LEC35: Secretory Pathway and Endocytosis (Part D: Endocytosis, Part E: Mechanisms of Vesicular Transport) Flashcards
basic function of smooth ER?
rough ER?
golgi complex?
smooth ER: biogenesis of lipids for membranes
rough ER: biosynthesis of protein
golgi: sorting, according to destination, of protein
regulated vs constituitive secretion purpose?
regulated secretion: need signal to get taken to plasma membrane
constituitive secretion: housekeeping molecules that’re constantly made, in different cell types
what is endocytosis?
process of how materals are moved from the outside to inside of the cell
occurs by invagination of the plasma membrane via endocytic pathway
2 processes do this: pinocytosis and phagocytosis
what is phagocytosis? what sized molecules undergo this?
ingestion of molecules >0.5 microns
occurs by professional scavenging cells like macrophages which can invaginate large areas of plasma membrane to encompass pathogenic bacteria
recycles things in blood and foriegn bodies, breaks foreign bodies down to constituent parts
what endocytic pathway do red blood cells undergo?
phagocytosis
up to 1011RBC are recycled daily by phagocytosis
what is pinocytosis? what sized molecules undergo this?
cells that invaginate and form smaller vesicles
helps **recycle membranes and transport materials into cells **
cell membrane invagination occurs constituitively in very small vesicles during pinocytosis
ingests materials < 0.5 microns in size
what type of process is receptor mediated endocytosis?
a type of pinocytosis
what are 2 examples of receptor mediated endocytosis?
1) LDL is endocytosed by its receptor
2) transferrin is endocytosed when binds to its receptor
* receptor mediated endocytosis is a form of pinocytosis*
what is LDL
low density lipoprotein, carries cholesterol in the blood
structure is large molecule of:
1) 1,500 molecules of cholesterol esters in the middle,
2) 1 huge protein, ApoB100, and
3) phospholipid/cholester coat w/ cholesterol on it
describe the process of receptor mediated endocytosis between LDL and the LDL receptor
1) LDL receptor is on plasma membrane
Apo-B protein of LDL binds to LDL receptor; this internalizes LDL w/ its receptor
2) LDL receptor w/ hundreds of LDL particle/receptor complexes enter cell, fuse w/ the endosome
3) b/c endosome pH=5.5, LDL is released from its receptor into endosome
LDL receptor is recycled back to plasma membrane
4) endosome and lysosome fuse, LDL transfers into lysosome
5) in lysosome, lysosomal enzymes attack LDL, break it down, return free cholesterol back into the cell
describe process of how Fe3+ moves into cells
via receptor mediated endocytosis, like w/ LDL
1) Fe3+ binds to transferrin, a protein in the blood
2) transferrin receptors on cell membrane bind transferrin-Fe3+
3) these receptor-transferrin-Fe3+ are bound into vesicles that fuse w/ the endosome
4) endosome pH = 5.5 does not release transferrin protein; only releases the bound Fe3+
Fe3+ thus is reduced from Fe3+ to Fe2+
5) Fe2+ is exported out of the endosome by a divalent methyl transporter
6) Apo-transferrin, transferrin w/o Fe3+, is recycled back to the membrane
transferrin w/o Fe3+ fuses w/ the membrane, is released into the extracellular space, returns to blood to pick up more iron
what is autophagy?
how intracellular components are delivered to lysosome for destruction and recycling
describe process of autophagy
1) organelle/protein aggregate is engulfed by a autophagosome, double membrane structure
2) autophagosome fuses with lysosome to form **autophagolysosome **
what is the relationship between autophagy and age?
direct relationship: if have better autophagy as you age, you live longer (shown in worm, fly, yeast)
how often does autophagy occur?
constant process
what is this?
EM of LDL receptor w/ LDL particles going through membrane invagination, encircling by vesicle, vesicle to membrane, and then pinched off
why does vesicle budding happen?
in response to need for cargo to be transported from 1 compartment to another in secretory pathway
e.g. ER to Golgi, Golgi to Lysosome, etc
what process is integrated alongside budding process?
cargo selection via “coat” proteins that bend the membrane into vesicles
what are different types of coat proteins? what type of vesicles does each coat?
1) clathrin: vesicles from trans-Golgi network to plasma membrane and to endosomes/lysosomes
2) COPI: vesicles from cis Golgi to ER
3) COPII: vesicles from ER to plasma membrane
when does vesicle’s coating form?
alongside formation of vesicles -
when receptor binds cargo, molecule of clathrin or COPI or COPII forms also
what is structure of clathrin molecule?
heavy and light chain
has polymerizing units, triskelions that interact w/ each other, polymerize into shape (see image) which drags membrane into a curve form, allows vesicles to form
properties of polymerization means vesicles have uniform 30 nm size
what processes are integrated w/ vesicle budding?
cargo selection via proteins that bind to both cargo itself and to ‘coat’ proteins that help beind membrane into vesicles
coat proteins = clathrin, COPI, COPII
describe process of when/how coat proteins bind to vesicles
1) in TGN, when receptor and cargo bind, receptor binds to Adaptin AP1 molecule
2) Adaptin binds clathrin
or
1) in plasma membrane, when receptor and cargo bind, Adaptin is AP2
2) AP2 binds clathrin
how do clathrin molecules aid w/ vesiclular formation? describe process of vesicle budding via clathrin increase
1) when there are more receptors on membrane, that means there are more clathrin molecules prsent
2) when clathrins are close to each other, they begin to polymerize
3) as they polymerize, they pull the membrane away into a vesicle
4) vesicle sits on the membraen until acted on by dynamin, protein that uses GTP hydrolysis to pinch vesicle off the membrane
what is dynamin? how would its action be inhibited?
protein that cuts clathrin-coated vesicle from the plasma membrane
works by GTP hydrolysis
so if you inhibit the GTPase, dynamin cannot complete action of pinching off the plasma membrane, even if clathrin coat has been made
what happens to clathrin coat when vesicle buds and pinches off the membrane? why does this matter?
coat falls off, disassembles
important b/c targeting mechanism is on the vesicle so if coat is covering vesicle then targeting doesn’t happen
what are COPI and II coats a part of?
ER retrieval pathway and movement of vesicles within golgi itself
what are the general functions of RAB and SNARE proteins?
target vesicles specifically to different membranes depending on the cargo they carry
what does RAB protein do? describe mechanism of action
protein responsible for executing targeting mechanism for vesicles
1) Rab binds to vesicle
2) Rab binds to long tethering protein called Rab effector, long fibrous protein tethered to target membrane
3) Rab effector bound to Rab protein brings vesicle into close apposition w/ correct membrane
4) fusion of membranes occurs via SNARE proteins
what do SNARE proteins do?
responsible for vesicle fusion
overcomes enegetically unfavorable fusion by interactions of V-SNARE proteins on the vesicle and T-SNARE proteins on target membrane
why is vesicle fusion energetically unfavorable?
the hydrphilic head groups and their associated water molecules represent a barrier to mixing of hydrophobic hydrocarbons
how does SNARE fusion occur after Rab targeting?
V-snare and T-snare get close to each other
interact in a coiled coil around each other
when snare proteins pull from either side, they pull vesicle and membraen close to each other
interaction of SNARE proteins squeezes out all the water moelcules and reducing thermodynamic barriers to lipid mixing
after fusion, SNARE proteins are disassembled w/ help of a specific chaperone, then recyled back to respective membrane systems
what is SNARE system analogous to?
how viruses get into cells by squeezing tighter and tighter until fusion of membraens occurs
what is NSF?
chaperone protein that undoes SNARE-v and SNARE-t winding/binding
