The large ribosomal subunit has several important structures essential to translation. These include the peptidyl transferase center (PTC) – which is the site where the peptide bond is formed – and a large, internal, water-filled tube through which the nascent polypeptide moves. This latter structure is called the Peptide Exit Tunnel, and it begins at the PTC and spans the body of the large ribosomal subunit. During translation, as the nascent polypeptide chain is synthesized, it passes through the peptide exit tunnel. It then emerges out in the solvent side, where the peptide chain is subsequently folded into a protein.
This tunnel formed by the 23S ribosomal RNA creates a large hydrophilic surface, containing tiny hydrophobic patches. The dimensions of the tunnel (approximately 10 nm × 1.5 nm) can accommodate growing, unstructured polypeptide chains, as well as solvent molecules. The interior of the peptide exit tunnel is not complementary to any peptide. Hence, the polypeptide chain does not “stick” to the walls and can easily slide through. Once it reaches a location in the exit tunnel where there is sufficient space, the nascent peptide chain starts to fold and may successfully form some α-helical regions. However, the majority of protein folding occurs once the polypeptide exits the ribosome.
