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When the small ribosomal subunit binds to mRNA’s 5’-cap, it looks for the complex of three eLFs bound to the poly-A-binding protein (PABP), which circularises the mRNA. It is then this circular mRNA that the small ribosomal subunit binds to. Transcription Initiation Complex

In low-nutrient environments, 4E-BP1 binds to PABP, inhibiting it from binding to the 5’-cap complex. I high nutrient environments 4E-BP1 is phosphorylated. 4E-BP1 Inhibition of PABP

Why (what benefit is conferred?) are the eLFs there? Why not just have the PABP bind to the 5’-cap to initiate transcription? And why 4E-BP1? Why not just phosphorylate PABP directly?

I mean I know that as is, PABP isn’t set up to be usefully phosphorylated, but why is it this way? It seems a whole bunch energy and resources wasted making these extra proteins, and more room for things to go wrong, more DNA that has to be made and carried around with no mutations and transcribed and translated correctly.

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The eukaryotic initiation factor (eIF) proteins are necessary for the ribosome to translate the mRNA. The main benefit is that they, when present, make it easier for the ribosome to find the AUG sequence, which is the start of the protein (as shown in this diagram) by "helping" the Methionine t-RNA find said sequence.

The reason why the mRNA doesn't bind to the PABP directly is for translation regulation. If the mRNA needed to just bind to the PABP and the mRNA for a protein is made when there is more than enough protein, then amino acids are being wasted to make the already abundant protein. If there exist some factors that can control the translation of protein, then the cell can better control protein translation.

Why 4E-BP1 is phosphorylated instead of PABP is because it evolved that way. I know that isn't a satisfying answer, but cells don't really evolve for efficiency, but for "good enough". If it is enough to survive, then why bother changing it?

I hope this answers your question!

Citation for Paper with Diagram: https://biosignaling.biomedcentral.com/articles/10.1186/s12964-020-00607-9

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