To create droplet-inducing peptide tags, we designed YK peptides comprising alternating tyrosine and lysine residues, following the fundamental principles of β-sheet peptide design (Fig. ). Alternating repeats of hydrophobic and hydrophilic amino acids yield an amphiphilic β-strand, which polymerically assembles into supramolecular fibers . In our previous study, we developed a Y15 peptide in which hydrophobic tyrosine residues were aligned on one face while lysine or glutamate residues were positioned on the opposite face. Despite the Y15-fused proteins demonstrating self-assembly within mammalian cells, they formed gellike structures lacking fluidity. We attributed this stable and irreversible assembly to the interplay of complementary electrostatic intermolecular interactions between cationic lysine and anionic glutamate residues.