Mechanisms of Secretion and Exocytosis Flashcards
Exocytosis
process by which intracellular membrane bound container fuses with PM
Consequences exocytosis
- Container contents released into extracellular space
2. PM expands with membrane components supplied by container
Cargo of exocytosis
- Soluble lumen and membrane proteins (derived from secretory pathway)
- Ions and sm molecules transported across membrane by specialized protein channels and transporter
2 types exocytosis
constitutive and regulated
constitutive exocytosis
after biogenesis transport carriers will travel to, dock, and fuse with PM without further signal input
Regulated exocytosis
after biogenesis transport carriers only fuse to PM in response to signal
Mammary alveoli
Example of constitutive exocytosis; can be manipulated to produce biological pharmaceuticals (use animals who are biologically engineered to produce certain pharmaceuticals in milk); better to do this way b/c cheaper and bc proteins need to fold correctly and this is correct mammalian environment
Examples regulated exocytosis
Sperm and egg fusion (calcium regulation), neuronal exocytosis (neurotransmitter w/ neurosecretory vessel), insulin secretion (endocrine cells make insulin, released by exocytic release hormone); contact dependent (cytotoxic T cells, contacts target then endocytosis and releases content)
paracrine signaling
signaling molecules released by cell only affect target cells in close proximity, conducting of electric impulses from 1 nerve cell to another via paracrine signaling
Ca2+ signal and exocytosis
Rise in intracellular calcium leads to exocytic vesicles fusing with PM
Regulated insulin secretion
- Pancreatic endocrine beta cells respond to elevated B/G
- Intracellular rise in ATP -> Closure ATP sensitive K+ channels
- K+ channel closes depolarizing cell -> influx Ca2+ from voltage gated ion channel
- rise intracellular Ca2+ -> insulin containing exocytic vesicles to fuse with PM
Insulin uptake into cell
- Insulin receptor on PM binds insulin
2. Signal causes relocalization of glucose receptors to PM boosting glucose uptake into cell
bilayer fusion
SNARE proteins responsible for bilayer fusion during vesicle transport
properties of SNARE proteins
- LG protein family w/ compartment specific location
- Membrane anchored (mostly TM proteins)
- Large cytoplasmic domain w amphipathic helices
- Ampipathic helices continuous w anchoring transmembrane domains
- sufficient for membrane fusion when reconstituted in liposomes
- complimentary sets on 2 membranes destined to fuse (v-SNARE and t-SNARE)
* stabile associated with membranes**
V-snares
on secretory vesicle membrane (synaptobrevin in case of neuronal exocytosis)
T-snares
on target PM (SNAP-25 and syntaxin t-SNAREs of neuronal exocytosis)
Molecular mechanism of SNARE protein action
very broadly, not tested on this
coiled-coil structure bc ampipathic alpha helices wrap around each other and form super helix with stable rigid superstructure
Assembly of coiled-coil structure
(4 helix bundle) drives bilayer fusion with complimentary sets of SNARE protein on vesicles and target compartments; common feature SNARE complex formation at other stages of membrane fusion in secretory and endocytic system
viral entry into cell
exploits endocytosis; goes through PM this way then into cytoplasm PH changes leads to conformational change and this is when it breaks out
SNARE protein complex assembly
drives exocytosis; v-SNARE and t-SNARE complimentary; 4 helical bundles form synaptic vesicle exocytosis
Enveloped viral mediated fusion vs SNARE mediated membrane fusion
Virus has to be both V snare and T snare and bring both elements together
botulism toxin
SNARE protein synaptobrevin is target of botulinum toxin protease (botulism is a neurotoxin); pathogenic bacteria Clostridium botulinum present in soil; birds can ingest toxin or eat invert containing toxin; cycle develops in botulism outbreak when fly larvae feed on animal carcasses and ingest toxin, ducks can develop botulism from eating 3-4 maggots
Toxins and SNARE proteins
other SNARE proteins are targets for other bactieral toxins (toxins are different proteases for different snare molecules)
membranes of exocytic vesicles
SNARE proteins not only protein molecule on membrane of exocytic vesicles others also contribute
contributions non snare proteins on membrane exocytic vesicle
- participate in docking and fusion of exocytic vesicles
- model organisms used to ID these proteins
- many of these proteins regulate SNARE fx (ex NSF)
- specialized transporters import small molecules and ions into exocytic vesicle lumen
NSF
ATPase chaperone protein that assists other proteins achieve correct conformation (this -> unfolding SNARE -> conf change to disengage it from coiled-coil bundle so that it can do its job multiple times otherwise it would bind and since it binds V stabile it would stay there)
Secretion of specialized secretory lysosome
vesicles fusing w PM during exocytosis usually Golgi derived but there are types specialized lysosomes in differentiated cells that undergo exocytosis
- Hematopoietic cells such as granulate exocytosis of CTL
- Melanosomes
- Sperm acrosome
Cargo specialized secretory lysosome
once such cargo is cytotoxic T cell which is contact dependent
Lysosomes and exocytosis
regular lysosomes of all cells have capability to undergo regulated exocytosis under special conditions; acting in repair of ruptured cell when lg quantity PM needed for resealing
Machinery used in lysosomal exocytosis
uses analogous molecular machinery of regulated exocytosis (ie SNARES and molecules that regulate SNARE machinery)
