Conserved loop of a phase modifier endows protein condensates with fluidity

Dipeptide repeats (DPRs) that are gene products from abnormal hexanucleotide repeat expansion in C9orf72 trigger amyotrophic lateral sclerosis (ALS) through unknown mechanism. This study highlights, importin Karyopherinβ2 (Kapβ2), which is responsible for nuclear transport and phase modification of RNA-binding proteins (RBPs), as a major DPR target. We demonstrate DPR accumulation in the nucleus via Kapβ2-mediated transport, which results in dose-dependent toxicity observed in nematode and yeast models. In vitro interaction studies exploiting chemical probe arrays and biophysical measurements reveal multivalent DPR binding to Kapβ2, including at the conserved acidic loop. Refractive index and fluorescence imaging coupled with biochemical assays unveiled that binding of excess DPRs to the acidic loop turns a phase modifier Kapβ2 into phase disrupter, resulting more condensed and viscous RBP condensates. Our findings provides molecular insight into C9orf72 -ALS related to age and repeat expansion.