Intracellular Membrane Traffic Flashcards
What is the secretory pathway?
Delivers new proteins, carbs, lipids, etc to extracellular space or plasma membrane. Exocytosis
Where do cells move plasma membrane components to in endocytosis?
Endosomes (endocytotic pathway)
Where can cells bring stuff from endosomes?
Lysosome (degradation) or rest of the cell
Where does the secretory pathway start and where does it move towards?
Starts in ER, goes out to golgi and cell surface (side route to lysosomes)
What’s the role of retrieval pathways?
Balance membrane flow (same # of lipids in sending / receiving areas)
How do vesicles know where to go?
Specific combination of molecular markers on cytosolic surface of membranes to differentiate them
What are vesicle coats for?
Getting specific molecules and going too specific places
What are the three types of coats on vesicles?
COP1, COPII, and Clathrin
Which vesicle coat buds from Golgi compartments?
COP1
Which vesicle coat buds from ER?
COPII
Which vesicle coat transports stuff from the plasma membrane and between endosomal/golgi compartments?
Clathrin
What are Clathrin subunits called? Where are they?
triskelions, outer coat
What type of proteins are on the inner layer of the clathrin coat?
Adaptor proteins
What are cargo receptors?
Transmembrane receptors that capture soluble cargo molecules inside the vesicle. Adaptor proteins trap them
How many subunits to the best characterized adaptor proteins have?
4; others are single chain
What is the purpose of multiple types of adaptor proteins?
To be specific for a different set of cargo receptors
What are the light chains of a triskelion for?
Link to the actitn cytoskeleton, generating force for membrane budding / vesicle movement. Phosphorylation regulates coat assembly
What are heavy chains of triskelions for?
Fom an outer shell that binds to adaptor proteins
What on the membrane do adaptor proteins bind to? Why?
Phosphoinositides (lipids). So that cargo receptors can come bind
What gives phosphoinositols their name?
They’re phospholipids with inositol sugars at the top
How are PIPS regulated?
Phosphorylation / dephosphorylation by kinases / phosphatases
What assembles at bud necks as clathin-coated buds grow?
Dynamin
What on Dynamin regulates how fast vesicles pinch off from the membrane?
Its GTPase domain
How is Dynamin recruited?
PIPs in the neck
What happens to the clathrin coat when a vesicle is leaving the membrane?
Vesicle loses the clathrin coat
What enzymes regulate coat formation?
Coat-recruitment GTPases
What turns coat-activating GTPases on and off?
On = guanine nucleotide exchange factors (GDP>GTP)
Off = GTPase-activating proteins (GTP>GDP, hydrolysis)
Which type of coat assembly is Sar1 responsible for?
Assembly of COPII at ER
If coats aren’t dissasembled by the time they get to their target membrane (and SAR1 is still bound) what happens?
Kinases phosphorylate the coat proteins, and coat disassembly is completed. Coats need to be gone before fusion
What is the function of Rab proteins and Rab effectors?
Guide vesicles to the right place once they leave their original membrane
What is the function of SNARE proteins and SNARE regulators?
Mediate fusion of lipid bilayers
What type of protein is Rab
Monomeric GTPase (it has one subunit and binds GTP)
Where are Rab proteins?
On transport vesicles, on target membranes, or on both
How are Rab proteins activated?
Bind to GTP, makes them insoluble in cytosol and bind to membrane of organelle or transport vesicle
What is the function of Rab effectors?
Grab GTP-activated Rab proteins
Why are SNARE proteins needed?
Water needs to be displaced from hydrophilic surface of membrane for membranes to fuse - this is energetically unfavorable
Where are v- and t-SNARES found?
v-snares = vesicle membranes
t-snares = target membranes
How do v-SNARES and t-SNARES bind together?
Helical domains on each protein wrap around each other
How are interacting SNARES pried apart for reuse?
NSF cycles bewteen membranes and cytosol, catalyzes disassembly
What pH are lysosomes? Why?
Low (5.0, about 2 lower than cytoplasm). So that hydrolytic enzymes can activate and eat things. Protect lysosomes from damaging cells (enzymes that leak out are inactivated at lower pH)
After macromolecules are digested in lysosomes, what happens to their useful products (amino acids, sugars, nucleotides)?
Transport proteins in lysosome membrane carry them to the cytosol
How does H+ get into lysosomes to keep pH low?
vacuolar h+ ATPases pump protons in
What happens when late endosomes and newly synthesized lysosomal hydrolases fuse?
Form endolysosomes, which fuse with each other to become lysosomes
Which lysosome degradation pathway specializes in nonspecific uptake of fluids / membrane particles attached to the plasma membrnae?
macropinocytosis
What is the benefit of autophagy over other forms of degradation (e.g. proteosomes)?
Autophagy can remove large objects from the cell
How are autophagosoes formed? What happens after it’s formed?
Double membrane surrounds cargo, then it merges with lysosome for degradation
What happens to endocytotic vesicles after they enter the membrane?
Fuse with the early endosome
What types of pinocytic vesicles form lipid rafts and originally were recognized by their ability to transport molecules across endothelial cells?
Caveolae
How do most aniaml cells take up cholesterol into the cell?
Receptor-mediated endocytosis
How do LDL’s get into the cell? What are they for?
Transmembrane receptors grab them and they get inserted. For getting cholesterol for membrane synthesis.
What do LDL receptors transport while they’re still going to the lysosome?
Cholesteryl ester. Turns into cholesterol in the lysosome
