5 Golgi Apparatus, Endocytosis, and Exocytosis

Describe the structure of the Golgi apparatus. What are its main compartments, and what are their functions?

• The Golgi apparatus is a central organelle within the endomembrane system responsible for processing, modifying, sorting, and packaging proteins and lipids. Its structure is characterized by a stack of flattened, membrane-bound sacs, or cisternae. The Golgi is typically organized into four main compartments:

  • Cis-Golgi network (CGN): This compartment is closest to the endoplasmic reticulum (ER) and functions as the entry point for proteins and lipids arriving from the ER in transport vesicles.

  • Cis cisternae: These are the cisternae closest to the CGN and are involved in early stages of protein modification.

  • Medial cisternae: These are the middle cisternae of the Golgi stack, where further modifications, such as glycosylation, take place.

  • Trans cisternae: These cisternae are farthest from the ER and are the site of final processing and sorting of proteins and lipids before they are packaged into vesicles for transport to their final destinations.

  • Trans-Golgi network (TGN): The TGN is a network of tubules and vesicles that buds from the trans cisternae and functions as the exit point of the Golgi.

Explain the cisternal maturation model of Golgi function. How do proteins and lipids move through the Golgi apparatus?

• The cisternal maturation model proposes that Golgi cisternae are dynamic structures that move from the cis face to the trans face of the Golgi. As cisternae mature, they carry their cargo of proteins and lipids with them. This model suggests:

  • New cis cisternae are continuously formed by the fusion of vesicles arriving from the ER.

  • As the cis cisternae mature, they move towards the trans face, undergoing a series of modifications.

  • Enzymes responsible for specific modifications are thought to be carried in vesicles that move in a retrograde direction, from later to earlier cisternae, maintaining the proper enzymatic composition of each compartment.

  • Finally, the trans cisternae break down into vesicles at the TGN, delivering the processed proteins and lipids to their final destinations.

What are the major types of glycosylation that occur in the Golgi apparatus? What are their functions?

• Glycosylation, the addition of carbohydrate chains to proteins and lipids, is a major modification that takes place in the Golgi apparatus. Several types of glycosylation occur in the Golgi, with each type having specific functions:

  • N-linked glycosylation: As mentioned in our previous conversation, N-linked glycosylation is initiated in the ER, where a preassembled oligosaccharide is attached to an asparagine residue. In the Golgi, this oligosaccharide is further modified by the addition and removal of sugar residues, resulting in a diverse array of N-linked glycans. These modifications are important for protein folding, stability, and function.

  • O-linked glycosylation: In O-linked glycosylation, sugar residues are attached to the hydroxyl group of serine or threonine residues. These modifications often play roles in cell signaling, cell adhesion, and protein recognition.

What is the role of the Golgi apparatus in lipid metabolism?

In addition to protein processing, the Golgi apparatus is also involved in lipid metabolism, particularly the synthesis and modification of sphingolipids, a class of lipids important for membrane structure and function.

How are lysosomes formed? What is the role of the Golgi apparatus in this process?

• Lysosomes, the cellular "recycling centers", are membrane-bound organelles containing a variety of hydrolytic enzymes for degrading macromolecules. Lysosome formation involves the following steps:

  • Hydrolytic enzymes are synthesized in the ER and transported to the Golgi.

  • In the cis Golgi, these enzymes are tagged with mannose-6-phosphate (M6P) residues, a specific recognition signal for lysosomal enzymes.

  • M6P receptors in the trans Golgi network (TGN) bind to the M6P-tagged enzymes.

  • The receptor-enzyme complexes are packaged into clathrin-coated vesicles that bud from the TGN.

  • These vesicles fuse with late endosomes, delivering the lysosomal enzymes.

  • The late endosome matures into a lysosome as the pH becomes more acidic and the hydrolytic enzymes become active.

Describe the process of vesicle budding. What are the main types of vesicles, and what proteins are involved in their formation?

• Vesicle budding is a fundamental process for intracellular transport, allowing molecules to move between the compartments of the endomembrane system. Different types of vesicles mediate transport between different compartments, with specific proteins involved in their formation:

  • COP-II coated vesicles: Transport cargo from the ER to the Golgi.

  • COP-I coated vesicles: Mediate retrograde transport within the Golgi and from the Golgi back to the ER.

  • Clathrin-coated vesicles: Transport cargo from the TGN to lysosomes, endosomes, and the plasma membrane.

Explain clathrin-dependent vesicle formation. What is the role of the triskelion and adaptor proteins?

• Clathrin-coated vesicles are formed through a highly regulated process involving clathrin, adaptor proteins, and dynamin:

  • Clathrin molecules, composed of three heavy chains and three light chains, assemble into a characteristic triskelion structure.

  • Triskelions polymerize to form a lattice-like coat on the cytosolic surface of the membrane, driving vesicle formation.

  • Adaptor proteins bind to both clathrin and specific cargo receptors, selecting the cargo to be incorporated into the vesicle.

  • Dynamin, a GTPase, constricts the neck of the budding vesicle and pinches it off from the donor membrane.

When and how do COP-I and COP-II vesicle budding events occur?

COP-I and COP-II coated vesicles are formed by similar mechanisms to clathrin-coated vesicles, but they use different coat proteins and adaptor proteins. COP-II vesicles bud from the ER and transport cargo to the Golgi, while COP-I vesicles mediate retrograde transport within the Golgi and from the Golgi back to the ER. The timing and regulation of these budding events are crucial for maintaining the proper flow of proteins and lipids through the endomembrane system.

What is the role of the Rab GTPase in vesicle targeting?

• Rab GTPases are a family of small GTP-binding proteins that play a crucial role in vesicle targeting and fusion. Different Rabs are associated with different compartments of the endomembrane system.

• A Rab on the surface of a transport vesicle interacts with its specific effector protein on the target membrane, ensuring that the vesicle is delivered to the correct destination.

Explain the mechanism of vesicle fusion mediated by SNARE proteins. What is the difference between V-SNAREs and T-SNAREs?

• SNARE proteins (soluble NSF attachment protein receptors) mediate the fusion of vesicles with their target membranes. There are two types of SNAREs:

  • v-SNAREs (vesicle-SNAREs), located on the vesicle membrane

  • t-SNAREs (target-SNAREs), located on the target membrane.

• v-SNAREs and t-SNAREs interact with each other, forming a tight complex that brings the vesicle and target membranes into close proximity, driving membrane fusion and the release of the vesicle's contents.

What are the main functions of lysosomes?

• Lysosomes are essential for a variety of cellular processes:

  • Degradation of cellular waste products: Lysosomes break down damaged organelles, misfolded proteins, and other cellular debris.

  • Digestion of extracellular materials: Lysosomes fuse with phagosomes, vesicles that engulf extracellular materials, to degrade the ingested materials.

  • Autophagy: Lysosomes are involved in autophagy, a process by which cells degrade their own components to recycle nutrients and eliminate damaged organelles.

How are materials degraded inside lysosomes?

The acidic environment of lysosomes, maintained by proton pumps that pump protons into the lumen, is optimal for the activity of the hydrolytic enzymes responsible for degradation. These enzymes include proteases, lipases, nucleases, and glycosidases, which break down proteins, lipids, nucleic acids, and carbohydrates, respectively.

What are the main differences between lysosomes and peroxisomes?

• Lysosomes and peroxisomes are both involved in cellular degradation, but they differ in several key aspects:

  • Enzyme content: Lysosomes contain a variety of hydrolytic enzymes, while peroxisomes contain oxidative enzymes that generate hydrogen peroxide (H2O2).

  • Origin: Lysosomes originate from the Golgi apparatus, while peroxisomes are thought to replicate by division.

  • Functions: Lysosomes are involved in general degradation and recycling, while peroxisomes are involved in specific metabolic pathways, such as fatty acid oxidation and detoxification.

What diseases are associated with lysosomal dysfunction?

• Defects in lysosomal function can lead to a group of inherited disorders called lysosomal storage diseases. These diseases are characterized by the accumulation of undegraded materials within lysosomes, leading to cellular dysfunction and a variety of clinical symptoms. Examples include:

  • Tay-Sachs disease: Caused by a deficiency in the enzyme hexosaminidase A, leading to the accumulation of gangliosides in the brain.

  • Gaucher disease: Caused by a deficiency in the enzyme glucocerebrosidase, leading to the accumulation of glucocerebroside in various tissues.

  • Pompe disease: Caused by a deficiency in the enzyme acid alpha-glucosidase, leading to the accumulation of glycogen in lysosomes.

Describe the different types of exocytosis.

• Exocytosis is a process by which cells release materials from their cytoplasm to the extracellular environment. There are different types of exocytosis, including:

  • Constitutive exocytosis: This is a continuous process that operates in all cells, delivering newly synthesized proteins and lipids to the plasma membrane.

  • Regulated exocytosis: This type of exocytosis is triggered by specific signals, such as the binding of a hormone or neurotransmitter to a cell surface receptor. It is responsible for the release of hormones, neurotransmitters, and other signaling molecules.

Explain the process of receptor-mediated endocytosis. Give an example.

• Receptor-mediated endocytosis is a selective process for the uptake of specific molecules from the extracellular environment. This process involves the following steps:

  • Ligands bind to specific receptors on the cell surface.

  • The receptor-ligand complexes cluster in clathrin-coated pits.

  • The pits invaginate to form clathrin-coated vesicles, which pinch off from the plasma membrane.

  • The vesicles uncoat and fuse with early endosomes.

  • The low pH of the endosome causes the ligand to dissociate from the receptor.

  • The receptor is recycled back to the plasma membrane, while the ligand is sorted for degradation or transport to other cellular compartments.

• An example of receptor-mediated endocytosis is the uptake of low-density lipoprotein (LDL), also known as "bad cholesterol", by cells.

What is pinocytosis? What are its types?

• Pinocytosis, also known as "cell drinking", is a non-selective process for the uptake of extracellular fluid and small molecules. There are two main types of pinocytosis:

  • Macropinocytosis: Involves the formation of large vesicles that engulf large volumes of extracellular fluid.

  • Clathrin-independent endocytosis: Involves the formation of small vesicles that take up smaller volumes of fluid.

What is the role of caveolae in endocytosis?

Caveolae are small, flask-shaped invaginations of the plasma membrane that are enriched in cholesterol and specific proteins, such as caveolins. They are involved in a specialized form of endocytosis that is distinct from clathrin-dependent and clathrin-independent pathways.

How are receptors recycled after endocytosis?

• After endocytosis, many receptors are recycled back to the plasma membrane to be used again. This recycling is important for maintaining the sensitivity of the cell to extracellular signals. Receptor recycling can occur through several pathways, including:

  • Direct recycling: The receptor is returned to the plasma membrane from the early endosome.

  • Recycling through the trans-Golgi network (TGN): The receptor is transported from the early endosome to the TGN and then back to the plasma membrane.

What are the connections between endocytosis and cell signaling?

• Endocytosis and cell signaling are interconnected processes that play crucial roles in regulating cell behavior. Endocytosis can:

  • Regulate the availability of signaling receptors: By removing receptors from the plasma membrane, endocytosis can downregulate signaling pathways.

  • Internalize signaling complexes: Endocytosis can internalize signaling complexes, allowing them to continue signaling from within the cell.

  • Generate signaling endosomes: Endosomes themselves can act as signaling platforms, recruiting and activating downstream signaling molecules.