Cell Biology: Protein Glycosylation and ER Functions

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Questions and Answers

What triggers the Unfolded Protein Response (UPR)?

Slow folding of proteins (correct)

Excessive protein degradation

Increased vesicle formation

High levels of protein synthesis

What sequence is necessary for N-linked glycosylation to occur?

Asn-X-Ser/Thr (correct)

X-Asn-Ser

Ser-X-Thr

Thr-X-Asn

Which of the following proteins will NOT exit the ER?

Properly folded proteins

Correctly assembled multimeric proteins

Proteins with an ER retention signal (KDEL) (correct)

Misfolded proteins retained by chaperones (correct)

What is the main function of chaperones in the ER?

To aid in protein folding and assembly Signup and view all the answers

Which part of the Golgi network faces the plasma membrane?

trans Golgi network Signup and view all the answers

What is the consequence of the Unfolded Protein Response (UPR) being triggered?

Increased synthesis of chaperones Signup and view all the answers

True or False: Vesicles from the ER fuse with the trans Golgi network.

True Signup and view all the answers

What happens to vesicles produced by the cis Golgi network?

They return to the ER or move through the Golgi Signup and view all the answers

What is a characteristic feature of proteins moving through the Golgi apparatus?

Different stacks have different assortments of enzymes. Signup and view all the answers

What is true about constitutive secretion?

It operates continuously and without a signal sequence. Signup and view all the answers

In which process do proteins aggregate and are stored until a signal is received?

Regulated secretion Signup and view all the answers

What initiates the release of insulin from pancreatic β cells?

An increase in blood glucose levels Signup and view all the answers

Which statement about exocytosis is correct?

It can occur via both constitutive and regulated pathways. Signup and view all the answers

The process of exocytosis is primarily responsible for moving which substances to the plasma membrane?

Proteins and lipids Signup and view all the answers

What is the role of SNARE proteins in the secretion process?

They facilitate the fusion of vesicles with membranes. Signup and view all the answers

What is the primary function of lysosomes in a cell?

Digestion of extracellular materials and old organelles Signup and view all the answers

What characterizes transcytosis in cellular processes?

Movement of receptors/cargo to a new plasma membrane area Signup and view all the answers

In receptor-mediated endocytosis, what determines the fate of receptor proteins after endocytosis?

The type of receptor involved Signup and view all the answers

Which term describes the process of engulfing large particles by immune cells?

Phagocytosis Signup and view all the answers

What is the visible result of autophagy within a cell?

Creation of an autophagosome enclosing an old organelle Signup and view all the answers

What is the primary structural feature of a clathrin subunit?

Three-legged structure called a triskelion Signup and view all the answers

Which protein is responsible for pinching off the clathrin-coated vesicle?

Dynamin Signup and view all the answers

What role do adaptins play in clathrin-coated vesicle formation?

They mediate the connection between coat proteins and cargo receptors Signup and view all the answers

What is the function of Kinesins in vesicle transport?

Moving cargo from the cell body toward the cell periphery Signup and view all the answers

Which type of coated vesicle is involved in anterograde transport from the ER to the Golgi apparatus?

COPII Signup and view all the answers

What occurs during the initial stages of vesicle formation?

Cargo receptors binding transport signals on cargo molecules Signup and view all the answers

What defines the function of Dynein in vesicle transport?

It moves cargo from the cell periphery back to the cell body Signup and view all the answers

What type of structure do clathrin molecules form during vesicle budding?

Basketlike cages Signup and view all the answers

What is the primary role of the Golgi apparatus in the vesicular transport pathway following cargo arrival from the ER?

Protein sorting and modification Signup and view all the answers

Which process involves the uptake of fluids and molecules in small vesicles?

Pinocytosis Signup and view all the answers

What is characterized by the uptake of particles into large vesicles called phagosomes?

Phagocytosis Signup and view all the answers

Which type of endocytosis is selective for specific macromolecules?

Receptor-mediated endocytosis Signup and view all the answers

What happens to vesicles that fuse with lysosomes after endocytosis?

They degrade their contents or release viral genomes. Signup and view all the answers

Which of the following is NOT a type of endocytosis?

Exocytosis Signup and view all the answers

What role do endosomes play in exocytosis and endocytosis?

They serve as sorting compartments. Signup and view all the answers

During which process can viruses enter cells, often resulting in the virus’s genome being released?

Endocytosis Signup and view all the answers

Study Notes

N-linked Glycosylation

The amino acid sequence Asn-X-Ser/Thr on a protein is required for N-linked glycosylation.
Each oligosaccharide chain is transferred as a whole unit to the Asn from a lipid called dolichol.
Oligosaccharides are further modified in the ER and Golgi apparatus.

ER Exit

The exit from the ER is controlled to ensure protein quality.
Proteins do not leave the ER if they:
- have an ER retention signal (KDEL sequence at the C-terminus)
- are misfolded
  - retained by chaperones
- are incorrectly assembled into multimeric proteins
  - retained by chaperones

Unfolded Protein Response (UPR)

The UPR is triggered if protein folding takes too long in the ER.
The UPR increases ER and folding machinery (chaperones) to help fold proteins quicker.
The UPR slows down ER protein synthesis to give folding machinery time to catch up.

Golgi Network

cis Golgi network: faces the ER
- buds vesicles that return to the ER or continue through the Golgi.
trans Golgi network: faces the plasma membrane
- buds vesicles headed to the plasma membrane or other compartments.
Vesicles from the ER do not fuse with the trans Golgi network.

Golgi Modifications

Different stacks of the Golgi contain different enzymes, allowing for progressive processing of proteins as they move from the cis to the trans face.
Proteins exiting the trans Golgi network are sorted based on their next destination.

Exocytosis

Constitutive secretion:
- the default pathway, does not require a specific signal sequence
- continuous process
Regulated secretion:
- occurs in some cells
- proteins aggregate and are stored in secretory vesicles until a signal is received.
An example of regulated secretion:
- Increased glucose levels in the blood trigger the release of insulin from pancreatic β cells.
- This process involves glucose metabolism, ATP-sensitive K+ channels, voltage-gated Ca2+ channels, and SNARE proteins.
In this process, increased ATP levels activate K+ channels and do not hyperpolarize the membrane.

Endocytosis

Endocytosis is the uptake of material into a cell by membrane invagination and internalization of the resulting vesicle.
Phagocytosis:
- uptake of particles into large vesicles (phagosomes)
- occurs in phagocytic cells (dendritic cells, macrophages, and neutrophils)
- Phagosomes fuse with lysosomes.
Pinocytosis:
- uptake of fluids and molecules in small vesicles.
- Occurs via clathrin-coated pits and vesicles.
- Balanced with exocytosis.
- Vesicles fuse with endosomes.
- Pinocytosis is generally a non-specific process
Receptor-mediated endocytosis:
- selective endocytosis of specific macromolecules.
- LDL, low-density lipoproteins (cholesterol + proteins)

Viruses and Endocytosis

Viruses can enter cells through receptor-mediated endocytosis.
These vesicles can fuse with lysosomes leading to the release of the viral genome into the cytoplasm and subsequent replication.

Endosomes

Endosomes are sorting compartments.
Receptors can be recycled to the same plasma membrane, directed to lysosomes for degradation, or transcytosed to a new plasma membrane area.

Clathrin Coats

Clathrin-coated vesicles are a major component of the cell's transport system.
The primary protein in clathrin-coated vesicles is clathrin.
Each clathrin subunit is made of three large and three small polypeptide chains that form a three-legged structure called a triskelion.
Triskelions readily assemble into polyhedral cages.

Vesicle Formation

Clathrin-coated vesicle formation is driven by the assembly of a protein coat.
Cargo receptors recognize transport signals on cargo molecules and bind to them.
Adaptins connect coat proteins to cargo receptors.
Clathrin proteins form basket-like cages that shape membranes into vesicles.
Dynamin, a GTP-binding protein, pinches off the vesicle.
The clathrin coat disassembles after the vesicle is pinched off.

Coated Vesicles

COP (coat protein) coated vesicles mediate membrane transport.
COPII mediates anterograde transport from the ER to the Golgi.
COPI mediates retrograde transport from the Golgi to the ER.
Clathrin participates in endocytosis at the plasma membrane and transport between the Golgi and endosomes.

Vesicle Movement

Kinesins move cargo from the cell body towards the cell periphery (anterograde transport).
Dynein moves cargo back toward the cell body (retrograde transport).

Vesicle Fusion

Protein-protein interactions mediate the fusion of transport vesicles with appropriate membranes.

Receptor-mediated Endocytosis

The fate of receptor proteins after endocytosis depends on the type of receptor.

Lysosomes

Lysosomes are the sites of intracellular digestion.
Lysosomes digest extracellular materials and old organelles.
They are acidic due to the activity of an H+ pump.
They contain transporters that export metabolites.
Lysosomal membrane proteins are glycosylated for protection.
The signal sequence for Lysosomal targeting is mannose-6-phosphate (M6P), recognized at the trans Golgi network.

Lysosomal Pathways

Autophagy: enclosure of an old organelle in a double membrane, creating an autophagosome.

Phagocytosis

Phagocytosis is the uptake of large particles by certain cells, primarily immune cells like macrophages and neutrophils.

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Description

Explore the critical processes of N-linked glycosylation, ER exit mechanisms, and the unfolded protein response in this informative quiz. Understand how proteins are modified and monitored for quality control within the cell. Test your knowledge on the Golgi network's role in protein processing.