Lecture 12: Exocytosis, Endocytosis, and recycling Flashcards
Exocytosis
Transport from the trans golgi network to the cell exterior
Constitutive vs regulated secretion
What are the three sorting possibilities in the TGN?
- Signal-mediated diversion to lysosomes via endosomes (mannose-6-phosphate receptors)
- Signal-mediated diversion to secretory vesicles (for regulated secretion)
- Constitutive secretory pathway
____ help mark organelles and membrane domains in the late secretory pathway
Phosphoinositides
Regulated secretory vesicles are also called secretory _____ based on their appearance under the electron microscope
granules or dense-core granules
How does proteolytic processing occur in the late Golgi and within secretory vesicles?
Different peptide hormones can be made from the same pre-protein in different cell types
What are the 5 stages of synaptic vesicle fusion?
Compound exocytosis? example?
Compound exocytosis - vesicle-vesicle fusion as well as vesicle PM fusion
Ex-mast cell degranulation
GLUT4 translocation is an example of ____. How does this work?
- regulated excretion
- Insulin binds to insulin receptor -> sends an intracellular signal that causes glucose receptors to reloacte to the PM -> causes an influx of glucose into cell
What are 4 examples of regulated exocytosis leading to plasma membrane enlargement?
Secretion is _____ in polarized cells. How is this possible?
- directional
- The apical plasma membrane of epithelial cells have a different protein content than the basolateral plasma membrane
What are two ways that polarized exocytosis and transcytosis occur?
Apical sorting may involve lipid microdomains called lipid rafts. How?
Only proteins with longer transmembrane domains can go into the raft phase.
Endocytosis
Internalization of external material including proteins located on the plasma membrane
3 types of endocytosis?
- Receptor-mediated endocytosis
- phagocytosis
- pinocytosis
Receptor-mediated endocytosis
- Cellular uptake of macromolecules usually en route to the lysosome
- Ingestion of metabolites
– Cholesterol through LDL and the LDL receptor
– Iron via the transferrin receptor
• Termination of cell surface events (ie signaling)
Phagocytosis
- “cellular eating”
- Ingestion of large particle such as microorganisms or dead cells (usually >250 nm in diameter)
- Usually triggered
Pinocytosis
• “cellular drinking”
- Ingestion of fluids and solutes (~100 nm in diameter)
- Constitutive
COPII coats vesicles in the ___
ER- heading out toward PM
COPI coats vesicles in the ___
TGN- heading back towards ER
Clathrin coats vesicles at the ___
PM, Early and late endosome
Mechanism of endocytosis
- Coat proteins are used to deform the membrane and form a coated vesicle
- primary coat protein is clathrin
Clathrin coats are composed of ___
the clathrin complex (a triskelion-3 heavy chains and 3 light chains) and clathrin adaptors
The diversity of clathrin coats (ie budding form different compartments) is generated by…
using different adaptors
____ proteins link integral membrane proteins to clathrin
Adaptor
What is responsible for uncoating clathrin coated vesicles?
- “Uncoating ATPase”
- Cytosolic HSP70 family member
How does the clathrin adpator AP2 work?
- When AP2 is bound to phosphoinositides and cargo recpetors, it opens and allows clathrin to bind
Unlike internal membranes, vesicles derived from the plasma membrane require help of _____for membrane fission
dynamin
What is dynamin? Whar does it do?
• Dynamin is a GTPase that constricts the neck of the newly formed coated pit to form a vesicle
Why do PM derived vesicles require the help of dynamin?
– Different phospholipid bilayer composition
– Increased membrane stiffness
– Regulation of internalization (sometimes a pit is good)
Low density lipoprotein (LDL) particles are responsible for…
import of cholesterol esters into the cell
How does import of extracellular cholesterol occur?
What are the three potential fates of endocytosed membrane proteins?
- degradation in the endolysosome
- recycling
- transcytosis
_____ form as early endosomes mature to late endosomes.
- Multivesicular bodies (MVBs)
- This process separates and sequesters material (particularly membrane proteins) to be degraded from material to be recycled.
membrane proteins are marked for inclusion into MVBs by ____
ubiquitin, which detaches just before formation of intralumenal vesicles
___ proteins are responsible for sorting into and formation of MVBs
ESCRT
MVB formation is the same process as ____
virus budding from the PM
How does synaptic vesicle recycling occur?
What are the specialized types of endocytosis?
- Phagocytosis
- Pinocytosis
- Transcytosis
Phagocytosis occurs by…
psuedopod formation
How are psuedopods formed?
Pseudopod formation is driven by actin polymerization and reorganization in response to specific phosphoinositide generation.
Macropinocytosis
What are caveolae?
Membrane invaginations, similiar to an endocytosed vesicle that has failed to detach from the PM
