Lecture 13 - Endomembranes & Vesicles Flashcards
what are the functions of vesicles protein coatings?
- curve the membrane to form the vesicle
- select the components to be carried in the vesicle
def: vesicle coating, acts as a scaffolding cage around the vesicle
outer layer
def: vesicle coating, acts as an adaptor between the outer layer and the lipid bilayer/cargo
inner layer
def: move materials from the ER “forward” to the ERGIC and Golgi complex
COPII coated vesicles
def: move materials from the ERGIC and Golgi “backward” to ER or from the trans Golgi to the cis Golgi cisternae
COPI coated vesicles
def: move materials from the plasma membrane “backwards”, and the TGN to the endoscopes/lysosomes/plant vacuoles
Clathrin-coated vesicles
in a COPII coated vesicle, what is budding initiated by?
the recruitment of small GGTP binding proteins (GTPases) = SAR1-GDP to a patch of donor membrane
what gives COPII vesicles their curvature?
Sec23 and Sec24 dimers, since they have curved conformation
def: not only curving protein, but also acts as an adaptor protein, associating with ER export signals recruiting proteins to the forming vesicle
Sec24
def: integral membrane protein crucial for the fusion of the vesicle to the correct target membrane
v-SNAREs
in COPII what forms the cage like outer layer?
Sec13 and Sec 31
for COPII vesicles, the outer layer is a relatively simple _____, with each vertex forming from the convergence of ____ ______ ____ dimers
lattice, four Sec13 and Sec31 dimers
what happens to the protein coat while in transit of COPII and why
it disassembles and release its contents back into the cytosol so the v-SNAREs are accessible and the vesicle can target the proper membrane
what chemical process mediates the disassembly of the protein coat of the vesicle?
hydrolysis of GTP, producing SAR1-GDP
what mediates the movement of vesicles while reaching their targets?
microtubules
the initial contact between a vesicle and the target membrane involves _________ proteins
tethering
what are the 2 groups of tethers?
- rod shaped fibrous proteins that form long bridges
- multi-protein complexes that hold two membranes close together
def: regulates membrane specificity and is responsible for most of membrane vesicle/target specificity, group of GTPases
Rab GTPases
how does Rab-GTP allow docking to occur?
- recruits the cytosolic tether proteins to the membrane surface
- recruits motor proteins to the vesicle to facilitate transport
SNARE proteins _____ _______ between vesicles and target membranes
mediate fusions
def: sorting and targeting of vesicles involves two families of SNARE proteins
SNARE hypothesis
def: SNARE proteins found on vesicles
v-SNARE
def: SNARE proteins found on target membranes
t-SNARE
T or F: v- and t-SNAREs are not complementary molecules to each other
False, they are complementary molecules
v- and t-SNARE __________ intertwine, pulling the membranes together
alpha helices
in vitro, the v-SNARE–t-SNARE interaction is strong enough to cause _______ ______, but in vivo, a rise in ____ _________ is thought initiate fusion
membrane fusion, Ca2+ concentration
T or F: After fusion, the v- and t-SNARE are still tightly associated, and are now in the same membrane
true
how do v- and t-SNAREs dissociate after membrane fusion has occurred?
- require NSF and SNAPs to pry apart the SNAREs, using energy from ATP hydrolysis
where do v-SNAREs go after the vesicle transport has occurred?
they are shuttled back to the previous compartment to repeat this process
what is the main difference between how COPI- coated vesicles are formed compared to COPII
they use different coat and adaptor proteins
def: aka a secretory pathway, vesicle contents are released to the exterior, vesicle membranes fuses with the plasma membrane
exocytosis
def: imports extracellular molecules by forming vesicles with the plasma membrane, in steady state with exocytosis
endocytosis
def: some vesicles move directly to the cell surface and immediately fuse with the plasma membrane, this process is unregulated and continuous and independent of external signals
constitutive secretion
what is an example of constitutive secretion?
mucus secretion by the intestinal lining
what does current evidence suggest about constitutive secretion?
that some tags may be required for it to occur
def: vesicles involved in this kind of exocytosis accumulate in the cell and only fuse with the plasma membrane in response to specific signals (like Ca2+)
regulated secretion
immature regulated secretory vesicles bud from the ____
TGN
def: the concentration of secretory proteins
condensation
where do mature secretory vesicles move to and remain there until receiving a signal?
move close to the site of secretion
what is membrane fusion triggered by
hormonal or chemical signals
def: when specific proteins need to be secreted from a limited region of the plasma membrane
polarized secretion
where is polarized secretion common?
in nerve cells and intestinal cells
how is polarized secretion regulated?
temporally
def: proteins in vesicles are released to the exterior of the cell as the vesicle fuses with the plasma membrane
exocytosis
proteins and lipids are added to the membrane via ________
exocytosis
def: essential nutrients are ingested by the cell as vesicles bud inwards off of the plasma membrane
endocytosis
proteins and lipids are removed from the membrane via ________
endocytosis
def: one of the primary ways that cells maintain membrane and fluid balance
bulk-phase endocytosis
does bulk phase endocytosis ingest or concentrate PARTICULAR molecules
no, non-specific
bulk phase endocytosis compensates for ________ ________ _____ by exocytosis and maintaining the surface to volume ratio
plasma membrane gain
def: process in which cells can selectively and efficiently acquire macromolecules
receptor-mediated endocytosis
as receptor-ligand complexes diffuse laterally they encounter specialized regions called ______ _____, sites for collection and internalization of these complexes
coated pits
T or F: receptors are sparse in coated pit regions
False, they are concentrated in coated pits at 10-20X their level in the plasma membrane
what does the accumulation of complexes in the coated pits trigger?
the accumulation of Clathrin-coat proteins on the cytosolic side of the membrane = inducing a curvature and invagination of the pit
each clathrin molecule consists of:
- 3 heavy chains
- 2 light chains
- joined at the centre to make a triskelion
T or F: COPII molecules extensively overlap, while Clathrin does not
False, Clathrin molecules extensively overlap, while COPII does not
def: promotes the assembly of clathrin cage and recruitment of membrane receptors to the buddying vesicle
AP2, adaptor protein 2
def: binding of this changes the conformation of AP2, making the cargo binding site accessible
Phosphoinositide
what is required as clathrin accumulates around the budding vesicle to constrict and close the vesicle?
dynamin
def: a cytosolic GTPase that constrict and separates the vesicle from the plasma membrane
dynamin
as GTP is hydrolyzed, the dynamin ring ________, separating the vesicle from the plasma membrane
tighten
def: ingestion of large (>500nm) macromolecules, cell parts, or whole organisms
phagocytosis
in humans, when is phagocytosis used
mostly just restricted to white blood cells
contact with the “target” triggers the onset of phagocytosis, as folds in the membrane, ________, surround the object forming an intracellular _______ ________
pseudopods, phagocytic vacuole
def: a hydrolytic enzyme, cleaves one molecule into two molecules, with an optima pH of ~5.0
acid hydrolase
what causes the lowered pH in lysosomes
V-Type ATPases in the membrane which pump in protons
def: the vesicle budding from the plasma membrane as a result of bulk-phase or receptor mediated endocytosis
endocytic vesicle
def: the primary sorting station in the endocytic pathway. endocytic vesicle fuse with the early endosome and material is either recycled back to the cell membrane or targeted for degradation
early endosome
def: an organelle containing the full compliment of acid hydrolyses, but who’s lumen has not reached pH 4.0-5.0. This is the site of intraluminal vesicle formation
late endosome
def: an organelle that is digestively active. There are 2 routes for a late endosome to mature into a lysosome
lysosome
what are the 2 routes of a late endosome to mature into a lysosome
- the ATPase pumps can lower the pH of the late endosome, which activates the enzymes, there by generating a new lysosome
- the late endosome can fuse with an existing lysosome
def: mediate the uptake of material that will be used by the cell, receptor will deliver the bound material to the early endosome, releasing it because of the lower affinity at higher [H+], then return to the cell membrane
housekeeping receptors
def: bind extracellular messengers that change the activity of the cell, these receptors will be degraded, reducing the sensitivity of the cell to further simulation
signalling receptors
phagocytic vacuoles become _______ by fusion with endosomes
lysosomes
phagocytosis and receptor-mediated endocytosis involve the degradation of _______ _______ brought into the cell
external materials
where do vesicles formed by receptor-mediated endocytosis fuse?
with vesicles of the TGN containing acid hydrolase
def: when there is only indigestible material left in the lysosome, it is called this
residual body
def: the digestion of old organelles and cell structures
autophagy
def: formation of an autophagic vacuole when an organelle becomes wrapped in a double membrane derived from the ER
macrophagy
def: a much smaller vacuole is formed surrounded by a single membrane
microphagy
