Secretory Pathway Flashcards
What can be the final destination of proteins in the secretory pathway?
ER, Golgi, plasma membrane, lysosome, outside of cell
What is the difference between regulated and constitutive secretion?
Constitutively secreted materials are always secreted and are secreted right away after leaving the trans-Golgi network. Regulated secretion involves vesicles that won’t be secreted unless there is a signal that triggers secretion
What are the 5 steps common to all proteins going to the lysosomes, plasma membrane, or outside the cell?
- Proteins get targeted to the ER with an ER signal sequence
- ER and cis-golgi vesicles fuse and form the new cis-golgi cisterna
- Missorted ER proteins and some vesicle proteins get sent back to the ER
- The cis-golgi cisterna undergoes maturation and moves towards the plasma membrane
- In each cisterna, resident Golgi proteins make modifications to proteins and then sent back to the right cisterna with retrograde transport
What are the 4 very general steps for how vesicles work?
They bud off from a donor organelle, get transported to the target organelle, “dock” to the target organelle, then fuse
How do vesicles bud off from a donor organelle?
When there is enough cargo, coat proteins polymerize and that causes the membrane to curve until the vesicle gets pinched off
What are the 3 types of coat proteins?
COPI, COPII, and clathrin
Why are coat proteins necessary? Why do they have to be removed?
They are necessary because the vesicle can’t form the circular shape without them, so it can’t bud off. The receptors on the vesicle can’t bind with the target because they’re under the coat proteins, so they have to be removed
What direction do COPI coated vesicles go?
Retrograde transport from the Golgi to the ER or from a later cisterna in the Golgi to an earlier one
What direction do COPII coated vesicles go?
Anterograde transport from the ER to the Golgi
What direction do clathrin coated vesicles go?
From the trans-Golgi network to the lysosomes, and from the plasma membrane to the lysosomes
How do we know that a G-protein is responsible for coat assembly and disassembly?
Coated vesicles accumulate when around non-hydrolyzable GTP
How do the G-proteins trigger the coat to be assembled or disassembled?
The coat is assembled when bound to GTP and disassembled when bound to GDP
What are the two G-proteins used for coat assembly and disassembly?
Sar1 and ARF
Which coat proteins use Sar1?
COPII
Which coat proteins use ARF?
COPI and clathrin
What key shape change occurs with Sar1 and ARF to allow for coat assembly?
When bound to GTP, they have a tail that becomes an anchor into the membrane and allows for the coat proteins to bind to it
What are the 4 steps in the formation of the coat of a vesicle?
- Sar1 or ARF interacts with a GEF and the tail anchors into the membrane
- They become a binding site for the coat proteins
- The GTP gets hydrolyzed and the tail retracts out of the membrane
- coat dissembles
What are the two proteins responsible for directly docking and fusing vesicles to their targets?
v-SNARES and t-SNARES
What is the G-protein that is involved in keeping the vesicle close enough to the target for the SNAREs to interact?
Rab
How does Rab work?
It can put a lipid anchor into the membrane of the vesicle when bound to GTP and interacts with effectors on the target membrane. That keeps the vesicle in place long enough for the SNAREs to interact
What happens if a vesicle tries to dock and fuse to the wrong place?
Nothing. The wrong t-SNAREs are around and the v-SNAREs won’t interact with them. The vesicle moves on
How do the SNAREs draw the vesicle close to the target membrane for fusion?
They coil up really tight and pull the vesicle closer as they coil up
What two proteins are needed to separate the coiled up SNAREs?
NSF and SNAPs, and they need ATP
How do motor proteins work?
They constantly hydrolyze ATP and change shape, which makes them “walk” along the cytoskeletal filaments
