Quantitative Western blots of total tau and MT-bound and -unbound tau in cultured astrocytes infected with T40-AV. Western blot analyses of tau from astrocytes infected with T40-AV for different lengths of time: total tau (A) and MT-bound and -unbound tau (B) probed with primary antibodies against phosphorylation-independent (17026), phosphorylation-dependent (PHF1 and 12E8), and non-phosphorylation-dependent (T1) anti-tau and anti-β-tubulin (β-tub) antibodies, followed by [¹²⁵I]-labeled secondary antibodies. Tau bound to MTs in the pellet (P) fractions comprised ∼65% of the total tau 8 hr after infection but it reduced to ∼35% at 24 hr, whereas tau in the pellet fractions increases twofold from 8 to 24 hr (B). Phosphorylated tau (PHF1 and 12E8) was mainly found in the supernatant (S) fractions (B).