Binding of phosphorothioate oligonucleotides with RNase H1 can cause conformational changes in the protein and alter the interactions of RNase H1 with other proteins.

We recently found that toxic PS-ASOs can cause P54nrb and PSF nucleolar mislocalization in an RNase H1-dependent manner. To better understand the underlying mechanisms of these observations, here we utilize different biochemical approaches to demonstrate that PS-ASO binding can alter the conformations of the bound proteins, as illustrated using recombinant RNase H1, P54nrb, PSF proteins and various isolated domains. While, in general, binding of PS-ASOs or ASO/RNA duplexes stabilizes the conformations of these proteins, PS-ASO binding may also cause the unfolding of RNase H1, including both the hybrid binding domain and the catalytic domain. The extent of conformational change correlates with the binding affinity of PS-ASOs to the proteins. Consequently, PS-ASO binding to RNase H1 induces the interaction of RNase H1 with P54nrb or PSF in a 2'-modification and sequence dependent manner, and toxic PS-ASOs tend to induce more interactions than non-toxic PS-ASOs. PS-ASO binding also enhances the interaction between P54nrb and PSF. However, the interaction between RNase H1 and P32 protein can be disrupted upon binding of PS-ASOs. Together, these results suggest that stronger binding of PS-ASOs can cause greater conformational changes of the bound proteins, subsequently affecting protein-protein interactions. These observations thus provide deeper understanding of the molecular basis of PS-ASO-induced protein mislocalization or degradation observed in cells and advance our understanding of why some PS-ASOs are cytotoxic.