Is ribosome-mediated protein folding co-translation or post-translational?
During protein synthesis, the growing polypeptide chain remains fairly flexible in the tunnels and channels on the 50S ribosomal subunit. Cross linking of growing polypeptide chain with the 50S particle showed many contacts, especially between the nucleotides in the domain V and the growing polypeptide chain21. In fact, an unusually large number of nucleotides in this domain remain exposed from the protein coat of the 50S subunit, presumably because the two major activities, polypeptide synthesis and its folding into active form, are largely due to their contacts with the charged tRNAs (the A and P sites), the growing polypeptide chain, etc.17. The polypeptide chain can move on the 50S subunit, making contacts with these nucleotides21, so that widely separated parts of it can come close together to form the folding intermediates having large part of its secondary structures formed and could even engage in tertiary structure formation25. Yet, the final level of folding takes place outsi
