Receptor-Mediated Endocytosis As Presented By Michael Chan

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

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

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

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

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

Mechanisms of RME Clathrin-coated pits 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

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

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

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

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