Receptor-Mediated Endocytosis

1. Receptor-Mediated Endocytosis As Presented By Michael Chan

2. Objectives • Define endocytosis • Explain endosomes • Illustrate receptor-mediated endocytosis • Give an example of receptor-mediated endocytosis

3. Endocytosis: • Uptake of extracellular materials by invagination of the plasma membrane to form a small membrane-bounded vesicle (endosome)

4. Endosomes • Network of tubules and vesicles that receive materials taken up by endocytosis • Two types of endosomes a. early endosomes b. late endosomes

5. Receptor-Mediated Endocytosis • Specific receptors on the cell surface bind tightly to the extracellular macromolecule that it recognizes (i.e. ligand) • Plasma membrane region containing the receptor-ligand complex buds inward and pinches off (i.e. transport vesicle)

6. Common Macromolecules that Eukaryotic Cells Internalize by RME : • Choloresterol-containing particles called LDL • Iron-binding protein transferrin • Protein hormones (e.g. insulin) • Glycoproteins

7. Mechanisms of RME • Clathrin-coated pits a. the invagination of the plasma membrane is covered on its cytoplasmic face by a layer of bristly, electron-dense material made of protein b. form from trans Golgi

9. Mechanisms of RME (cont.) • Vesicles a. spontaneous polymerization of clathrin triskelions b. lose coats after endocytosis (early endosomes)

13. LDLs and Cholesterol Metabolism • First studied and best understood • Cholesterol is a hydrophobic molecule that cannot be transported in the blood in the free state • Carried as a giant complex, called a low-density lipoprotein (LDL)

14. LDL Particle • Surface is a monolayer of phospholipid and unesterified cholesterol • Hydrophobic core is rich in fatty acid esters of cholesterol • Hydrophobic apo-B protein is embedded in the membrane

18. Questions? Comments?