Fig. 1. Tau increases SFK activity. (A-D) Tyrosine phosphorylation of tubulin in the presence of tau and Fyn or Src. (A) E. coli tau and/or Fyn were incubated and added to microtubules. After centrifugation, the supernatant (S) and microtubule pellet (P) were probed with anti-phosphotyrosine (see Materials and Methods). While microtubules were phosphorylated by Fyn (lane 4), tau increased the level of phosphorylation (lane 8). Autorad was deliberately overexposed to show tyrosine phosphorylated tau in lanes 5 and 8 (asterisk). (B) N-terminal fragment of tau and Fyn were incubated, added to microtubules, and probed as described in A. (C) Microtubules isolated from COS7 cells transfected with full-length tau and/or Fyn were probed with anti-phosphotyrosine (top panel) or anti-tubulin (bottom panel). (D) Tau and Src were incubated, added to microtubules, and probed as described in A. (E) Tau activates autophosphorylation of Src. 0.2 or 2 µM tau (lanes 1 and 2, respectively) was incubated with Src and probed for active Src, total Src, and tau as described in Materials and Methods.