The Low Complexity Domain of the FUS RNA Binding Protein Self-assembles via the Mutually Exclusive Use of Two Distinct Cross-β Cores

The low complexity (LC) domain of the fused in sarcoma (FUS) RNA binding protein self-associates in a manner causing phase separation from an aqueous environment. Incubation of the FUS LC domain under physiologically normal conditions of salt and pH leads to rapid formation of liquid-like droplets that mature into a gel-like state. Both examples of phase separation have enabled reductionist biochemical assays allowing discovery of an N-terminal region of 56 residues that assembles into a labile, cross-{beta} structure. Here we provide evidence of a non-overlapping, C-terminal region of the FUS LC domain that also forms specific cross-{beta} interactions. We propose that biologic function of the FUS LC domain may operate via the mutually exclusive use of these N- and C-terminal cross-{beta} cores. Neurodegenerative disease-causing mutations in the FUS LC domain are shown to imbalance the two cross-{beta} cores, offering an unanticipated concept of LC domain function and dysfunction. Significance StatementSingle amino acid changes causative of neurologic disease often map to the cross-{beta} forming regions of low complexity (LC) domains. All such mutations studied to date lead to enhanced avidity of cross-{beta} interactions. The LC domain of the fused in sarcoma (FUS) RNA binding protein contains three different regions that are capable of forming labile cross-{beta} interactions. Here we describe the perplexing effect of amyotrophic lateral sclerosis (ALS)-causing mutations localized to the LC domain of FUS to substantially weaken its ability to form one of its three cross-{beta} interactions. An understanding of how these mutations abet uncontrolled polymerization of the FUS LC domain may represent an important clue as to how LC domains achieve their proper biological function.