Endocytosis Flashcards
What is endocytosis?
Cells absorb molecules by engulfing them in membrane vesicles
What are the two fates for molecules engulfed by endosomes?
Plasma membrane
Lysosomes for degradation
What are the 3 endocytic pathways?
Receptor-mediated endocytosis
Phagocytosis
PInocytosis
Describe phagocytosis.
Whole particles are broken down by enzymes like cathepsins
Describe pinocytosis.
Cell engulfs material that has already been broken down
Non-specific
Describe phagocytosis?
Mechanism for the removal of pathogens and cell debris
How is a phagolysosome formed?
Particle is taken up by the phagosome
Fuses with lysosome
What is autophagy?
Cells digest their own cytoplasmic constituents within lysosomes
What is an autophagosome?
Double-membraned vesicle
What is cargo?
Transport vesicles fusing with one another and carrying membrane components and soluble molecules
What do cytosolic surfaces of each compartment contain?
Molecular markers
How do transport vesicles function effectively and maintain compartmental diversity?
Specifically take up appropriate cargo
Fuse only with correct target membranes
What allows the cell to segregate membranes into different sub-domains?
Specialized coat proteins
What are coat proteins 2 functions?
Form a specific patch
Deform membrane patch
What are 3 coat proteins?
Clathrin
COPI
COPII
Describe Clathrin’s structure.
3 large and 3 small polypeptide chains (triskelion)
Legs joined by trimerization domain
36 triskelions form a polyhedral lattice
N-terminal domains interact with inner layer of adaptor proteins
What is the role of adaptor proteins?
Interact with cargo receptors that bind specific, soluble cargo molecules
What is a retromer?
Heteropentameric protein complex
What is the role of retromers?
Assemble on endosomes to form vesicles
Help recycling transmembrane receptors from endosome to trans-Golgi network
When does retromer assembly only occur?
It can bind to tails of cargo receptors
It can interact with a curved phospholipid bilayer
It can bind to specific phosphoinositide
How do phosphoinositides confer specificity?
Help determine when and where coats assemble
Used as molecular markers of compartment identity
Control trafficking
What is Dynamin’s function?
Assembles around neck of clathrin coated buds
Contains phosphoinositide binding domain (tethers)
Recruits other proteins to assist in destabilizing lipid bilayers (vesicle release)
What is the role of Rab proteins?
GTP-bound Rabs interact with Rab effector proteins on target membrane (guide vesicle targeting)
What is the function of SNARE proteins?
Pair to dock the vesicle to target membrane
Catalyse fusion of 2 opposed bilayers
What are the 4 steps of receptor-mediated endocytosis?
- Clathrin coated pits and vesicles uptake specific macromolecules in ECF
- Cargo proteins bind specific receptors in clathrin coated pits
- Clathrin and adaptor proteins are removed from vesicle and recycled
Uncoated vesicle docks with its destination compartment via SNARE proteins
What are lipid rafts?
Specialised cholesterol and sphingolipid-enriched microdomains
What are the 4 types of Clathrin-independent-endocytosis (CIE)?
Dynamin dependent
Dynamin independent
macropinocytosis
phagocytosis
What do invaginations of the cell membrane depend on?
Actin filaments
What is the role of LDL?
Transport cholesterol to peripheral tissues and regulate synthesis of cholesterol at these sites
How is LDL transported?
Receptor mediated endocytosis
What are 2 regions of a LDL receptor?
Cytoplasmic region
Exoplasmic region
What is hypercholesterolaemia?
Defective LDL receptor gene prevents binding to adaptor proteins
What are endosomes?
First destination of uncoated vesicles
Heterogeneous membrane-rich set of membrane enclosed tubules/vesicles
What are 3 classes of endosomes?
Early
Late
Recycling
How do early endosomes mature into late endosomes?
Become more acidic by V-ATPase which breaksdown receptor/ligand complex
Where do most ligands go after endosomes?
Lysosomes for degradation
