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During intracellular proteins synthesis, all proteins are made by free ribosomes in the cytoplasm and some, but not all ribosomes (those which make membrane or secretory proteins) move to the endoplasmic reticulum (ER) and attach to it during translation (called co-translational transport).

Why are proteins that will be exported to the outer of cell are not made in the ER directly? What is the function of ER in intracellular protein synthesis? What is the purpose of co-translational transport?

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Why are proteins that will be exported to the outer of cell are not made in the ER directly? What is the function of ER in intracellular protein synthesis? What is the purpose of co-translational transport?

This is a eukaryotic cell that we are talking about.Once the mRNA is synthesized , it has to move out from the nucleus to the cytoplasm.During its exit, SRP (Signal Recognition Particle) recognizes specific signal sequences in the mRNA and forms a complex with that mRNA.then SRP complex targets ER membrane proteins for cotranslational-transport.

If the secretory proteins are not moved into the ER , disulfide bonds would not be formed from cysteine residues . Because the necessary Enzymes required for this process are found only and only in the ER (namely PDI and ERO1).

Plus, these secretory proteins often have to be glycosylated to perform their action correctly.The Enzymes are located in the ER.

Moreover I believe that these secretory proteins will aggregate and degraded later if they are not transported into the ER.

I do not know WHY this process exists this way. That is Evolution and it is just about adaptation to the environment and not necessarily based on pure logical reasons.

Further readings : Molecular Cell Biology Lodish et al 8th ed at sections 13-1 and 13-3. In addition see : Protein translocation across biological membranes , Targeting proteins to membranes: structure of the signal recognition particle Post-translational translocation Regarding to Ero1 and DPI Formation and transfer of disulphide bonds in living cells

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    – tyersome
    Dec 20, 2020 at 1:29
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The secreted proteins are not made in the ER because ribosomes cannot get into the ER. Even individual ribosomal subunits are much larger (at least 10-fold greater in volume, if not hundreds of times greater) than a typical protein, so it is much easier to translocate the protein being produced by the ribosomes across the ER membrane than to translocate the ribosomes themselves. Even the protein being produced must be at least partially unfolded as it is being translocated.

That's not even taking into account that the mRNAs would need to be translocated as well if the ribosomes were to translate them while in the ER. The mRNA is again many times the mass of the protein (a nucleotide is ~3-4 amino acids "worth" of mass, and there are at least 3 nucleotides per amino acid encoded, more if you add the untranslated regions).

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  • $\begingroup$ Why don't secreted proteins use post-translational translocation? $\endgroup$
    – Natdanai Boss
    Oct 23, 2020 at 2:53
  • $\begingroup$ Trying to translocate the protein after it has already been completely synthesized and folded would require disruption of an already stable structure, as well as presumably expose lots of aggregation-prone peptides. With co-translational translocation, only a short stretch of amino acids has been translated but not passed through the pore, and this is protected from the surrounding environment by the seal between the ribosome and the pore. This also means that the folding process inside the ER is able to act like a kind of "ratchet" that helps pull the protein through. $\endgroup$
    – biohacker
    Oct 23, 2020 at 7:41
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    $\begingroup$ This looks like a good answer, but answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. This is a good example of how to format references. ——— You may also want to take the tour and then consult the help center pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$
    – tyersome
    Nov 1, 2020 at 18:38
  • $\begingroup$ @NatdanaiBoss many proteins are indeed post-translationally translocated. I am no expert on this subject, but here's a review and here's a recent paper that describes the structure of the translocation complex. $\endgroup$
    – gaspanic
    Dec 20, 2020 at 12:07

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