ATP forms complexes with Mg(II) within cells . We confirmed the ATP-dependent self-assembly of YK13 peptides under 5 mM MgCl2 conditions (Supplementary Fig. 12-14, 16). Notably, negative control of YK13(K6Y/Y7K), which did not induce droplet formation inside cells, showed an increase in neither turbidity nor ThT fluorescence under Mg(II) conditions (Supplementary Fig. ). This consistency indicates that, within living systems, endogenous poly-anions mediate the bridging of YK peptides through interactions with the cationic side chains of lysine, arranged along one face of the YK peptide's β-strand, thereby stabilizing the β-sheet structure.