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I'm learning the basics of cell biology and one thing about the SNAP-SNARE complex is confusing me a lot.

I read that the SNAP-SNARE complex has nothing to do with endocytosis. But my question is: endocytosis and exocytosis both ultimately lead to the formation of a vesicle; and a vesicle, once formed, must fuse with a target membrane. The SNAP-SNARE complex mediates fusion of vesicles with target membranes, so wouldn't that mean that it controls the fusion of both endocytic and exocytic vesicles with their respective target membranes?

Am I making a gigantic blunder here? It would be great if someone could clarify this.

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The very definitions of endocytosis and exocytosis seem to have been misunderstood here.

Endocytosis involves the cell membrane invaginating (folding inwards) and pinching off to form a vesicle containing some material present in the environment. This vesicle may then fuse with a lysosome and the material inside it will be degraded.

Exocytosis, on the other hand, involes the fusion of a vesicle inside the cell with the cell membrane to release its contents into the external environment.

So, endocytosis does not involve the process of fusion of a vesicle with a target membrane. The fusion of a vesicle with a target membrane is, in fact, exocytosis, and this process requires the SNAP-SNARE complex.

So, I think we are justified in claiming that the SNAP-SNARE complex pertains only to exocytosis.

Reference: Essential Cell Biology by Alberts et al

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