Zinc binding to Tat promotes discrete Tat-tubulin complexes under non-assembly conditions. A. Characterization of Tat-tubulin interaction by analytical ultracentrifugation, in PG buffer. Continuous sedimentation coefficient distribution C(S) of tubulin (5 μM) in the absence (black solid line), or in the presence of 10 μM holo-Tat (blue dashed line) or 10 μM apo-Tat (red dotted line). Inset: Full range C(S) of tubulin (5 μM) in the presence of 10 μM apo-Tat (red dotted line). Tat contributed to less than 10% of the signal. B. Electron micrograph of 5 μM tubulin in the presence of 10 μM apo-Tat, in PG buffer.