Liquid-Liquid Phase Separation-mediated formation of amyloid fibrils from DcpS scavenger enzymes

Decapping Scavenger (DcpS) enzyme was initially identified by its ability to hydrolyze the cap structure resulting from mRNA decay. Human DcpS is an established target for acute myeloid leukemia (AML) and hepatic metastasis. Recently, the protein has been linked to neuronal development regulation and implicated in certain developmental neurological disorders. Here we demonstrate for the first time that DcpS undergoes misfolding in vitro, leading to the formation of amyloid-like fibrils. Fibrillization was observed for human and nematode (C. elegans) DcpS using transmission electron microscope (TEM) imaging, Thioflavin T (ThT) fluorescence assay, Fourier-transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy, differential scanning fluorimetry (DSF), and dynamic light scattering (DLS). Additionally, the DcpS ^INS15 insertional mutant linked to the Al-Raquad syndrome, exhibited accelerated fibril aggregation kinetics compared to the wild type protein. Moreover, we show that the DcpS species investigated in this study undergo liquid-liquid phase separation (LLPS) prior to amyloid-formation. We propose that the LLPS phase transition underlies the intricate kinetics (e.g. lack of a clearly-resolved lag phase) of the misfolding process. As the physiological implications of the here-reported propensity of DcpS to lose its biological function through the coupled LLPS-fibrillization transition remain to be elucidated, this work lays the groundwork for further studies on this phenomenon.