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One of the methods of mRNA localization, for example, is random diffusion of mRNAs where the mRNA binding proteins are localized to a certain part of the cell. However, I was taught that the ribosome can translate mRNAs before they've even finished being exported from the nucleus. So what stops localized mRNAs from being translated all over the cell before they reach their designated location?

I was thinking maybe they're bound by hnRNPs that inhibit their translation, which dissociate when they're bound by localized mRNA binding proteins, but I could be wrong.

Or is it that they are translated all over the cell, but the concentration at their locale is just way higher in their locale?

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  • $\begingroup$ Could you give a specific example of the type of localization you are referring to, please. Are you talking about mitochondrial or nuclear localization or something else? At the moment your question is too broad and imprecise. $\endgroup$ – David Nov 30 '19 at 12:33
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    $\begingroup$ Sorry, I guess a good example could be Calmodulin Kinase mRNA being transported to the synapse of a neuron on the cytoskeletal microfilaments. So in the case of that mRNA, how is it not transcribed at all on route to the synapse? I see that Nicolai has answered that particular example below though. $\endgroup$ – Ben Dec 1 '19 at 1:48
  • $\begingroup$ That’s an example I’m not familiar with. I must check out the reference from @Nicolai. $\endgroup$ – David Dec 1 '19 at 9:37
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The answer to your general question is 'it depends':

  • A lot of RNA localisation to the ER and mitochondria happens co-translationally (based on the protein targeting sequences), so these mRNA actually depend on translation to be localised. However, RNA localisation to mitochondria can also happen due to targeting sequences in the mRNA 3'UTR - in these cases translational repression would be possible, but it's unlikely and I don't think anyone ever checked it in this particular case. (see i.e. this review)

  • the translation of many mRNAs that are actively transported along the cytoskeleton is inhibited during transport, either by specific RNA binding proteins or larger RNP (ribo-nucleo-protein) complexes (see another review).

There are also bound to be some specific mRNA that have very specific regulation mechanisms and others that are exceptions.

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