Endocytosis Flashcards
what is endocytosis?
the inward budding of the plasma membrane to form a vesicle/endosome
what is pinocytosis?
- ‘cell drinking’
- plasma membrane recycling
- transcytosis - movement of molecules from apical to basolateral
- pulling in extracellular fluid
what is constitutive and receptor mediated endocytosis?
- inital acute response and then internalise the response
- signalling
- nutritional
what is secretory vesicle endocytosis?
partially fuse to release and then rapidly endocytose
what phaocytosis?
- ‘cell eating’
- large particles
what are the criteria for endocytosis?
- either clathrin dependent or independent endocytosis
- type of of endocytosis can be defined by the molecules involved
what are the principles of endocytosis?
- deform the membrane
- include or exclude proteins/lipids in the budding compartment
- ‘budding’ of the compartment and from the plasma membrane
- uncoat the vesicle and deliver (fuse) the cargo to another compartment
why does they membrane need to be deformed?
- the lipid bilayer is extremely stable
- its very flat but needs to form a bud
what are amphipathic helices?
- polar and hydrophobic amino acids on opposite sides of the helix
- top half of the lipid bilayer becomes deformed (get a curve)
what is a loop insertion?
loops with a hydrophobic amino acid
what are classic lattices?
- bend the membrane, forces it into a curve
- lattices bind cargo proteins but the lattice forms a curved polymer
what are BAR domain proteins?
- proteins bind the bilayer via a curved surface
- e.g. BAR domain containing proteins
- binds in 2 different places
what does including or excluding cargo involve?
- two protein complexes/molecular hubs
- clathrin
- adaptor protein 2 (AP-2)
- these protein complexes form molecula hubs binding cargo at the plasma membrane
what does AP2 do?
- binds a huge number of proteins
- these protein complexes from molecular hubs
- binds cargo at the plasma membrane with the aid of accessory proteins
- can bind the same proteins
what do AP2 and clathrin allow?
allows the membrane to deform and can the internalise the transmembrane cargo proteins
what are cargo adaptors?
- proteins that link cargo into the clathrin-coated pits
- AP2 is the adaptor protein at the plasma membrane
what do appendage domains bind?
- they bind accessory proteins
- appendages
- the hing regions bind to clathrin
- binds motifs
what do trunk domains do?
bind to membranes and cargo
what is the process of AP2?
- initially in a closed confirmation
- AP2 docks with the plasma membrane where it binds the plasma membrane lipid PI(4,5)P2
- 4 x PI(4,5)P2 per AP2 allows cargo binding following a conformation change
- once AP2 is bound to cargo it can then bind to clathrin
what is the structure of the clathrin coat?
- consists of triskelions: 3 heavy and 3 light chains
- assemble to form a clathrin cage around a membrane
- hexagons and pentagons like a football
what are the steps of clathrin mediated endocytosis?
- AP2 and clathrin help to deform the membrane
- receptor binds to ligand, binds AP2
- AP2 binds PI(4,5)P2 and recruits clathrin
- form clathrin triskelions allowing the membrane to deform
- receptor is concnetrated in the vesicle
- the stalk is cleaved
- uncoating of the clathrin and AP2
- these can be recycled
what do clathrin and AP2 bind?
- bind accessory proteins
- accessory proteins are not one to one
how is clathrin dependent endocytosis achieved?
through sequential protin-protein interactions at the site of vesicle budding
what is budding?
- vesicle scission
- forms a clathrin cage but still attached to the plasma membrane
what is the process of budding?
- hexameric GTPase
- forms a ring around the neck of the budding vesicle
- uses GTP hydrolysis to achieve fission
how does the does the vesicle finally bud off?
- constrict (narrows in width)
- twists in opposite directions
- distance between subunits become smaller
- pinches the membrane
what is shibire?
- temperature mutant, vesicles get pinched off and so line up at the plasma membrane
what happens when the receptor associates with the clathrin-coated pit?
- endocytosis
- uncoating
- fusion with early endosome
how does fusion with early endosome occur?
- maturation to late endosome (near Golgi, more acidic thatnEE and different Rabs)
- fusing with/trafficking to lysosome for degradation
- recycling to plasma membrane via transport vesicles that bind from EE and can fuse to form recycling endosomes
what are the different receptor fates?
- receptor recycles between an intracellular compartment and plasma membrane
- transcytosis
- degradation
what does the LDL receptor bind to?
ApoB
- association NPXY motif with AP2+ localises it to CCP
what is the clathrin-coated pit?
- coated vesicle
- uncoating and then fusion with an endosome
how is the LDL receptor recycled?
- acidic pH in endosome, dissociates ligand from receptor
- receptor is recycled back to plasma membrane
- LDL is sent to lysosome for degradation
what happens in an inherited LDL trafficking disorder?
- familial hypercholesterolemia (one of 2 defects)
- mutations of NPXY motif
- mutation of AP2 adaptor
what is the process of EGF receptr degradation?
- only associates with CCP on ligand binding (fast endocytosis)
- EGF, tetrapeptide internalisation sequence interacts with AP2
- internalisation involves tyrosine kinases activity
- most EGF receptors are degraded in the lysosome
- leading to receptor and ligand down-regulation/degradation
