To further study the potential role of PIKfyve in K18 PFFs uptake and seeding, we used YM-201636, a selective pharmacological PIKfyve inhibitor, shown to reduce the synthesis of PI 3,5 P 2 and to a lower extent PI 4,5 P 2 (49). To confirm its selectivity, we characterized this compound in a kinome profiling at 1 μM (Fig. S6). The selectivity profile confirmed strong binding to PIKfyve and no binding to the other two PIP5K subfamilies (note that the KINOMEscan from Discov-erX does not contain assays for the isoforms PIP5K1B and PIP4K2A-full **data** set is provided in Fig. S6).