ALS/FTLD-Linked Mutations in Glycine Residues of FUS Lead to Immiscibility with Wild-Type FUS

Abstract

The formation of pathogenic inclusions of RNA-binding proteins in neurons is a hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia (FTLD). One prominent protein in these inclusions is Fused in sarcoma (FUS), and over 70 mutations in Fus are linked to ALS/FTLD. In patients, all Fus mutations are heterozygous, indicating that the mutant drives pathology despite the presence of wild-type FUS. Here, we demonstrate that ALS FUS mutations in glycine (G) strikingly drive formation of droplets that are immiscible with wild-type FUS whereas arginine (R) mutants form miscible condensates with wild-type FUS. Remarkably, immiscibility between wild-type and G mutants begins at the earliest stages of FUS nucleation. In contrast to G mutants, the miscible R mutants physically interact with the wild-type FUS such that wild-type FUS rescues the mutant defects by reducing droplet size and recovering dynamic interactions with RNA. This result suggests disparate molecular mechanisms underlying pathogenesis and differing rescue potential depending on the type of mutation.