CFM 2: Endocytosis

Type of endocytosis where large particles such as bacteria are ingested by an actin-based mechanism (aka "cell eating")

Type of endocytosis where fluid and solutes are ingested via endocytic vesicles

Flask-shaped invaginations of specialized domain in the plasma membrane, rich in cholesterol and coated with the protein caveolin, which under certain circumstances mediate uptake of specific cargoes.

Actin-driven mechanism in which cells are able to take up large volumes of extracellular fluid

• Phagocytosis: "cell eating", large particles, actin-driven
• Pinocytosis: "cell drinking", fluids and solutes, includes:
•       Macropinocytosis: actin-driven, large volumes of extracellular fluid
•       Clathrin-mediated endocytosis
•       Caveolin-mediated endocytosis
•       Clathrin-and-caveolin independent endocytosis: unknown coat protein

• 3 clathrin heavy chains, 3 clathrin light chains
• Spontaneously forms a "cage" structure in physiological conditions

AP2

Heterotetramers usually, but there are some monomeric adaptor proteins; bind accessory proteins

• Cargo molecules bind to receptors on the plasma membrane
• Adaptor proteins bind to the receptors on the cytosolic side; regulated endocytosis often involves addition of a reversible post-translational sorting tag on the cytosolic domain of the receptor.
• Clathrin bound to adaptor proteins; assembly of clathrin coat drives vesicle formation
• The clathrin-coated pit invaginates, forming a narrow neck
• The neck is cleaved
• Clathrin dissociates, leaving you with naked transport vesicle

• Wraps around the narrow neck of the invaginating clathrin-coated pit; it, together with accessory proteins that it recruits, destabilize interacting lipid bilayers so vesicle pinches off from membrane
• Dynamin is a GTPase

• Sorting endosome
• Ligand and receptor are separated; receptor to be recycled

• Early: 6
• Late: 5.5
• Lysosome: 5

After ligands and receptors are separated, receptors go to be recycled in recycling endosomes, and ligands go to late endosomes and ultimately to lysosomes to be discarded.

lower

enzyme in lysosome that relieves free cholesterol from LDL once LDL is in the lysosome

• NPC2 binds free cholesterol in lysosome that was relieved from LDL by LAL, since cholesterol is hydrophobic
• NPC1 interacts with NPC2 and helps it migrate across the lysosomal membrane
• Defects in either can lead to accumulation of cholesterol in lysosome, leading to lysosomal storage diseases

• LDL binds to LDLR
• Clathrin coated pit forms
• Vesicle forms
• Goes to early endosome; low pH in early endosome releases LDL from LDLR
• LDLR goes to recycling endosome to go back to plasma membrane
• LDL goes to late endosome, then to lysosome
• In lysosome, LAL frees cholesterol from LDL
• Cholesterol, which is hydrophobic, binds NPC2
• NPC1 helps NPC2/cholesterol complex to migrate across lysosomal membrane

• Transferrin (with iron) binds to transferrin receptor
• Clathrin coated pit forms
• Vesicle forms
• Goes to early endosome
• Lower pH releases iron into endosome
• Apotransferrin now bound to receptor, goes with receptor to endosome to be recycled
• Apotransferrin gets released when pH is raised back to 7 extracellularly

Familial Hypercholesterolemia

Autosomal recessive hypercholesterolemia

• A protein secreted by the hepatocyte that binds to LDL receptors, but stays bound to them in endosomes, targeting LDLRs for degradation in lysosome
• Fewer LDLRs --> more LDL in bood --> hypercholsterolemia

Inhibit

• Receptor dimerizes, tail is phosphorylated, leads to ubiquitinylation of the cytoplasmic domain of the receptor
• Ubiquitinylation leads to the receptor being sequestered into the lumen of the endosome (membrane with receptor pinches off and forms a vesicle within a vesicle); this is mediated by ESCRTs
• Signaling stops and the receptors are degraded by the lysosome

Complexes that recognize ubiquitinylated of cytoplasmic tails of receptors, invaginating the endosomal membrane into the lumen and concentrating the receptors on the membranes of internal vesicles of the multivesicular body

• GLUT4 is major glucose transporter expressed mainly in muscle and adipose tissue
• GLUT4 is stored in specialized recycling endosomes
• In response to insulin, GLUT4 translocates to cell membrane and increases glucose uptake
• In type II diabetes, cell does not respond to insulin, so no more GLUT4, can't uptake glucose

• Zipper mechanism: Bacteria express adhesion that binds to host cell adhesion protein, tricking it into thinking it is forming a cell junction, but ultimately the host cell just surrounds the bacterium.
• Trigger mechanism: Bacteria express type III secretion apparatus, injecting effector molecules into the host cell that affect cytoskeletal elements, causing lamellipodia to surround bacteria

• 1. Escape endosome or phagosome
• 2. Prevent fusion with lysosome
• 3. Fuse with lysosome and survive in newly formed phagolysosome

• B subunit of toxin binds to receptor on host cell membrane
• B subunit is cleaved autocatalytically, with one fragment released from receptor and another staying
• 7 large fragments of B subunit form heptameric ring
• A subunit binds to this ring
• Complex is taken in by endocytosis
• Low pH of endosome induces conformational change in ring, which then forms a pore in endosome, releasing A subunit of toxin into cytosol

• Virus has phospholipid membrane around it
• Can bind non-specifically to many cell surface receptors and get endocytose
• Certain parts of glycoprotein get cleaved in low pH in endosome, opening up binding site for NPC1
• This allows nucleocapsid to be released into cytosol