Long-range electrostatic forces govern how proteins fold on the ribosome

Protein biosynthesis and folding are tightly intertwined processes regulated by the ribosome and auxiliary factors. Nascent proteins can begin to fold on their parent ribosome but formation of the native state is often inhibited well beyond the emergence of the necessary residues from the exit tunnel. The dominant forces driving this apparent destabilisation have remained unclear. We investigate this here, combining NMR experiments and atomistic simulations of a folded nascent chain on and off the ribosome. While its native structure and internal dynamics remain unaltered, co-translational folding is disfavoured due to intermolecular electrostatic repulsion between the negatively charged ribosome surface and nascent protein. Partially folded intermediates are less destabilised, resulting in their high populations. Specifically, we show that the polypeptides net charge is the dominant factor determining nascent folding thermodynamics, with smaller contributions from charge distribution. Consequently, positively charged proteins can fold on the ribosome without populating stable intermediates. These findings reconcile conflicting observations of previous studies and establish the general physical principles underpinning de novo protein folding.