Transporting Stuff in Cell: Vesicles

By yuzuktasiyan | Understanding Cells | 30 May 2021

We all know the cell is dynamic, and the organelles too. The cell must tightly regulate the rate of delivery to, and removal from, individual
organelles.

If there were more outgoing than incoming vesicles at a particular organelle, the compartment would disappear over time. If an organelle received too many vesicles, it would grow too big, most likely at the expense of other organelles. Also, the inappropriate delivery of cargo and membranes may even change the organelle identity and interfere with its proper function. We got it, controlling vesicles is important.

Figure I: Formation and Transport of a Vesicle

So, what is the mechanism by which transport vesicles are formed? What are the mechanisms by which selectivity is conferred?

Let's start with the identity of a vesicle. The major factor affecting the identity is the origin of the vesicle which determines the chemical property of the vesicle's membrane. Then, the direction of the transport and fusion are will be determined by molecular matching (receptor/ligand) during the journey of the vesicle.

I can say the interesting part is the formation of the vesicle. Because directing a bunch of chemicals from somewhere to somewhere and calling it life is more like a magic trick. There is three-step of forming a vesicle: budding, docking/tethering, and fusion.

Figure 2: Vesicle Formation

At a smooth surface, some budding proteins ( Clathrin, COPI, or COPII) create tension and it causes a bulb formation. This is because the attached proteins have an angle. The addition of coating proteins creates a great tension on the membrane surface such that it releases a bubble which is our vesicle. This is it, and all the coating proteins leave. Some others catch the vesicle at the other end of the way. There Rab proteins on the vesicle which is recognized by tethering proteins. The specific combination of Rab and tethering proteins ensure that the vesicle is binding to the correct organelle. Then, the SNARE proteins start their dancing, like a snake. Their interaction creates tension both in the vesicle and target organelles membranes and forces them to fuse. This is it, the cargo is delivered. THIS IS WHAT HAPPENS WHEN YOU BUY SOMETHING ON AMAZON.

Pictures

1) Molecular Biology of the Cell ( Garland, 2015)

2) www.cell.com/fulltext/S0092-8674(03)01079-1

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Understanding Cells

The cells are our bodies' simplest units. But they are so complicated that, the life of many scientists, included me, is spent to understand its mechanisms!