Lecture 16: Vesicular Transport Flashcards
1
Q
Vesicular transport
A
- Protein transport between ER, Golgi complex, plasma membrane and vesicles is achieved through vesicular transport.
- The vesicles bud off of the donor compartment and then travel between compartments in the cell along defined, regulated pathways and fuse specifically with their targets
2
Q
Vesicular transport #2
A
- Represents movement between compartments that are topologically similar
- Budding and fusion are not inverses of each other
- > In budding, the lumen face touches first
- > In fusion, the cytosolic face touches first
3
Q
What does vesicular sorting depend on?
A
- Depends on the assembly of a special protein coat formed at specific locations along a given donor compartment
- 3 code proteins that are responsible for this targeting
4
Q
COPII
A
- Responsible for vesicle traffic between the ER and the Golgi
- Coats ER-to-Golgi vesicles
5
Q
COPI
A
- Responsible for vesicle traffic within the Golgi
- Coats vesicles moving from Golgi to ER, Golgi to plasma membrane (secretory vesicles), and within the Golgi
6
Q
Clathrin
A
- Responsible for vesicle traffic between the plasma membrane and the endosomes; in some cases the Golgi
- Mainly involved in transport to/from and within the endosomal compartments
7
Q
Coat Proteins Represent the Initial Step in Vesicle Formation
A
- Transport vesicles bud off as coated vesicles that have a distinctive cage of proteins covering their cytosolic surface.
- > The cage of proteins is there to help the vesicle form and to target it
- Before the vesicle fuses with a target membrane, the coat is discarded, to allow the two cytosolic membrane surfaces to interact directly and fuse.
- Different coat proteins are involved in transport between different organelles
8
Q
Phospholipids containing inositol head groups mark organelles and membrane domains
A
- Inositols can get phosphorylated at various locations by different lipid kinases (often located in distinct organelles, providing specificity)
- > The inositols can be phosphorylated at different positions and they can be differentiated as well
- PI’s can recruit various proteins that possess lipid binding domains
- These lipid binding domains usually only recognize a specific type of PI
9
Q
Regulation of coat assembly and vesicle stability
A
- Adapter proteins bind to membrane proteins and recruit coat proteins
- > Often bind to cargo receptors – transmembrane proteins that bind soluble cargo proteins for transport
- Coat recruitment GTPases control coat assembly
- > Monomeric GTPases regulate many steps in vesicular traffic
- > Sar-1 regulates COPII assembly
- > Arf proteins regulate COPI and clathrin assembly
10
Q
Regulation of coat assembly and vesicle stability #2
A
- Sar1-GDP = cytosol = inactive
- > Contains an amphipathic helix hidden in this form
- Sar1-GTP = ER membrane-bound = active
- Sar1-GEF causes Sar1-GDP to exchange GDP for GTP, which causes a conformation change causing the amphipathic helix to show
- Active Sar1-GTP then promotes the assembly of coat complexes
- GTP hydrolysis causes coat disassembly after budding
- > The GTPase regulates both coat assembly and disassembly
11
Q
Vesicle docking and targeting
A
- Membrane traffic needs to proceed in an orderly way
- > Transport vesicles must be highly selective in recognizing the correct target membrane with which to fuse.
- Specificity in targeting: surface markers that identify vesicles according to their origin and type of cargo.
- Target membranes display complementary receptors that recognize the appropriate markers.
12
Q
SNAREs
A
- One of the two classes of protein that control recognition of donor vesicles by acceptor membranes
- Provide specificity & catalyze vesicular the fusion of with the target membrane.
- v-SNAREs: vesicle
- t-SNAREs: target membrane
- The interaction between v-SNAREs and t-SNAREs drives fusion between the vesicle and the target membrane
13
Q
Rabs (GTPases)
A
- One of the two classes of protein that control recognition of donor vesicles by acceptor membranes
- Work together with other proteins to regulate the initial docking and tethering of the vesicle to the target membrane; on the vesicle
14
Q
Cargo Recruitment
A
- Entry into vesicles leaving the ER is usually a selective process
- Membrane proteins: have exit signals in their cytosolic ‘tails’ that are recognized by coat proteins
- Soluble Proteins: bind to cargo receptors that have exit signals in their cytosolic ‘tails’
15
Q
Vesicular tubular clusters
A
- Transport vesicles leaving the ER fuse together to form intermediate compartments called these
- Travel towards the cis Golgi via motor proteins on microtubule tracks
- Generate coated vesicles going back to the ER (COPI coat) – retrograde transport
16
Q
ER retrieval signals
A
- Membrane ER resident proteins: retrieval signals in their cytosolic tails & recognized by COPI coat proteins
- Soluble ER resident proteins: Retrieval signals within their structure & bind to receptors
- > EX: KDEL sequences
- > Bind to KDEL receptors
17
Q
CISTERNAE = STATIC (Vesicular Transport Model)
A
- Vesicles travel between them
- The identity of each cisternae is fixed (cis stays cis and trans stays trans
18
Q
CISTERNAE = DYNAMIC (Cisternal Maturation Model)
A
- Move upward, changing their properties slightly as they migrate
- The membrane of the Golgi will mature into the membrane that is next to them (like a treadmill)
- These models are not mutually exclusive; some proteins can move by the static model, while others can travel via the dynamic model
19
Q
Transport from the Trans Golgi Network to the Cell Exterior
A
- Many proteins leaving the Golgi need to get to the plasma membrane.
- Vesicles carrying such proteins fuse with the plasma membrane via exocytosis.
- Vesicle cargo:
- > Membrane proteins and the lipids: become part of the plasma membrane
- > Soluble proteins: are secreted into the extracellular space
20
Q
Constitutive secretory pathway
A
- One of 2 pathways for transport from the Trans Golgi Network to the Cell Exterior
- The vesicles bud off of the Golgi and fuse with the plasma membrane and the contents get secreted into the extracellular space
- This doesn’t require any external signals and it happens all the time
21
Q
Regulated secretory pathway
A
- One of 2 pathways for transport from the Trans Golgi Network to the Cell Exterior
- Special vesicles that store secretory proteins are synthesized and move to the plasma membrane
- They then wait for an extracellular signal to then fuse with the plasma membrane and then released their contents
22
Q
Regulated Secretion
A
- Cells that are specialized for secreting some of their products rapidly and “on demand” concentrate and store these products in secretory vesicles.
- These secretory vesicles only release their contents to the cell exterior in response to extracellular signals.
23
Q
Clathrin-Coated Vesicles
A
- Used for transport between plasma membrane, endosomal system, and Golgi
- Clathrin was the first coat protein to be identified:
- > Composed of 3 copies each of heavy chain and light chain
- > Arranged in a “triskelion” (three-armed pinwheel shape)
24
Q
Clathrin coats
A
- Clathrin
- Adaptor proteins
- Transmembrane cargo receptors
- Together, they capture and package cargo molecules within the donor compartment into a budding vesicle
- Clathrin coat is rapidly lost shortly after forming
25
Q
Dynamin
A
- The pinching-off of clathrin-coated vesicles is controlled in part by this cytoplasmic protein
- Lipid binding (specific phosphoinositide) & GTPase
- Wraps (oligomerizes) around the stem of the budding vesicle
- Brings inner leaflet membranes of vesicles together
- Fusion of these membranes severs the vesicle from the donor compartment.
- GTP hydrolysis regulates rate of vesicle pinching-off
26
Q
Lysosomal Enzyme Cargo
A
- Soluble
- Carry a unique marker: mannose 6-phosphate (M6P) groups
- Added to N-linked oligosaccharides in cis-Golgi
- Recognized by M6P-receptor in TGN and packaged into clathrin-coated vesicles for delivery to lysosomes
27
Q
Targeting of Lysosomal Enzyme Cargo to Lysosomes
A
- M6P groups are recognized by cargo receptors and recruited into clathrin-coated vesicles
- > Acidic compartment of early/late endosomes dissociates M6P receptors from cargo
- > Receptors recycled back to Golgi.
28
Q
Endosomes
A
- Intermediate organelles in vesicular transport pathways
- Vary in size/shape
Receive cargo from Golgi and PM - 3 main classes:
-> Early
-> Late
-> Recycling - Early endosomes ‘mature’ into Late endosomes, which ‘mature’ into lysosomes in part by becoming increasingly acidic
29
Q
Endosomes #2
A
- Also characterized by the presences of internal vesicles
- Such late endosomes are called ‘multivesicular bodies’
- > One of the ways that vesicle membranes can be recycled for other purposes
- The budding and internalization of vesicles from the plasma membrane is called endocytosis
30
Q
Vesicle cargo
A
- Membrane proteins and the lipids:
- > Are removed from the plasma membrane
- > Some will be Recycled back to the surface
- > Some will be Degraded
- Soluble proteins
- > from extracellular space
- > carried in the lumen
- Common membrane protein cargo: receptors +/- their ligands
31
Q
Phagocytosis (“cellular eating”)
A
- The ingestion of large particles, such as microorganisms or dead cells via
- Vesicles called phagosomes (generally >250 nm in diameter)
32
Q
Pinocytosis (“cellular drinking”)
A
- The ingestion of fluid and solutes
- Vesicles called pinocytic vesicles (about 100 nm in diameter).
- Includes receptor-mediated endocytosis
- Most eukaryotic cells are continually ingesting fluid and solutes by pinocytosis
- Cholesterol gets into cells via receptor-mediated endocytosis
33
Q
Epidermal growth factor receptor (EGFR)
A
- Gets degraded, along with its ligand, but transferrin receptor (TfnR) gets recycled to plasma membrane
- The degradation of EGFR ends the signal
34
Q
Transcytosis
A
Molecules internalized at one end of a ‘polarized’ cell are transported to a different end (and vice versa)
