of COPII adaptor proteins in COPII vesicle formation?

Initiation of ER membrane deformation. Signup and view all the answers

What is an essential characteristic of membrane-bound proteins with exit signals?

They act as cargo receptors for soluble proteins. Signup and view all the answers

What dual role does the coat of a vesicle serve during cargo transport?

It concentrates cargo and deforms the membrane. Signup and view all the answers

Which of the following is typically true about exit signals found on cargo proteins?

They are often amino acid sequences on the proteins. Signup and view all the answers

What consequence can occur when resident ER proteins are encapsulated within vesicles?

They can potentially return to the ER. Signup and view all the answers

How do vesicles ensure they reach the correct target compartment?

By recognizing specific receptor molecules. Signup and view all the answers

What is one reason resident ER proteins might end up in vesicles despite not having exit signals?

They can occasionally be trapped during vesicle formation. Signup and view all the answers

Which mechanism does NOT participate in the trafficking of proteins to cellular compartments?

Folding in the cytosol followed by translocation Signup and view all the answers

What is the role of vesicles in the endomembrane system?

Connect membrane compartments and transport proteins Signup and view all the answers

Which of the following correctly describes the relationship between membrane topology in the ER and the plasma membrane?

The leaflet facing the cytoplasm in the ER also faces the cytoplasm in the plasma membrane Signup and view all the answers

What type of proteins do vesicles transport?

Soluble proteins, membrane proteins, and lipids Signup and view all the answers

What characterizes the endomembrane system?

A collection of membranes that communicate with each other Signup and view all the answers

How are proteins typically delivered to membrane-enclosed organelles?

By both translocation and vesicular mechanisms Signup and view all the answers

In terms of membrane topology, where does the part of a protein that is inside the ER end up?

It is directed outside of the cell Signup and view all the answers

Which statement is true regarding the communication between membranes in the endomembrane system?

Transport vesicles play a critical role in this communication Signup and view all the answers

Study Notes

Vesicular Transport and the Endomembrane System

Proteins are synthesized in the cytosol and trafficked to specific cellular compartments.
Three mechanisms transport proteins to their destinations:
- Fully-folded proteins moving through pores.
- Unfolded polypeptide chains moving through protein translocators.
- Delivery by transport vesicles.
The endomembrane system includes the extracellular space and all membrane-enclosed compartments.
Vesicles facilitate communication between these compartments.
Vesicle transport involves the movement of soluble proteins, membrane proteins, and lipids.

Membrane Topology

The leaflet facing the lumen of the ER or Golgi is the leaflet facing the outside of the cell.
The leaflet facing the cytoplasm of the ER or Golgi faces the cytoplasm of the plasma membrane.
Proteins within the lumen of the ER or Golgi are also on the outside of the cell.
Proteins facing the cytoplasm in the ER or Golgi also face the cytoplasm in the plasma membrane.

Vesicle Transport: Sorting

Vesicles must be sorted to ensure they reach the correct targets.
Each organelle maintains its own identity and protein/lipid compositions.

Vesicle Transport: The Basics

Vesicles bud from a donor compartment and fuse with a target compartment.
Cargo includes soluble proteins, membrane proteins, and lipids.

Vesicle Budding: Protein Coats

Vesicle budding is driven by the assembly of a protein coat.
Different types of coats are used for specific steps in vesicle transport.
COPII coats are responsible for ER to Golgi vesicle traffic.

COPII-Coated Vesicle Formation

The monomeric GTPase Sar1 is responsible for COPII coat assembly.
Sar1’s activity is spatially regulated by an ER-associated Sar1-GEF.
Sar1-GTP recruits COPII adaptor proteins, which:
- Select cargo proteins for packaging.
- Initiate ER membrane deformation.
- Recruit outer coat proteins to form a bud.

Cargo Selection

Cargo proteins are selected based on exit signals, which are often specific amino acid sequences.
Both soluble and membrane-bound proteins have exit signals.
Membrane-bound proteins with exit signals can either:
- Reside in the membrane of another organelle.
- Act as cargo receptors for soluble proteins and be recycled back to the ER.

Coat Function

The inner coat concentrates cargo.
The outer coat, with its curved structure, deforms the membrane and shapes the vesicle.

Vesicle Targeting

Vesicle targeting involves a sequential system to ensure accuracy.
First, Rab proteins on the vesicle surface are recognized by tethering proteins on the target membrane.
Second, v-SNAREs on the vesicle interact with complementary t-SNAREs on the target membrane.
Unique Rab-tether and v-SNARE-t-SNARE pairs are specific to each vesicle-target membrane pair.

Vesicle Fusion with Target

SNARE proteins help dock the vesicle to and fuse it with the target membrane.
SNAREs catalyze the fusion process.
Lipid flow from one bilayer to another requires the membranes to be within 1.5 nm of each other.

Cargo Release

Upon fusion, the cargo receptor releases soluble cargo.

Upcoming Topics

ER exit and quality control.
The Golgi apparatus.
Exocytosis.

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Description

Explore the intricate processes of vesicular transport and membrane topology within the endomembrane system. This quiz covers protein synthesis, transportation mechanisms, and the orientation of proteins in the endoplasmic reticulum and Golgi apparatus. Test your knowledge of cellular compartments and their functions!