Exocytosis Flashcards
What is exocytosis? What is the destiny of exocytosed proteins?
Fusion of of the vesicles with the PM, protein and lipid secretion.
- Transport vesicles with lipids and proteins destined for the plasma membrane normally leave the trans Golgi network in a steady stream.
- Soluble proteins inside the vesicles are secreted to the extracellular space.
How are proteins sorted in the trans-Golgi and where do they go?
- Proteins with the M6P marker are diverted to lysosomes (via late endosomes) in clathrin coated transport vesicles.
- In unpolarized cells, proteins with no special features are delivered to the cell surface by a constitutive secretory pathway, most proteins, default pathway, immediate delivery.
- Proteins with secretory vesicle signals are a part of regulated secretory pathway that is only in specialized secretory cells.
How are secretory vesicles formed?
Secretory proteins become segregated and highly concentrated in secretory vesicles by two mechanisms:
- they aggregate in the ionic environment of the trans Golgi network
- excess membrane and lumenal content present in immature secretory vesicles are retrieved in clathrin coated vesicles as the secretory vesicles mature
Why are some proteins proteolytically processed during the maturation of secretory vesicles?
Many peptide hormones and neuropeptides and secreted hydrolytic enzymes, are synthesized as inactive protein precursors from which the active molecules have to be liberated by proteolysis . The cleavages begins in the TGN, and it continue in the secretory vesicles and sometimes in the extracellular fluid after secretion has occurred.
Describe the delivery of basolateral plasma membrane proteins in the gut lumen.
Basolateral PM proteins contain sorting signals in their cytoplasmic tail: conserved tyrosine and and two adjacent leucines.
These amino acids are recognized by coat proteins that package them into appropriate transport vesicles in the
TGN (also function in endosomes to redirect the proteins back to the basolateral PM after they have been endocytosed). Lipid Rafts May Mediate Sorting of glycosphingolipids and GPI-anchored proteins to the apical PM.
Glycosphingolipids = protect the cell from digestive enzymes and low pH.
GPI-anchored proteins and lectins stabilize the rafts.
Describe the delivery of neuronal proteins and cargo. Briefly coment on the synaptic vesicle secretion.
Two types of secretory vesicles:
- secretory vesicles in the standard way released by the regulated secretory pathway
- synaptic vesicles
Most synaptic vesicles are generated not from the Golgi membrane but by local recycling from the plasma membrane in the nerve terminals. The membrane components of the synaptic vesicles are initially delivered to the plasma membrane by the constitutive secretory pathway and then retrieved by endocytosis. But instead of fusing with endosomes, most of the endocytic vesicles immediately fill with transmitter to become synaptic vesicles.
The membrane components of a synaptic vesicle include carrier proteins specialized for the uptake of neurotransmitter from the cytosol, where neurotransmitters are synthesized. Once filled with neurotransmitter, the vesicles return to the
PM, where they wait until the cell is stimulated . After
they have released their contents, their membrane components are retrieved in the same way and used again.
Define constitutive exocytosis and exocytosis in a regulated fashion.
In the regulated pathways, molecules are stored either in secretory vesicles or synaptic vesicles, which do not fuse with the plasma membrane to release their contents until an appropriate signal is received. Secretory vesicles bud from the trans Golgi network . The secretory proteins they contain condense during the formation and maturation of secretory vesicles. Synaptic vesicles, which are confined to nerve cells and some endocrine cells, form from endocytic vesicles and from endosomes, and they are responsible for the regulated secretion of neurotransmitters. Only in specialized secretory cells.
Constitutive secretory pathway operates in all eucaryotic cells , mediated by continual vesicular transport from the trans Golgi network to the PM.
