Holo-Tat promotes and stabilizes microtubules under assembly-conditions, at a tubulin concentration below the critical concentration (Cr). A. Effect of Tat on tubulin (6 μM) assembly, as measured by turbidimetry at 350 nm. Measurements were performed in the absence (black solid line), or in the presence of 4 μM holo-Tat (blue dashed line), 8 μM holo-Tat (red dashed-dotted line), 8 μM zinc sulfate (purple dotted line), or 16 μM zinc sulfate (green dashed-dotted-dotted line), in PMG buffer at 37°C. At the time indicated by the first arrow, samples were cooled to 10°C. The second arrow represents one hour of incubation at 4°C. The trace with 8 μM apo-Tat was indistinguishable from the control and was thus not represented. B. Electron micrographs of 6 μM tubulin in the presence of 8 μM holo-Tat, or 16 μM zinc sulfate, in PMG buffer at 37°C and after cold depolymerisation at 10°C or 4°C.