Intracellular Compartments and Vesicular Trafficking Flashcards
What are the two directions that vesicles can move from the ER?
Anterograde: from the ER to Golgi, plasma membrane and to lysosomes
Retrogrades: From endosomes to golgi to ER
Is the inside of vesicles more similar to the cytosol or extracellular space?
Vesicle interiors are topologically the same as extracellular space
What are the 5 major steps of vesicular transport?
1) Cargo selection/loading
2) Vesicle budding
3) Vesicle transport
4) Vesicle targeting
5) Vesicle fusion/ cargo delivery
How does the clathrin coat impact the membrane shape?
The binding of Clathrin induces a vurvature into the membrane which begins the formation of a transport vesicle
True or false: Vessicles passively move throughout the cytosol.
FALSE
Vesicles are relatively large and require active transport by motor proteins
What is the function of Rab proteins?
Rab proteins connect transport vesicles to motor proteins
Rab proteins also target vesicles to the appropriate destination point and interact with Rab effectors (tethering proteins) once they are near the target membrane
What are the three different coat proteins and when does each play a role in vesicle transport?
COPI: Retrograde movement
COPII: Anterograde movement
Clathrin: Exo/Endo-cytic vesicles between golgi, endosomes and plasma membrane
What role does Sar1 play in the formation of vesicles?
Sar1 providees a site to which COPII coat proteins can bind (Coat recruitment GTPase)
Sar1 is activated by Sar1-GEF which exchanges GDP for GTP.
The Sar1-GTP complex is membrane bound and marks the transitional ER for COPII binding
What amino acid sequence is the “ER retention motif”?
KDEL
This sequence is unique to proteins that should remain in the ER lumen. If the protein is accidentally sent to the golgi network, it will be returned via retrograde trafficing
How are specific cargo molecules selected for export in vesicles?
Receptors specific to the cargo molecules collect the cargo at the membrane and recruit adaptor proteins and clathrin coat proteins for vesicle formation
What are the different mechanisms by which integral membrane proteins can be selected for transport?
The cytoplasmic tails of the receptors are marked via:
Phosphorylation
Amino acid motifs
Ubiquitination
Describe how LDL receptors make cholesterol available to the cell.
LDL, a lipoprotein complex used to transport cholesterol between cells, binds to LDL receptors on the cell surface
Endocytosis via clathrin coated vesicles is initiated
Clathrin leaves and acidic endosomes are formed
LDL is hydrolyzed in lysosomes and cholesterol is made available to the cell
What is the result of a mutation that prevents LDL receptors from binding to clathrin adaptor proteins?
“Hypercholesterolemia”
Less cholesterol can be uptaken into the cell
Most dangerous clinical result: atherosclerosis
Describe the structure of clathrin
Triskelion structures with 3-fold symmetry
Clathrin molecules self associate into icosahedrons
What does Dynamin do?
Dynamin is a snake-like protein that helps with the vesicle fission process
ATP hydrolysis tightens the dynamin coil which “pinches off” the budding vesicle
Do coat proteins impact vesicle targeting?
No. Coat proteins are removed after the budding stage.
The specificity of vesicular transport is determined by entities below the coat proteins on the vesicles
What role does phosphatidyl inositol play in vesicular targeting?
Phosphatidy inositol has 5 OH groups that can be selectively phosphorylated
The phosphorylation pattern dictates which proteins can bind and thus the destination of that vesicle
What is a Rab5 membrane domain?
Markings on target membranes (and vesicular surfaces) are built up to encourage specific vesicle targeting/recognition
These domains are made up of active Rab5-GTP, Rab effector proteins, and phosphatidyl inositol
What are the SNARE proteins?
Proteins that promote vesicle fusion by positioning the vesicle close to the membrane.
v-SNARE (on vesicules) interacts with t-SNARE (target membrane)
When bound: 4 stranded alpha helical coiled coil
How are SNAREs taken apart following vesicle fustion?
They must be pried apart by accessory proteins and NSF which require ATP energy
How does BOTOX work?
Botulism toxin binds to neurons at neuromuscular junctinos and cleaves SNAREs resulting in paralysis because acetylcholine vesicles cannot fuse
Describe the structure of the golgi apparatus.
Organized into 5 different compartments
cis Golgi network, cis cisterna, midial cisterna, trans cisterna, and trans Golgi network
What are the two pathways of protein secretion from the Golgi appartus?
Constitutive secretory pathway: vesicle fusions are unregulated
Regulated secretory pathway: do not fuse without an extracellular signal
How are secretory storage granules formed?
Forward transport moves cargo into vesicles while reverse transport removes unintended cargo from the vesicles
Increasing concentrations of homogenous cargo are formed by this process
