Figure 10.

Dynamic instability of microtubules before and after phosphorylation of tau with PKA. The initial solution contained tubulin (25 μM), tau (10 μM), and PKA (1.5 U/μl, activity as in Figure 8). The images were taken 8 min after raising the temperature to 37°C. Left, isoform htau23; right, isoform htau23 with Ser214 mutated into Ala. Top, no ATP; bottom, prephosphorylated with 2 mM ATP. The top images show that microtubules grow well under the stabilizing influence of tau. The bottom left shows that microtubule formation is almost totally suppressed when PKA is allowed to phosphorylate tau (note that PKA has no direct effect on tubulin; our unpublished observations). The bottom right shows that the effect of PKA is mainly due to the phosphorylation of Ser214; when this residue is mutated to Ala, microtubules assemble normally.