Transient ion-mediated interactions regulate subunit rotation in a eukaryotic ribosome.

While it is known that ions are required for folding of RNA, little is known about how transient/probabilistic ionic interactions facilitate biologically-relevant conformational rearrangements. To address this, we developed a theoretical model that employs all-atom resolution, with a simplified representation of biomolecular energetics, explicit electrostatics and ions (K, Cl, Mg). For well-studied RNA systems (58-mer and Ade riboswitch), the model accurately describes the concentration-dependent ionic environment, including (bidentate) chelated and hydrated (diffuse/outer-shell) ions. With this foundation, we applied the model to simulate the yeast ribosome and quantified the ion-dependent energy landscape of intersubunit rotation. These calculations show how the energetics of rotation responds to millimolar changes in [MgCl], which shift the distribution between rotation states and alter the kinetics by more than an order of magnitude. We find that this response to the ionic concentration correlates with formation and breakage of ion-mediated interactions (inner-shell and outer-shell) between the ribosomal subunits. This analysis provides a physical basis for understanding how transient ion-mediated interactions can regulate a large-scale biological process.