exocytosis Flashcards
what are the two types of exocytosis?
constitutive: happens continuously and does not require a trigger
regulated: happens in response to a trigger. only specialized cells use (neurons, goblet cells, pancreatic beta cells, odontoblasts)
how does constitutive exocytosis occur and examples?
cargos are packaged in the trans golgi and are secreted immediately via exocytosis all around the perimeter of the cell.
fibroblasts, osteoblasts, and chondrocytes are some examples of cells that perform constitutive exocytosis
how does regulated exocytosis occur and examples?
cargos are packaged in to vesicles and vesicles are stored until the appropriate trigger comes along. trigger may be an action potential or a chemical messenger molecule (secretagogue)
if action potential, VGCC’s open and increase intracellular Ca2+
what determines where cargo goes?
sorting signals. proteins destined for lysosomes have M6P modification. proteins destined for regulated exocytosis have a sorting signal.
proteins lacking a signal automatically get packed for constitutive secretory pathway
how do proteins become concentrated within secretory vesicles?
after vesicle budding, bilayer is retrieved from the vesicle by clathrin mediated budding, this concentrated cargo. vesicles may also contain a matrix of proteoglycans that reduces osmotic pressure and allow the high concentration of cargo
some secreted proteins are synthesized as larger proteins called
pre-pro-proteins. some proteins are to small to be properly co-translationally translocated into the ER so they are first made as pre-pro-proteins and then are cut to the correct size. The specific modifications made to a pre-pro-protein can vary between cell types
what does it mean for a cell to be polarized?
different domains of the PM contain different complements of proteins. polarization is maintained by tight junctions that create a molecular fence between apical and basolateral domains. the axon hillock in neurons functions as a molecular fence
how do vesicles get to the correct domain for exocytosis? two pathways
direct sorting pathway: the protein is sent directly to the appropriate domain of the PM from the trans golgi
indirect sorting pathway: the protein is initially sent to an inappropriate domain (basolateral) of PM and then retrieved to an early Endosomes and trafficked to appropriate domain
how do vesicles fuse with PM?
vesicles must dock very near the PM (2 nm).
Rab (vesicle) and Rab Effector (target) meet and the vesicle gets closer to the membrane. Then SNAREs can interact and pull even closer, Rab is released. Winching action occurs between the v-SNAREs and t-SNAREs which brings the vesicle within 2 nm of the target membrane and allows for fusion. The SNAREs untwist by NSF enzyme and can be re-used
what is synaptotagmin
the Ca2+ sensor for regulated exocytosis. only when calcium is present will fusion occur. it is a v-SNARE
what is the process of vesicle fusion in nerve terminals specifically? name the proteins
- Docking: synaptobrevin (v-SNARE) and syntaxin (t-SNARE) intertwine
- Priming: SNAP25 peripheral protein helps with SNARE winching and complexin modulates SNARE bundle
- Fusion Pore Opening: Ca2+ ions activate synaptotagmin and allow for fusion to occur
- fusion complete: cargo is released as fusion is complete
why is it important that vesicles can be recycled and refilled near the site of exocytosis?
synaptic terminals can be very far away from a neurons soma so it would take much longer to refill from there. instead, vesicles are refilled by recycling endosomes located within the presynaptic terminal and this is much faster.
How are vesicles refilled?
through V-type H+ pumps
what is kiss and run exocytosis?
the vesicle does not fully fuse with the PM, it only fuses enough to open and release cargo
