Vesicles

Different kinds of vesicles abound in cells. They come in all sizes and content but they all transport something, be it towards the nucleus or towards the cell membrane or between cell organelles. Anyway, one thing is common among these vesicles; they usually have coats and markers. The coats generally indicate their source while the markers indicate their destination.

For example,3 types of coats are used by cells when they transport substances between the ER and Golgi: coatomer I (COP I), coatomer II (COP II) and clathrin.

Vesicles carrying newly formed proteins from the ER are coated with COP I while COP II coats vesicles that transfer substances between the different aspects of the Golgi apparatus. If a vesicle has to transport substances from the Golgi back to the ER however, COP II will coat this vesicle.

Usually if a vesicle contains newly finished product from the Golgi then clathrin is used to coat this. Newly synthesized lysosomal enzymes for example are clathrin-coated. Clathrin also coats newly formed endocytic vesicles. Actually these coats all help in the vesicle formation and later on in the transport process.

There are of course different markers for different destinations of these vesicles. So some vesicles are marked for lysosomes only or for “export”only.

Vesicle formation is actually a complex process involving many steps. What is presented here is a highly shortened version. And not all clathrin coats are the same.