Toxic PR_n poly-dipeptides encoded by the <i>C9orf72</i> repeat expansion block nuclear import and export.

Abstract

The toxic proline:arginine (PR_n) poly-dipeptide encoded by the (GGGGCC)_n repeat expansion in the <i>C9orf72</i> form of heritable amyotrophic lateral sclerosis (ALS) binds to the central channel of the nuclear pore and inhibits the movement of macromolecules into and out of the nucleus. The PR_n poly-dipeptide binds to polymeric forms of the phenylalanine:glycine (FG) repeat domain, which is shared by several proteins of the nuclear pore complex, including those in the central channel. A method of chemical footprinting was used to characterize labile, cross-β polymers formed from the FG domain of the Nup54 protein. Mutations within the footprinted region of Nup54 polymers blocked both polymerization and binding by the PR_n poly-dipeptide. The aliphatic alcohol 1,6-hexanediol melted FG domain polymers in vitro and reversed PR_n-mediated enhancement of the nuclear pore permeability barrier. These data suggest that toxicity of the PR_n poly-dipeptide results in part from its ability to lock the FG repeats of nuclear pore proteins in the polymerized state. Our study offers a mechanistic interpretation of PR_n poly-dipeptide toxicity in the context of a prominent form of ALS.