Learning Outcomes

• Define key terms: endocytosis, exocytosis, cis, medial, trans Golgi.
• Understand endocytosis and exocytosis processes.
• Identify organelles involved in exocytosis: directionality included.
• Compare and contrast types of endocytosis: Constitutive vs Regulated Secretions, Phagocytosis, Pinocytosis, Receptor-mediated.
• Detail Golgi Apparatus and lysosome structure and respective functions.
• Explain LDL endocytosis steps.
• Differentiate between LDL and HDL.
• Illustrate vesicle budding and fusion processes in detail.

Endomembrane System and Vesicular Transport

• Components include membranes and organelles that package, modify, and transport proteins.
• Sequence of trafficking: ER → Golgi → other compartments using vesicular transport (budding and fusion of vesicles).

Exocytosis and Endocytosis

• Exocytosis:

  • Vesicles containing proteins/molecules fuse with the plasma membrane, releasing contents outside the cell.

• Endocytosis:

  • Plasma membrane buds inward to internalize proteins/molecules, forming vesicles.

Steps in Protein Transport via Exocytosis

1. ER:
   • Proteins undergo covalent modifications (e.g., disulfide bond formation, glycosylation).
2. Golgi apparatus:
   • Further modifications and sorting before reaching final destinations (e.g., plasma membrane or lysosome).
3. Secretory vesicles:
   • Carry proteins to plasma membrane for exocytosis.

The Golgi Apparatus Structure and Function

• Composed of flattened membrane-enclosed structures called cisternae.
• Has three key regions:
  • Cis (adjacent to ER), Medial (central), Trans (towards plasma membrane).
• Functions:
  • Modifies proteins/lipids, sorts them for transport.

Types of Exocytosis:

• Constitutive Secretion:

  • Occurs in all eukaryotic cells continuously; releases soluble proteins, provides plasma membrane with newly synthesized lipids and proteins.
  • Example: Zona Pellucida in mammalian oocytes for sperm-egg recognition.

• Regulated Secretion:

  • Specific to specialized cells (e.g., β cells of pancreas, neurons).
  • Proteins stored in secretory vesicles.
  • Vesicles fuse only upon receiving signals (hormones or neurotransmitters).

Types of Endocytosis

• Phagocytosis:
  • Uptake of large particles (e.g., bacteria); performed by macrophages and neutrophils.
• Pinocytosis:
  • Uptake of small droplets of fluid.
• Receptor-mediated:
  • Utilizes receptor proteins for specific targeted molecule capture.

Receptor-mediated Endocytosis of LDL

• LDL (Low-Density Lipoprotein):
  • Transports cholesterol to target cells (e.g., gonads).
  • Phospholipid-protein shell and cholesterol core composition.

Steps of LDL Uptake:

1. LDL binds to its receptor on the cell surface and internalizes as a coated vesicle.
2. The vesicle fuses with an endosome which matures into a lysosome.
3. Free cholesterol is released to the cell.
4. The receptor recycles back to the cell membrane.

Role of HDL

• HDL (High-Density Lipoprotein) brings cholesterol to the liver for excretion, contrasting with LDL's function of delivering cholesterol to target tissues such as the ovaries.

Lysosomes

• Membrane-bound organelles unique to animal cells.
• Contain hydrolytic enzymes operating optimally at pH 4-5.
• Functions as the cell's "garbage disposal," degrading endocytosed materials and damaged organelles.

Mechanisms of Membrane Fusion and Vesicular Traffic

• Three key steps:
  1. Budding from donor membranes.
  2. Traveling through the cytoplasm.
  3. Fusing with target membranes.

Step 1: Budding Mechanism

1. Cargo binds to membrane-bound receptors, adaptin links it to clathrin.
2. Clathrin assembles into a basket shape, forming a bud.
3. Dynamin proteins assist in pinching off the vesicle.
4. Clathrin coat disappears post-budding.

Fusion Mechanism

1. Tethering: Rab proteins on vesicles bind to tethering proteins on membranes.
2. Docking: v-SNAREs on vesicles interact with t-SNAREs on target membranes.
3. Fusion: v-SNAREs and t-SNAREs catalyze membrane fusion, delivering cargo.

Exam Style Questions

Understanding the different roles of adaptins, clathrin, and dynamin in vesicle budding as well as the outcomes if any were omitted is critical to the comprehension of the vesicle trafficking process. Questions may require you to explain their functions and predict results of their absence:

1. Adaptins: Necessary for bud formation; without them, buds will not form.
2. Clathrin: Forms the basket structure essential for budding; absence leads to no bud formation.
3. Dynamin: Critical for pinching off the vesicle; without dynamin, the bud will form but remain attached to the membrane (no release).

Homework Exercises

1. What are the outcomes of omitting adaptins, clathrin, or dynamin in vesicle budding?
2. Discuss issues that arise from the presence of two kinds of v-SNAREs during transport from the Golgi and suggest solutions.