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When an animal cell is going through endocytosis it cell surrounds a food particle, and the membrane swallows it, creating a vesicle within the cell.

However, what happens to the embedded transmembrane proteins previously in the cell membrane? Do they shut down? Are they degraded? Or do they just not cause a problem that the cell just leaves them be?

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    $\begingroup$ Do you have a specific protein in mind? As @ScienceIsGolden points out in their answer, different proteins respond to this situation differently. $\endgroup$
    – James
    Dec 17 '19 at 9:25
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Endosome formation.

For the sake of keeping this somewhat brief, I'm going to stay somewhat surface level.

When a vesicle is endocytosed, it is called a primary endocytic vesicle. When multiple primary endocytic vesicles fuse together, they form a larger entity called an early endosome.
Early endosomes can also be called "sorting endosomes" because different portions of the vesicle are sorted into different destinations, including these things called "recycling endosomes."

Transmembrane proteins in endosomes

In my experience, transmembrane proteins largely get sorted into these recycling endosomes and are then re-inserted into the cell membrane. To my knowledge and from some cursory review on the topic, this is a common outcome for transmembrane proteins; they are eventually recycled and placed back into the cell membrane via exocytosis after being sorted into a recycling endosome.

Further reading

I'm sure others can add more explanation and some of the alternative routes these proteins can take, but in the interest of brevity and clarity, I thought it best to just stick with what I've come to know as most common. Also, if you'd like a helpful source, see if you can get access to the following paper through a school or library service: Elkin, S. R., Lakoduk, A. M., & Schmid, S. L. (2016). Endocytic pathways and endosomal trafficking: a primer. Wiener Medizinische Wochenschrift, 166(7-8), 196-204.

In a phenomenon that I think is really cool, some receptors are thought to actually keep functioning after they are endocytosed and form these things called "signalling endosomes" (search "signalling endosomes NGF and TrkA" to read more).

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  • $\begingroup$ Lovely answer. I've made some superficial edits to make it clearer, but feel free to roll back. Could you point me toward some article explaining the reinsertion process? As far as I am aware, unless it is a post-translationally inserted tail-anchored-TMP or a mitochondrial TMP, TMPs can only be inserted via the Sec61 translocon at the ER. I would be interested to learn more about how this mechanism works. $\endgroup$
    – James
    Dec 17 '19 at 9:34
  • $\begingroup$ Hi James! Thanks for the edits, definitely made the answer more digestible! I tracked down an article that will hopefully get you the info you're looking for: ncbi.nlm.nih.gov/pmc/articles/PMC3038567 The specific section to look at is "From the ERC to the plasma membrane." This article and others point largely to the protein Arf6 for this process, but this specific paper does the most to explain its role in re-insertion. Hope that helps! $\endgroup$ Dec 17 '19 at 21:07

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