Microtubule assembly competence of tau*. Tubulin (15 μM) was incubated with tau* (1.5 μM, —), unmodified tau (1.5 μM, - -), or no tau (- - -) at 35°C, and microtubule assembly was monitored by absorbance at A350 caused by light scattering. The reaction buffer was 50 mM Pipes, pH 6.8/1mM EGTA/1 mM Mg2SO4/1 mM GTP.