Residue-Specific Binding Mechanisms of Thioflavin T to a Surface of Flat β-Sheets within a Peptide Self-Assembly...

Thioflavin T (ThT) is a popular fluorescent dye for detecting amyloid, a protein aggregate with a β-sheet-rich structure that causes many neurodegenerative diseases. Despite the dye's popularity, a detailed understanding of its molecular binding mechanism remains elusive. We previously reported a protein model that can bind ThT on a single-layer β-sheet and revealed that a channel formed by aromatic rings with a confined length enhanced ThT binding. One of the mutants of the model system, 5-YY/LL, showed the highest affinity with a low micromolar dissociation constant. Here, we investigate the residue-specific mechanism of binding of ThT to 5-YY/LL. We introduced tyrosine to phenylalanine and tyrosine to histidine mutations into the channel. The mutants revealed that the fifth position of tyrosine (Y₅) is important for binding of ThT. Positive charges introduced by histidine under a low-pH condition at the channel repel the binding of cationic ThT. Furthermore, we found a positive to negative conversion in the vicinity of the binding channel increases ThT fluorescence 4-fold. A detailed understanding of the ThT binding mechanism will enhance our ability to develop amyloid-specific small molecules.