1.9 - trafficking - Bioch / OB

Question 1

trafficking

Answer 1

refers to the processes by which proteins are directed to specific locations within or outside of the cell after they are synthesised

Question 2

vesicle trafficking route

Answer 2

from endoplasmic reticulum to the golgi apparatus to the plasmalemma or to the lysosome

Question 3

Step 1: Translation and Entry into the ER

Answer 3

Signal sequences (short peptides) on the nascent proteins direct ribosomes to the rough ER (rER).

As the protein is being synthesized, it enters the ER lumen through a translocon.

In the ER, proteins are folded and undergo post-translational modifications, such as glycosylation.

Question 4

Step 2: ER to Golgi Transport

Answer 4

proteins are packaged into COPII-coated vesicles that bud off from the ER and move toward the Golgi apparatus via vesicular transport

Question 5

Step 3: Processing in the Golgi Apparatus

Answer 5

Cis-Golgi: Receives vesicles from the ER.

Trans-Golgi Network (TGN): Directs proteins to their final destination.

Further modifications occur, such as:
Glycosylation (addition of sugar chains)

Sorting into different vesicles depending on their final target.

Question 6

Step 4: Sorting and Vesicular Transport

Answer 6

sorted in vesicles to its destination

Question 7

key destinations for proteins from golgi apparatus

Answer 7

Plasma Membrane

Secretion (Extracellular Space)

Lysosomes

Endosomes

Other Organelles (e.g., peroxisomes, mitochondria)

Question 8

What is the purpose of protein sorting and vesicular transport

Answer 8

To direct proteins to their correct cellular destinations (e.g., plasma membrane, secretion, lysosomes, endosomes, organelles) after leaving the Golgi apparatus

Question 9

How are lysosomal proteins sorted?

Answer 9

Lysosomal proteins are tagged with mannose-6-phosphate (M6P) in the trans-Golgi, recognized by M6P receptors, and sent to lysosomes in clathrin-coated vesicles

Question 10

What role do endosomes play in protein sorting?

Answer 10

Endosomes act as sorting centers, directing proteins to their final destinations (e.g., lysosomes for degradation or recycling back to the plasma membrane).

Question 11

What types of vesicles are involved in protein transport?

Answer 11

Clathrin-coated vesicles: Transport to lysosomes or endosomes.

COPII-coated vesicles: Transport from the ER to Golgi.

COPI-coated vesicles: Retrograde transport from Golgi back to ER.

Question 12

What proteins mediate vesicle fusion with target membranes?

Answer 12

SNARE proteins facilitate vesicle fusion, aided by Rab GTPases for targeting specificity

Question 13

How does the cell ensure that misfolded proteins are not secreted?

Answer 13

Misfolded proteins are recognized in the Golgi and are sent to lysosomes for destruction instead of secretion, through changes in vesicle coat protein configuration.

Question 14

What is the primary function of trafficking proteins to the plasma membrane?

Answer 14

add materials to the plasma membrane or to secrete proteins into the extracellular space, maintaining cellular functions and communication.

Question 15

What is constitutive secretion, and how does it function?

Answer 15

continuous process by which proteins are transported from the Golgi apparatus to the plasma membrane and secreted into the extracellular space without the need for specific signals

Question 16

Describe the mechanism of constitutive secretion.

Answer 16

In constitutive secretion, vesicles containing proteins bud off from the trans-Golgi network and move directly to the plasma membrane, where they fuse and release their contents into the extracellular space

involves SNARE proteins

Question 17

What is regulated secretion, and how does it differ from constitutive secretion?

Answer 17

occurs in response to specific stimuli, where proteins are stored in secretory vesicles and released only when a signal, such as a rise in calcium ions (Ca²⁺), triggers their fusion with the plasma membrane

Question 18

Explain the mechanism of regulated secretion.

Answer 18

In regulated secretion, proteins are packaged into secretory vesicles that accumulate near the plasma membrane but do not immediately fuse. Upon receiving a specific signal (e.g., a rise in Ca²⁺ concentration), these vesicles undergo fusion, releasing their contents into the extracellular space.

Question 19

Provide examples of regulated secretion in biological systems

Answer 19

Neurotransmitter release at synapses (e.g., acetylcholine at neuromuscular junctions).

Hormone secretion from endocrine glands (e.g., insulin from pancreatic beta cells).

Digestive enzyme release from pancreatic acinar cells.

Question 20

What role do secretory vesicles play in trafficking to the plasma membrane?

Answer 20

Secretory vesicles are membrane-bound compartments that transport proteins from the Golgi apparatus to the plasma membrane, facilitating both constitutive and regulated secretion.

Question 21

What is the primary function of the microtubule network in cells?

Answer 21

The microtubule network provides structural support and serves as tracks for the movement of vesicles and organelles within the cell.

Question 22

What are motor proteins, and what role do they play in cellular trafficking?

Answer 22

specialized proteins that transport vesicles and organelles along the microtubule network, ensuring proper distribution of cellular content

Question 23

Name the two types of motor proteins involved in cellular trafficking and their functions.

Answer 23

Dyneins: Responsible for retrograde transport, moving vesicles towards the microtubule organizing center (often the cell center).

Kinesins: Responsible for anterograde transport, moving vesicles away from the microtubule organizing center towards the plasma membrane (plasmalemma).

Question 24

What is the vesicular roundabout, and where is it typically located in the cell?

Answer 24

vesicular roundabout is a microtubule organising center, usually present as a single structure in each cell, facilitating the organisation and transport of vesicles

acts as a central hub for directing the movement of vesicles, coordinating the efficient trafficking of materials to various regions of the cel

Question 25

What is receptor-mediated endocytosis?

Answer 25

It is a process by which cells internalize specific molecules (ligands) by binding them to receptors on the cell surface, allowing for efficient and selective uptake

Question 26

What are the main steps in receptor-mediated endocytosis?

Answer 26

Ligand Binding: Ligands bind to their specific receptors on the plasma membrane.

Membrane Invagination: The membrane folds inward to form a pocket around the ligand-receptor complex.

Vesicle Formation: The pocket pinches off to create a clathrin-coated vesicle containing the bound ligands.

Uncoating: Clathrin proteins are removed from the vesicle.

Fusion with Endosome: The vesicle fuses with an early endosome for sorting.

Recycling: Receptors may be recycled back to the membrane, while ligands are directed to lysosomes or used in the cell.

Question 27

What are ligands in receptor-mediated endocytosis?

Answer 27

Ligands are specific molecules (such as hormones, nutrients, or signaling molecules) that bind to receptors on the cell membrane, initiating the endocytosis process

Question 28

What is the role of clathrins in endocytosis?

Answer 28

Clathrins are proteins that coat the cytoplasmic side of the invaginated membrane, forming a basket-like structure. This helps stabilize and shape the vesicle during its formation and internalization

Question 29

Why is receptor-mediated endocytosis important for cells?

Answer 29

allows for selective uptake of critical molecules, regulating nutrient intake, cellular signaling, and maintaining homeostasis. This process is crucial for functions such as nutrient absorption, hormone signaling, and immune responses.

Question 30

What is an endosome and its role?

Answer 30

a membrane-bound vesicle formed after the fusion of a clathrin-coated vesicle with the cell membrane.

sorts the internalized materials, directing them either to lysosomes for degradation or back to the plasma membrane for recycling.

Question 31

What is transcytosis?

Answer 31

cellular transport process that involves the uptake of substances (such as proteins or nutrients) at one side of a cell, followed by their transport across the cell, and release on the opposite side

Question 32

What are the main steps involved in transcytosis?

Answer 32

Uptake: Ligands bind to receptors on the cell surface, and the membrane invaginates to form an endocytic vesicle.

Transport: The vesicle is transported across the cell via the cytoskeleton, often using microtubules and motor proteins (like kinesins).

Release: The vesicle fuses with the opposite membrane, releasing the ligands into the extracellular space.

Question 33

Can you provide examples of transcytosis?

Answer 33

Antibody Transport: Maternal antibodies are transported across the placenta to the fetus.

Nutrient Absorption: Transcytosis in intestinal epithelial cells allows the uptake of nutrients from the gut lumen into the bloodstream.

Question 34

What are signal sequences in proteins?

Answer 34

short, specific peptide sequences located at the N-terminus of newly synthesised proteins that direct the protein to its appropriate cellular location

Question 35

What is a Signal Recognition Particle (SRP)?

Answer 35

An SRP is a ribonucleoprotein complex that recognizes and binds to the signal sequence of a nascent polypeptide as it emerges from the ribosome during translation.

Question 36

mechanism of SRP

Answer 36

When the SRP binds to the signal sequence, it halts translation temporarily.

The SRP-ribosome complex is then directed to the SRP receptor on the ER membrane, allowing the ribosome to dock onto the translocon.

Once this occurs, translation resumes, and the nascent polypeptide is translocated into the ER.