Effect of Tat on tubulin assembly above the critical concentration (Cr) of tubulin. A and B. Effect of Tat on tubulin (15 μM) assembly, as measured by turbidimetry at 350 nm. Measurements were performed in the absence (black solid line), or in the presence of 2 μM (blue dashed line), 3 μM (red dotted line), or 4 μM (green dashed-dotted line) of A) apo-Tat or B) holo-Tat, in PMG buffer at 37°C. At the time indicated by the arrow, samples were cooled to 10°C. C. Electron micrographs of 15 μM tubulin in the absence or the presence of 4 μM apo-Tat, or 4 μM holo-Tat at 37°C and after cold depolymerisation at 10°C, in PMG buffer.