Coomassie blue staining, Western blotting, and the binding of N-tau to AD P-tau, PHF, and in vitro hyperphosphorylated N-tau. (A) N-tau (50 ng) and 4-μg aliquots of AD P-tau and PHF were subjected to SDS/PAGE, transferred to poly(vinylidene difluoride) membrane, and immunodetected with Tau-1 Ab. One strip was stained with Coomassie blue (Cb). Before immunostaining, the blots were treated with (+) or without (−) alkaline phosphatase (AP, 100 units/ml) for 3 h at 35°C. (B) Increasing concentrations of AD P-tau and duplicates of three different preparations of PHF, and as a control N-tau (10–100 μg tau content), were spotted on nitrocellulose membrane and overlaid with 5 μg/ml τ4L or with BSA and developed with Tau-1 Ab. To verify that the proteins bound to the membrane, one strip without any overlay was developed with a mixture of three phospho-independent polyclonal Abs to total tau. (C) Quantitations of the tau bound to AD P-tau (■) and to PHF (▴). (D) N-tau was hyperphosphorylated in vitro, as described in ref. ; one aliquot was probe-sonicated for 1 min to disrupt the tau filaments formed during the incubation period (). The sonicated sample was centrifuged at 55,000 × g for 20 min, and the pellet was discarded. The nonsonicated and the supernatant of the sonicated samples were dotted in triplicates on three nitrocellulose membrane strips. These strips were processed as described above. The quantitations are given as mean ± SD. ∗, P < 0.005.