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Secretory Pathway (Secretory Vesicles (All originate in the TGN.
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Secretory Pathway
Secretory Vesicles
All originate in the TGN.
Constitutive vesicles transit to the surface as soon as they are made. Difficult to see
Regulated vesicles are stored and wait for stimulus. Frequently dense and identifiable
Shape and morphology is dependent on protein content
COPII coats vesicles that transport cargos from the ER to the Golgi
COP1 coats vesicles that transport cargos within the Golgi and from the Golgi back to the ER
ER resident chaperones contain a KDEL retrieval sequence which is bounded by the KDEL receptor In the Golgi and then transported back into the ER via COP1-coated vesicles
Specificity depends on Rab Proteins. Once the vesicle arrives at its target, the v-SNARE binds to the t-SNARE
SNAREs aren't specific to the ER/Golgi like COPI and COPII
Golgi apparatus
Consists of 3-5 disk-shaped, flattened membrane compartments which are layered on each other to form a stack
The cis Golgi Network (CGN) is the entrance site coming from the ER
The trans Golgi Network (TGN is the exit site for vesicles traveling to the cell surface
The cisternae within a stack are distinct compartments that are not connected and have unique enzymes and molecular composition
Fenestrae are small wholes in the cisternae sheets
N-linked oligosaccharides added in the ER get more or lose more carbohydrates
O-linked glycosylation, sulfating, and phosphorylation
Rough ER
A tubular and reticular membrane compartment that has numerous ribosomes lining its cytosolic surface
It is found in all cells, but is particularly abundant in professional secretory cells: pancreatic and acinar cells
Synthesis of integral membrane proteins. Synthesis of all proteins begins in the cytosol, but for new secretory and integral membrane proteins, protein translocation pauses until ribosomes dock the ER
The first amino acids translated by ribosomes consists of a hydrophobic signal sequence, which is recognized by the signal recognization particle (SRP).
Binding of SRP to the signal sequence arrests translation and promotes interaction of the complex to an SRP receptor on the ER in association with a protein translocation channel (protein translocated)
Translocation requires ATP and a proton gradient across the membrane
the machinery is able to differentiation between hydrophobic and hydrophilic regions of the peptide chain.
The overall topology of the protein is determined by the initial hydrophobic region that is inserted
A single pass transmembrane protein with the C-terminus facing the cytoplasm is synthesized with a hydrophobic signal sequence at the N-terminus.
The signal sequence is cleaved off by signal peptidases once the remainder of the protein is translated
Translocation of a single transmembrane protein with the N-terminus located in the cytosol is initiated by an internal hydrophobic start-transfer signal.
The internal sequence is NOT cleaved and becomes the transmembrane domain
After the signal sequence is removed, the protein is folded using Chaperones. They can also target misfiled proteins to the ERAD degradation pathway. In ERAD, misfolded proteins are retrotranslocated across the ER membrane into the cytoplasm where they are deglycosylated, ubiquinated, and degraded by proteosomes.(CFTR is degraded with ERAD)
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Lipidation: a few proteins receive lipid modifications based upon motifs. Myristoylation and GPI-linkage happen immediately in the ER. Palmitoylation occurs in the plasma membrane
Smooth ER
Connected to the Rough ER but lacks ribosomes.
It is present in all cells but is most prominent in specialized cell types with cell-specific functions: cells devoted to production of steroids and detoxification
Synthesis of lipids and membranes. All bilayers get added phospholipids and not made de novo.
New phospholipids can be added into the cytoplasmic side of existing bilayers. The lipids on the opposite side use Fiippases, Floppases, and Scramblases
Produces cholesterol and steroid hormones