Abstract
Lassa virus protects its viral genome through the formation of a ribonucleoprotein complex in which the nucleoprotein (NP) encapsidates the single-stranded RNA genome. Crystal structures provide evidence that a conformational change must occur to allow for RNA binding. In this study, the mechanism by which NP binds to RNA and how the conformational changes in NP are achieved was investigated with molecular-dynamics simulations. NP was structurally characterized in an open configuration when bound to RNA and in a closed form in the absence of RNA. Our results show that when NP is bound to RNA, the protein is highly dynamic and the system undergoes spontaneous deviations away from the open-state configuration. The equilibrium simulations are supported by free-energy calculations that quantify the influence of RNA on the free-energy surface, which governs NP dynamics. We predict that the globally stable states are qualitatively in agreement with the observed crystal structures, but that both open and closed conformations are thermally accessible in the presence of RNA. The free-energy calculations also provide a prediction of the location of the transition state for RNA binding and identify an intermediate metastable state that exhibits correlated motions that could promote RNA binding.