Precise ligand-selective mechanism at the fab domain of a tau-recognizing antibody.

Insoluble aggregated tau protein in the form of paired helical filaments is a causative agent of the neurofibrillary tangles observed in Alzheimer's disease (AD). The hexapeptide 275VQIINK280 located in the microtubule-binding domain of tau plays a crucial role in the abnormal aggregation process. Therefore, targeting the VQIINK sequence with a tau aggregation inhibitor may be a promising therapeutic approach for AD. A previous study demonstrated that the Fab domain of the tau antibody (Fab2r3) inhibits tau aggregation by binding to the VQIINK sequence. By determining the three-dimensional structures of the Fab2r3-VQIINK peptide complex and apo Fab2r3, we elucidated the recognition mechanism between Fab2r3 and the VQIINK peptide. However, the basis for the selectivity of Fab2r3 for VQIINK was not completely clear. Therefore, the objective of this report is to investigate the selective binding mechanism of Fab2r3 against VQIINK peptide. Through isothermal titration calorimetry, we show that Ile-4 in the VQIINK peptide is crucial for the selectivity of Fab2r3. X-ray structural analysis of three complexes of Fab2r3 with Ile-4 mutated peptides (VQIVYK, VQILNK, and VQIFNK) suggested that the rigid conformation of a hydrophobic pocket in Fab2r3 plays a vital role in ligand selectivity. These findings may explain the effectiveness of Fab2r3 as a tau aggregation inhibitor.