Tethers form structures that extend away from the compartment membrane into the cytosol. This may help tethers interact with vesicles arriving from the previous membrane compartment.
A second set of proteins that helps correctly target vesicle to the appropriate membrane is the SNAREs. SNAREs also mediate fusion between membranes. SNARE proteins on vesicles and membrane compartments interact with specificity.
By localizing those SNAREs which interact only to vesicles and their target membrane, cells ensure that vesicles fuse to their correct target membrane.
SNARE proteins mediate fusion between vesicles and their target membrane compartment. SNARE proteins contain long regions that form helical structures.
The energy released through complete pairing of vSNAREs and tSNAREs is thought to drive fusion between vesicle membrane and compartment membrane, though the exact mechanism remains unclear.
Some vesicles dock on their target membrane but do not fuse. For example, secretory vesicles store proteins and other small molecules until the cell is signaled to release them. External signals trigger the removal of the pairing inhibition, allowing the vesicles to fuse with the plasma membrane. Upon reaching the trans-Golgi network, most proteins are targeted to their final destination.
The default pathway appears to be transport to the plasma membrane, as the plasma membrane needs to continuously replace lipids and proteins.
Other proteins are sorted to lysosomes and secretory vesicles. The signal to send a protein to the lysosome involves the sugar side chain. Most lysosomal proteins contain mannose 6-phosphate which is added in the cis-Golgi. The receptor that binds mannose 6-phosphate resides in the trans-Golgi network and recruits coat proteins to the trans-Golgi network.
Clathrin forms the coat around these vesicles, and the vesicles accumulate lysosomal proteins before budding from the trans-Golgi network. These vesicles fuse with endosomes. The lumen of endosomes has a low pH causing the mannose 6-phosphate receptor to dissociate from lysosomal proteins. The mannose 6-phosphate receptor is returned to the trans-Golgi network and the vesicle containing the lysosomal proteins matures into a functional lysosome.
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Intracellular transport Start this free course now. Free course Intracellular transport. Figure 11 Secretory pathways. Molecules in the trans Golgi network are sorted into secretory vesicles either for regulated secretion if they contain appropriate signal sequences or for constitutive secretion if they do not.
The lysosomal enzymes and the macromolecules to be digested that came in from early endosomes are transferred to lysosomes. Recycling of cell surface receptors and mannose- 6 - phosphate receptors by endosomes. For example, some lysosomal proteins end up in vesicles that are secreted. Cells have a retrieval pathway to catch these errant lysosomal enzymes. This rather elaborate system is simply a consequence of the normal operation of the sorting pathways.
The mannosephosphate receptors are at the surface because the targeting process that brings most of them back to the trans Golgi network is also not perfect. Overview : There are two different secretory pathways, the Regulated Pathway and the Constitutive Pathway.
In the regulated pathway proteins are consolidated into vesicles that are stored in the cell until they are secreted in response to a specific signal. In the constitutive pathway vesicles continuously form and carry proteins fron the Golgi to the cell surface.
Overview : In the lysosomal pathway proteins destined for lysosomes e. Here the lysosomal proteins are separated from their receptors and are sent by vesicles to lysosomes red arrows and the receptors are returned to the Golgi blue arrow. Overview : in the endocytotic pathway, external proteins bind to receptors in the plasma membrane and are incorporated into vesicles that are carried to early endosomes green arrow , where the receptors are separated and returned to the cell surface blue arrow.
Proteins from early endosomes is then moved to late endosomes and then to lysosomes green arrows where the proteins are digested by the lysosomal enzymes. Figure Macrophage engulfing two red blood cells. Notice the collar-like pseudopodium creeping over the surface of the red blood cells red arrows. Given what you know about the cytoskeleton, what do you think is happening inside the macrophage to make this happen? Click to enlarge. The protein binds to receptors on the cell surface and in internalized in clathrin-coated vesicles.
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