Condensation of the N-terminal domain of human topoisomerase 1 is driven by electrostatic interactions and tuned by its charge distribution

Liquid-liquid phase separation (LLPS) is pivotal in forming biomolecular condensates, which are crucial in several biological processes. Intrinsically disordered regions (IDRs) are typically responsible for driving LLPS due to their multivalency and high content of charged residues that enable the establishment of electrostatic interactions. In our study, we examined the role of charge distribution in the condensation of the disordered N-terminal domain of human topoisomerase I (hNTD). hNTD is densely charged with oppositely charged residues evenly distributed along the sequence. Its LLPS behavior was compared with that of charge permutants exhibiting varying degrees of charge segregation. At low salt concentrations, hNTD undergoes LLPS. However, LLPS is inhibited by high concentrations of salt and RNA, disrupting electrostatic interactions. Our findings show that, in hNTD, moderate charge segregation promotes the formation of liquid condensates that are sensitive to salt and RNA, whereas marked charge segregation results in the formation of aberrant condensates. Although our study is based on a limited set of protein variants, it supports the applicability of the "stickers-and-spacers" model to biomolecular condensates involving highly charged IDRs. These results may help generate reliable models of the overall LLPS behavior of supercharged polypeptides.