Microbiology 14: (Prigent) Endocytosiss Flashcards
3 Main mechanisms of endocytosis
Phagocytosis
- involves rearrangement of actin cytoskeleton
Macropinocytosis
- involved rearrangement of actin cytoskeleton
Micropinocytosis
- mediated by proteins e.g. Clathrin
Features of clathrin mediated cytosis
Active in nearly all cells (not mammallian RBCs)
Occurs through plasma membrane invaginations with characteristic clathrin coat
Clathrin coated pits (CPCs) occupy about 2% of cell surface
CCVs internalist membrane area equivalent to entire surface every 1-2 hour -> very dynamic system
Examples of ligands brought into cell via clathrin mediated endocytosis
example of viruses + toxins
Serum transport proteins and antibodies
e.g. Transferrin, LDLPs, IgG
Hormone receptors
e.g. Insulin, Epidermal growth factor, Growth hormone, Prolactin
Toxins
e.g. Cholera
Virus
e.g. HIV
Mechanism for formation of a CCV
Structure of clathrin?
Triskelion, composed of 3 heavy chains and 3 light chains
Describe AP2 adaptor complex structure and function
Forms a molecular link between YXXdelta-Receptor tails and clathrin
- Causes conformational change to permit bi nding of clathrin
Describe AP2 adaptor complex function
Forms a molecular link between YXXdelta-Receptor tails and clathrin
- Causes conformational change to permit binding of clathrin
Describe AP1 adaptor function
Recruit the M6P receptor into CCVs at the trans golgi
Function of Dynamin?
GTPase
-> mediates ‘Pinching-off’ of clathrin coated vesicles at the cell membrane to complete formation
Properties and functions of (early) endosomes?
Membrane bound compartment inside cell
First station to which ligands and receptors are delivered by CCVs
Acid pH ~6.0
Key sorting station in endocytic pathway
Describe the LDL receptor cycle
LDL particle binds to LDL receptor at cell surface
- incorperated into clathrin coated pits via NPXY sorting sequence -> vesicle forms
Coated vesicle then uncoats and forms early endosome
LDL dissociates from receptor -> receptor is recycled to membrane
LDL transferred to lysosome and degraded into constituents
Describe transferrin receptor cycle
Iron binds to Apo-transferrin to become transferrin
Binds to transferrin receptor in clathrin coated pit
-> vesicle formation
Iron dissociated from transferrin-receptor complex and leaves vesicle via transporter proteins
Transferrin and receptor and recycled back to cell surface
What happens to the EGF receptor cycle in high levels of EGF?
Instead of receptor recycling, EGFR is ubiquitinylated and trafficked to multi vesicular bodies (MVBs) where is it sequestered and degraded
