Question

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?

Answer 1

Ribosomes do not phosphorylate amino acids within nascent polypeptide chains; such phosphorylations are post-translational modifications that occur after the polypeptide exits the ribosome.

Step-by-step explanation:

The process of protein synthesis in cells involves the formation of polypeptides by ribosomes through the translation of mRNA. There is no known mechanism for ribosomes themselves to phosphorylate amino acids within the nascent polypeptide chains before they exit the ribosomal complex.

The ribosome is comprised of rRNA and proteins, with the large subunit containing the nascent polypeptide exit tunnel; however, post-translational modifications such as phosphorylation typically occur after the polypeptide chain has been released into the cytosol.

During translation, ribosomes move along an mRNA molecule, translating it codon by codon, and catalyze peptide bond formation between amino acids through the peptidyl transferase activity of the rRNA. This creates a growing polypeptide chain that exits the ribosome through the exit tunnel in the large subunit. Phosphorylation and other post-translational modifications often occur afterward, facilitated by specific enzymes in the cytoplasm or endoplasmic reticulum.