Question

How are transport vesicles made?

How do they select their cargo?

How do they find their target membrane?

How do they fuse with the target membrane?

Answer 1

A vesicle is the small and spherical compartment which, is separated from the cytosol by a lipid bilayer. Many of the vesicles also made in the endoplasmic reticulum and the Golgi apparatus. These are made by the process of endocytosis. The process of how the vesicles made, select their cargo, finds their target membrane and fuse with the target membrane is shown here.

1. A vesicle form when the membrane bulges out and pinches off. This process occur when coat protein assemble at the membrane. They force the bilayer of lipid to begin to bend. This process helps in making the vesicle. There are three type of coat proteins. The vesicles are mostly clathrin-coated. Clathrin is a coat protein which, allow the uptake of extracellular molecules from the plasma membrane by the endocytosis.

2. cargo is the target that has to be carried by the vesicles to their target sites. Cargo selection is usually mediated by the COPII and AP-2/clathrin vesicle coat complexes. Coat assembly support localized and selective cargo sorting. A conserved set of GTPase switch proteins controls assembly of different vesicle coats.

3. All the three coated vesicles contain a small GTP-binding protein. There are two sets of small GTP-binding proteins for vesicle secretion. One is ARF and sar I and another is Rab protein. ARF protein exchanges bound GDP for GTP and then bind to its receptor. The different Rab GTPases and Rab effectors control docking of different vesicles on target membrane.

4. Activated rabs release GDI, attach to the membrane via covalently attached lipid grpou at their C-termini and are incorporated into transport vesicles. Rab-GTP recruits effectors that can promot vesicle formation, vesicle transport and vesicle fusion with target membrane.