3- Transport by vesicles: when proteins are made on the rough endoplasmic reticulum (RER), they get loaded into the Golgi apparatus. They are then sorted, modified and packaged in vesicles made from the budding-off of the Golgi membrane and discharged.

Sorting signals directs the protein to the organelle. The signal is usually a stretch of amino acid sequence of about 15-60 amino acids long.

There are at least three principles that characterize all vesicles mediated transport within cells:

                      i.        The formation of membrane vesicles from a larger membrane occurs through the assistance of a protein coat such as clathrin that engulfs the protein because an adapter protein such as adaptin binds both to the coat and to the cargo protein bringing both close together.

Clathrin-coated vesicle transport. Click on image for credit.

The adaptin traps the cargo protein by biding with it’s receptors. After assembly particles bind to the clathrin protein they assemble into a basket-like network on the cytosolic surface of the membrane to shape it into a vesicle. Their final budding-off requires a GTP-binding protein called dynamin.

                    ii.        The process is facilitated by a number of GTP-binding proteins (ex; dynamin) that assemble a ring around the neck of a vesicle and through the hydrolysis of the phosphate group of GTP to GDP until the vesicle pinches off. In other words, GTP is one of the main sources of cellular energy for vesicle movement and fusion.

Dynamin powered by GTP pinches the vesicle off the membrane. Click on image for credit.

 OR

Another picture of the same process. Click on image for credit.

                   iii.        After a transport vesicle buds-off from the membrane, it is actively transported by motor proteins that move along cytoskeleton fibers to its destination. The vesicle then fuses with a target membrane and unloads the cargo (protein). But in order to fuse a vesicle with the membrane of another compartment, they both require complementary proteins, which in this case is soluble N-ethylmalei mide-sensitive-factor attachment protein receptor or, ahem, SNARE present in the membrane – one for the vesicle (vesicular SNARE) and one for the target membrane (t-SNARE).

SNARE helping direct transport vesicles to their target molecules. Click on image for credit.

Each organelle and each type of transport vesicle is believed to carry a unique SNARE. Interactions between complementary SNAREs helps ensure that transport vesicles fuse only with the correct membrane.

Membrane fusing does not always follow immediately after docking (unloading of cargo), however it often waits for specific molecular signals.

Btw, based on coat protein, transport proteins can be classified into three types:

 

Coated Vesicle Types.

COP stands for Coatamer proteins (COP) which is the second class of coat protein that mediates budding-off of vesicles from large membranes. It can be of two types I (anterograde; moving in a forward direction. Ex; from ER to Golgi) & II (retrograde; moving in a backward direction. Ex: from Golgi back to ER).