Lecture 9: Protein and Vesicular Transport Flashcards
where do proteins synthesised by ribosomes on the RER go
- nuclear membrane
- secretory vesicles
- peroxisome membrane
- packaged by Golgi to go to plasma membrane
what tells the protein where to go
signal sequence
what do proteins need to do to pass through the membrane
unfold and then refold when they’re through the membrane
what are the proteins that make up the nuclear pore
nucleoporins
what are the steps of nuclear import
1) protein’s nuclear localisation signal is recognised by importin, which binds to proteins within the cytosolic fibrils
2) importins carry the protein into the nucleus by disrupting the gel-like mesh of nucleoporins, making a channel
3) once inside, an interaction with Ran protein causes release of cargo proteins and returns importin back outside the pore
what is Ran and how does it work
- GTP binding protein
- when bound to GDP, Ran binds to importin and takes it into the nucleus while bound to NTF2
- inside the nucleus, Ran GEF exchanges GDP for GTP
- this releases importin back to cytosol
how does a mitochondrial protein get through the outer membrane
signal sequence recognised by import receptor protein, which will interact with a protein translocator in the outer membrane
how does a mitochondrial protein get through both membranes to the matrix
- gets through the outer membrane
- travels along membrane to a spot where both membranes are in close proximity
- interacts with protein translocator on inner membrane is unfolded into primary structure and fed through membrane
- once inside the matrix, signal sequence is cleaved off and chaperones fold protein back into its final shape
how are proteins transported to the endoplasmic reticulum
- protein translocator binds signal sequence during translation and threads polypeptide through as a loop
- signal sequence is cleaved off and left in membrane to degrade
- as polypeptide feeds through, chaperone BiP binds, ready to help fold protein back within the ER
how do the ribosomes know they have to transport the protein to the ER
- as the ribosomes start manufacturing the protein, the signal sequence is recognised by signal recognition particles (SRPs)
- SRP binds to SRP receptor on surface of ER lumen
- signal sequence transferred across to protein translocator, starting protein import
how are single-pass transmembrane proteins imported into the ER membrane
- signal sequence begins transfer into membrane
- when stop-transfer sequence reaches protein translocator, protein is stopped from being fed through
- signal sequence removed, releasing inner end of the sequence into the ER
- outer part remains outside ER and stop-transfer sequence becomes transmembrane domain
how are double-pass transmembrane proteins imported into the ER membrane
- instead of the signal sequence being at the end of the amino acid sequence, it’s embedded within
- translocator protein keeps feeding it through until it hits stop-transfer sequence, and ejects protein into the membrane
what is glycosylation
adding a branched oligosaccharide side chain to an appropriate asparagine
what is glycosylation important for
- inflammatory response
- could enable viral immune escape
- promoting cancer cell metastasis
- regulating apoptosis
what happens if a protein misfolds
- retained in ER by chaperones until it can be folded correctly, or exported into cytosol for degradation
- excess of unfolded proteins triggers the unfolded protein response
- increases size of ER, number of chaperones and decreases transcription
how do vesicles travel from the ER to other places
- bud off ER and each level of Golgi membrane to gt to the Golgi apparatus
- bud off Golgi into early and late endosomes or lysosomes to get to plasma membrane
- exocytosed vesicles replenish lipids and cell surface receptors at surface
what are the pathways of vesicular transport
- constitutive: unregulated, continued secretion of proteins from the cell
- regulated: secretory vesicles store high concentrations of protein and wait until an extracellular signal stimulates their secretion
what are coat proteins
- proteins that aid in vesicle budding
- after budding, the coat is shed to expose lipids on the vesicle which permits fusion with the target membrane
which coat proteins help transport from Golgi to lysosomes
clathrin and adaptin
which coat proteins help transport from plasma membrane to endosome
clathrin and adaptin
which coat proteins help transport from ER to Golgi
COPI
which coat proteins help transport from Golgi to ER
COPII
what is clathrin
- coat protein made up of 3 chains that form basket like cages
- involved in shaping the curvature of the budding membrane
- clathrin coated pits form on the membrane
what is dynamin
- a GTP binding coat protein that assembles around the neck of budding vesicles to pinch it off
- squeezes lipids together until they fuse and flow into each other
how is the protein recognised at the target membrane
- tethering proteins recognise Rab protein on vesicle and pull it towards the membrane
- v-SNARE binds to complementary t-SNARE on target membrane, which wind around each other to pull vesicle close to membrane
- squeezes away water molecules, allowing lipids to flow and form a continuous layer
- SNAKES unwind to be used again
how do phagocytes engulf pathogens
- forms pseudopods which is the membrane expanding
- viruses can also bind to receptors on the cell membrane and be taken up by receptor mediated endocytosis
- end up in clathrin coated vesicles
- low pH allows release of viral genome into cytoplasm
how are receptors degraded
- delivered to early endosome, then late endosome, then lysosome where they get destroyed and degraded
- important when the signal needs to be turned off
how are receptors recycled or transcytosed
- ligand dissociates in early endosome
- can return to same membrane they came from (recycling)
- or sometimes go to a different domain (transcytosis)
what is autophagy
breakdown of the cell’s own proteins and organelles by enclosing them in a membrane that fuses with the lysosome
