A pathological hallmark of Alzheimer disease and other tauopathies is the formation of neurofibrillary tangles mainly composed of bundles of fibrils formed by microtubule-associated protein Tau.Here we study the effects of Zn2+on abnormal aggregation and cytotoxicity of a pathological mutant ΔK280 of full-length human Tau.As revealed by Congo red binding assays,transmission electron microscopy,immunofluorescence,Western blot,and immunogold electron microscopy,pathological concentration of Zn2+dramatically accelerates the fibrillization of ΔK280 both in vitro and in SH-SY5Y neuroblastoma cells.As evidenced by annexin V-FITC apoptosis detection assay and MTT reduction assay,pathological concentration of Zn2+remarkably enhances ΔK280 fibrillization-induced apoptosis and toxicity in SH-SY5Y cells.Substitution of Cys-291 and Cys-322 with Ala,however,essentially eliminates such enhancing effects of Zn2+on the fibrillization and the consequent cytotoxicity of ΔK280.Furthermore,Zn2+is co-localized with and highly enriched in amyloid fibrils formed by ΔK280 in SH-SY5Y cells.The results from isothermal titration calorimetry show that Zn2+binds to full-length human Tau by interacting with Cys-291 and Cys-322,forming a 1:1 Zn2+-Tau complex.Our data demonstrate that zinc dramatically accelerates abnormal aggregation of human Tau and significantly increases Tau toxicity in neuronal cells mainly via bridging Cys-291 and Cys-322.Our findings could explain how pathological zinc regulates Tau aggregation and toxicity associated with Alzheimer disease.