We propose that the cationic YK13 peptide assembles into fibril structures through the interaction with anionic species such as ATP present at the millimolar level in living cells (Fig. ). Initially, we prepared the purified YK peptides by standard Fmoc solid-phase synthesis (Supplementary Fig. ). To assess the self-assembling propensities of YK13 peptide, we used thioflavin-T (ThT) dye or X-34 dye , fluorogenic dyes that bind to amyloid-like structures. Prior to the addition of ATP, the level of fluorescence was comparable to that of the blank control, whereas the addition dramatically enhanced the fluorescence intensity (Fig. and Supplementary Fig. and). This ATP-responsive assembly of YK13 was also monitored as an increase in turbidity (Supplementary Fig. ). In CD measurement, YK13 peptide solution with ATP exhibited positive Cotton effects at 211 and 238 nm, potentially attributable to alignment of the phenol group of tyrosine residues (Supplementary Fig. ). FT-IR measurements showed broad bands at 1700-1600 cm -1 from YK13, whereas the mixture of YK13 and ATP exhibited a strong sharp peak that appeared around 1635 cm -1 , indicative of a βsheet secondary structure 31 (Supplementary Fig. ). Moreover, from negatively stained TEM, the bundled nanofibril structures were observed (Fig. and Supplementary Fig. ).