Lecture 10: Secretory & endocytic pathway Flashcards
roadmap of vesicle transport
nuclear envelope -> ER -> golgi apparatus -> transport vesicles -> membrane -> early/late endosome -> lysosome
vesicle transport
asymmetry of the lipid bilayer is preserved, as is the orientation of membrane proteins
transport is selective -> only certain specific proteins are selectively incorporated into transport vesicles
maintains membrane topology
typical transport vesicle “life-cycle”
- Budding
- Movement
- Tethering
- Fusion
budding: cargo selection and membrane shaping
- selection of lumenal cargo to be transported is mediated by cargo receptors
- cargo adaptors capture transmembrane protein cargos (including cargo receptors) by binding “sorting signals” in their cytoplasmic tails
- coat proteins bind adaptors to shape membrane into a vesicle
- vesicle pinches from membrane
- vesicle is uncoared and ready to fuse with target membrane
- adaptor proteins and coat proteins refer to “the coat” of a vesicle
purpose of coats
coats + their associated cargo adaptors define the protein composition of transport vesicles
cargo receptors are “recycled” to be reused
COP = coat protein
each cargo adaptor recognizes different sorting signals
movement
vesicles (and organelles) are moved along microtubules by molecular motors
Tethering
- molecular motor attached to MT motve vesicle w/ Rab-GTP to tethering protein
- tethering protein attaches to Rab-GTP and pulls vesicle in closer to target membrane (docking)
- v-snare attaches to t-snare and secures vesicle to membrane
- Rab-GAP hydrolyzes GTP -> GDP and releases Rab-GDP
Fusion
“SNAREs” drive the fusion of transport vesicles w/ their target membrane
process of membranes actually fusing together and cargo receptor releasing cargo into the target membrane
role of SNARES in membrane fusion
- v & t snares start off seperate
- transport vesicle docks and snares attach
- membrnaes coalesce
- membrances fuse
- snmare bundle disassebled (requires ATP hydrolysis)
after fusion and SNARE bundle disassembly, v-SNARES are reycled back to donor compartment in different vesicles (i.e. COPI vesciles return the COPII vesicle v-SNARE back to the ER)
functions + structure of the golgi apparatus
- elaboration of the n-linked oligosaccharide chains “glycosylation” (process of refining the proteins - adding sugars)
- sorting to multiple destinations in the trans Golgi Network (“TGN”) -> end of golgi -> endosomes, lysosomes, plasma membrane
constitutive secretion
no signal is required, bul transport, as soon as proteins are made they can exit
unregulated exocytosis
regulated secretion
controlled by a signal, receptor mediated (ex: epinephrine/adrenaline is recognized by receptor and startes secreting proteins)
regulated exocytosis
endocytosis
the taking in of material by invagination of the plasma membrane
how does LDL perform endocytosis
LDL (low density lipoprotein) receptor on the cell surface binds LDL and internalizes it through clathrin coated pits
what is an example of receptor mediated endocytosis (RME)
LDL uptake steps
- endocytosis of LDL
- uncoating
- fusion with endosome (low pH about 6.0) * if the endosome is neutralized to pH 7, both the LDL and receptor get trapped there and the receptor does not recycle
- transfer LDL to lysosome
- vesicle exits endosome and buds off, LDL receptors return to plasma membrane
“RME” is also used to remove membrane proteins from the surface, when no longer needed, ex: Epidermal Growth Receptor
which viruses exploit endocytosis to gain entry into cells?
Flu virus, HIV, polio virus, hepatitis C virus, coronavirus
phagocytosis
type of endocytosis
engulfment of large particles (like bacteria) and delivery to the lysosome for degradation
phagocytosis involves:
-engulfing the particle into a phagosome by enclosing it with the plasma membrane
-fusion with a lysosome where the lysosomal enzymes will break down the particle
lysosomes
acidic due to the presence of a “proton” (hydrogen ion) pump
lysosomes contain hydrolytic enzymes that can break down any biological molecule
lysosomal enzymes function optimally at pH 5 (acid hydrolases)
contain membrane transporters so that products of degradation can be reused by the cell
autophagy
cells use autophagy to digest organelles and cytoplasmic material
