Structural fuzziness of the RNA-organizing protein SERF1a determines a toxic gain-of-interaction

Members of the MOAG/SERF protein family have been attributed a neuropathologic significance because of their ability to enhance the proteotoxic polymerization of amyloidogenic proteins such as alpha-Synuclein (aSyn). However, the cellular function remains unknown. Here, we identify SERF1a as an RNA-organizing protein i) with no structural homology to canonical RNA-binding proteins, ii) with an RNA-chaperoning activity which favours the incorporation of RNA into nucleoli and liquid-like artificial RNA-organelles, and iii) with a high degree of conformational disorder in the RNA-bound state. We demonstrate that this type of structural fuzziness determines an undifferentiated interaction of SERF1a with aSyn and RNA. Both molecules bind to one identical, positively charged site of SERF1a by an analogous electrostatic binding mode, with similar binding affinities, and without any observable disorder-to-order transition. The absence of primary or secondary structure discriminants results in SERF1a being unable to distinguish between nucleic acid and amyloidogenic protein, leading the pro-amyloid aSyn:SERF1a interaction to prevail under conditions of cellular stress. We suggest that structural fuzziness of SERF1a accounts for an adverse gain-of-interaction which favours toxic binding to aSyn at the expense of non-toxic RNA binding, therefore promoting a functionally distorted and pathogenic process. Our results provide a direct link between structural disorder, amyloid aggregation and the malfunctioning of RNA-associated cellular processes, three hallmarks of neurodegenerative diseases.