The effect of ruthenium red on the assembly and disassembly of microtubules and on rapid axonal transport.

The assembly of microtubules was found to decrease in proportion to the mount of added ruthenium red, indicating a high affinity of ruthenium red for the microtubule system. An equimolar amount of ruthenium red per tubulin dimer inhibited the microtubule assembly completely and disassembled existing microtubules. Binding of ruthenium red to tubulin is accompanied by a shift in the absorption maximum fro 535 to 538 nm. The binding is very strong, as shown by the finding that ruthenium red could not be displaced from tubulin by gel chromatography on Sephadex G-25, or by the addition of Ca2+ or Mg2+. The binding of ruthenium red to tubulin did not affect the single colchicine site, nor the Mg2+ site(s), as shown by use of Mn2+ as an EPr probe. Ruthenium red also interfered with microtubules in an intact cell system, as it inhibited rapid axonal transport in the frog sciatic nerve, measured by the accumulation of [3H]leucine-labelled proteins in front of a ligature.