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The ribosome synthesizes proteins by translating the mRNA. The nascent peptide needs to go through the nascent polypeptide exit tunnel in the large ribosomal sub-unit before it reaches the cytosol where it becomes exposed to possibe modification. Is there a ribosomal protein capable of phosphorylating the amino acids of nascent polypeptides before they exit the ribosome?

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No — there is no ribosomal protein with protein kinase activity.

It is difficult to provide conclusive evidence for a negative statement, but the two objective points I can make in support of my view are:

  1. All eukaryotic (and prokaryotic) ribosomal proteins have been sequenced in a wide range of organisms. None shares the characteristic motifs of protein kinases.

  2. There are hundreds of different protein kinases in eukaryotic cells, and these show a wide range of specificities towards different proteins. Many proteins are not phosphorylated at all. Hence, it is difficult to imagine what purpose a ribosomal protein with protein kinase activity could serve.

However…

The protein RACK1 (Receptor for Activated C-Kinase 1) is closely associated with the small subunit of many eukaryotic ribosomes, even though it is not designated a ribosomal protein under the standard Sx numbering system. The substrates of the activated C-kinase for which it is the receptor are not the proteins being synthesized on the ribosomes, but apparently cellular proteins involved in signalling pathways.

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