proteins and vesicular transport Flashcards
Where do proteins synthesised by ribosomes in the cytosol go?
- remain in cytosol
- go to mitochondria, peroxisomes, nucleus
Where to proteins made at the ribosomes at the ER go?
- for secretion
- plasma membrane
- lysosomes
How do cells know the correct location for a protein?
- part of the amino acid sequence contains a signal sequence
- proteins without a signal sequence remain in the cytosol
When do proteins need to be unfolded?
when they are transported across or into membranes
When do proteins get to remain folded?
when transported through pores or by vesicles
What are nuclear pores made of?
complex of around 30 different proteins called nucleoporins
What does importin do?
- recognises nuclear localisation signal (NLS) (which then bind to proteins within the
cytosolic fibrils) - carries protein into nucleus
How does importin carry proteins into the nucleus?
disrupts gel-like mesh of the proteins lining the channel
How does importin release the protein and return back to the cytosolic side of the pore?
- interacts with Ran
How do signal recognition particles direct ribosomes to the ER (to then lead to protein import into ER)?
- signal recognition particle (SRP) binds to signal sequence on ribosome
- SRP binds to receptor on ER and gets released
- ribosome passes to protein translocator which binds the signal sequence
How are proteins glycosylated?
- appropriate asparagine (Asn) enters ER lumen
- it is glycosylated by covalent addition of branched oligosaccharide side chain
Which proteins are usually glycosylated?
- during transport to the ER they often are
- most cytosolic proteins are not
What is the unfolded protein response (UPR)
- when there is an excess of unfolded proteins in the ER
- sensor proteins activate and stimulate expansion of the ER, more chaperones and less new proteins entering ER
Where are endocytosed vesicles usually delivered to?
- to early endosomes
- then to lysosomes via late endosome
What is the constitutive pathway of exocytosis?
- continual secretion of soluble proteins from cell
- this route replaces proteins and lipids in membrane
What is the regulated pathway of exocytosis?
- secretory vesicles store proteins until and extracellular signal stimulates their secretion
What do coat proteins do?
they shape the membrane into a bud and directly/indirectly capture the cargo proteins for transport
What is clathrin for?
involved in shaping the budding membrane
What do adaptins do?
they select the cargo molecules for vesicles
What is dynamin and what does it do?
- a GTP binding protein
- it assembles around the neck of budding vesicles to pinch off the vesicle
How are proteins transported into the nucleus?
- with specific signals to be actively transported across nuclear pores
- translocated in their fully folded confirmation
How are proteins transported into the mitochondria?
- signal sequence is recognised by an import receptor on outer membrane
- protein translocators pass protein through the membranes
- import receptor and translocator and protein diffuse to area in close proximity to second translocator
- once inside protein folds and signal sequence is cleaved off
How are proteins imported into the ER?
- protein translocator binds to signal sequence during translation and threads protein through membrane as a loop
- signal peptide is removed by signal peptidase
- molecular chaperone BiP binds ready to help fold protein within the ER
What are the two types of proteins that can be embedded in the ER membrane?
- single-pass transmembrane protein
- double-pass transmembrane protein
How are single-pass transmembrane proteins embedded in the ER membrane?
- N-terminal ER sequence initiates transfer
- transfer halted by stop-transfer sequence
- once embedded, N-terminal signal sequence is removed by signal peptidase
How are double-pass transmembrane proteins embedded in the ER membrane?
- internal sequence acts as start-transfer signal (also anchors protein in membrane)
- protein translocator stops once stop-transfer sequence reached
What are the two possible pathways for exocytosis?
- constitutive
- regulated
What do clathrin coated vesicles form from?
- cell membrane
- golgi apparatus
Which molecules help direct transport vesicles to their target membranes?
- SNAREs
- Rab proteins
- tethering proteins
How are vesicles recognised at a target membrane?
- tethering protein binds to Rab on vesicle
- vesicle docks on correct target membrane
- v-SNARE on vesicle binds to complementary t-SNARE
How do vesicles fuse with the membrane once docked?
- v-SNAREs and t-SNAREs bring the lipid bilayers close together
- they wind together squeezing water molecules out and allowing lipid bilayers to flow together
- after fusion, SNAREs pried apart to be reused
What are the 3 possible fates for receptor proteins following endocytosis?
- recycling
- transcytosis
- degradation
What is recycling of receptor proteins?
receptors are returned to the same plasma membrane from which they came
What is transcytosis of receptor proteins?
receptors are returned to a different area of the plasma membrane
What is degradation of receptor proteins?
receptors are delivered to lysosomes to be degraded
What is autophagy?
breakdown of cells own proteins/organelles by enclosing them in a membrane that fuses with the lysosome
