Charge interactions can dominate coupled folding and binding on the ribosome

Interactions between emerging nascent polypeptide chains and the ribosome can affect cotranslational protein folding. However, it has remained unclear how such interactions can affect the binding of nascent chains to their cellular targets. We thus investigated, on the ribosome, the interaction between the two intrinsically disordered proteins of opposite charge, ACTR and NCBD, which form a high-affinity complex in a coupled folding-and-binding reaction. Using fluorescence correlation spectroscopy and arrest-peptide-mediated force measurements in vitro and in vivo, we find that the ACTR-NCBD complex can form cotranslationally, but only with ACTR as the nascent chain and NCBD free in solution, not vice versa.

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