RNA-binding protein diversity and NLS arginines regulate FUS mixing in mRNA-rich compartments.

Despite being prone to condensation, many RNA-binding proteins (RBPs) do not form large condensates in cells. This issue is still widely researched, particularly because aggregation of RBPs, such as FUS, is the hallmark of some neurodegenerative diseases. Elevated RNA levels and protein chaperone activity have already emerged as key factors preventing aberrant phase separation. Here, we explored the role of RBP diversity in mRNA-rich condensates. While FUS and its partners form distinct compartments when probed one by one, increasing RBP diversity buffers FUS spatial segregation. In addition, we found that frequently mutated arginine residues in the nuclear localization signal (NLS) at the C-terminal end promote FUS mixing with multiple RBPs. Therefore, we anticipate that pathological NLS mutations in FUS not only alter its active nuclear import but also regulate FUS interactions with its partners in mRNA-rich compartments with putative consequences for the onset and progression of FUS-related neurodegenerative diseases.