C9orf72 arginine-rich dipeptide repeat proteins disrupt importin β-mediated nuclear import

Disruption of nucleocytoplasmic transport (NCT), including mislocalization of the importin {beta} cargo, TDP-43, is a hallmark of amyotrophic lateral sclerosis (ALS), including ALS caused by a hexanucleotide repeat expansion in C9orf72. However, the mechanism(s) remain unclear. Importin {beta} and its cargo adaptors have been shown to co-precipitate with the C9orf72-arginine-containing dipeptide repeat proteins (R-DPRs), poly-glycine arginine (GR) and poly-proline arginine (PR), and are protective in genetic modifier screens. Here, we show that R-DPRs interact with importin {beta}, disrupt its cargo loading, and inhibit nuclear import in permeabilized mouse neurons and HeLa cells, in a manner that can be rescued by RNA. Although R-DPRs induce widespread protein aggregation in this in vitro system, transport disruption is not due to NCT protein sequestration, nor blockade of the phenylalanine-glycine (FG)-rich nuclear pore complex. Our results support a model in which R-DPRs interfere with nuclear transport receptors in the vicinity of the nuclear envelope.