Receptor-Mediated Receptor-Mediated EndocytosisEndocytosis

As Presented By As Presented By

Michael ChanMichael Chan

ObjectivesObjectives

Define endocytosisDefine endocytosis Explain endosomesExplain endosomes Illustrate receptor-mediated Illustrate receptor-mediated

endocytosisendocytosis Give an example of receptor-Give an example of receptor-

mediated endocytosismediated endocytosis

EndocytosisEndocytosis::

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

EndosomesEndosomes

Network of tubules and vesicles Network of tubules and vesicles that receive materials taken up that receive materials taken up by endocytosisby endocytosis

Two types of endosomesTwo types of endosomes

a. early endosomesa. early endosomes

b. late endosomesb. late endosomes

Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

Specific receptors on the cell Specific receptors on the cell surface bind tightly to the surface bind tightly to the extracellular macromolecule that it extracellular macromolecule that it recognizes (i.e. ligand)recognizes (i.e. ligand)

Plasma membrane region Plasma membrane region containing the receptor-ligand containing the receptor-ligand complex buds inward and pinches complex buds inward and pinches off (i.e. transport vesicle)off (i.e. transport vesicle)

Common Macromolecules that Common Macromolecules that Eukaryotic Cells Internalize by Eukaryotic Cells Internalize by

RME :RME :

Choloresterol-containing Choloresterol-containing particles called LDLparticles called LDL

Iron-binding protein transferrinIron-binding protein transferrin Protein hormones (e.g. insulin)Protein hormones (e.g. insulin)GlycoproteinsGlycoproteins

Mechanisms of RMEMechanisms of RME

Clathrin-coated pits Clathrin-coated pits a.a. the invagination of the the invagination of the plasma membrane is covered on plasma membrane is covered on its cytoplasmic face by a layer of its cytoplasmic face by a layer of bristly, electron-dense material bristly, electron-dense material made of proteinmade of proteinb.b. form from trans Golgiform from trans Golgi

Mechanisms of RME (cont.)Mechanisms of RME (cont.)

VesiclesVesicles

a.a. spontaneous polymerization of spontaneous polymerization of clathrin triskelionsclathrin triskelions

b.b. lose coats after endocytosis lose coats after endocytosis (early endosomes)(early endosomes)

LDLs and Cholesterol LDLs and Cholesterol MetabolismMetabolism

First studied and best understoodFirst studied and best understood Cholesterol is a hydrophobic Cholesterol is a hydrophobic

molecule that cannot be molecule that cannot be transported in the blood in the free transported in the blood in the free statestate

Carried as a giant complex, called Carried as a giant complex, called a low-density lipoprotein (LDL)a low-density lipoprotein (LDL)

LDL ParticleLDL Particle

Surface is a monolayer of Surface is a monolayer of phospholipid and unesterified phospholipid and unesterified cholesterolcholesterol

Hydrophobic core is rich in fatty Hydrophobic core is rich in fatty acid esters of cholesterolacid esters of cholesterol

Hydrophobic apo-B protein is Hydrophobic apo-B protein is embedded in the membraneembedded in the membrane

Questions?Questions?

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