Rapid disassembly of cold-stable microtubules by calmodulin

Purified cold-stable microtubules from the rat brain are insensitive to podophyllotoxin and to millimolar concentrations of free calcium. However, in the presence of calmodulin at concentrations substoichiometric to that of tubulin, calcium causes rapid microtubule disassembly. The half-maximal effective calcium concentration in the presence of calmodulin is 100 microM. With 800 microM free calcium, the half-maximal effective concentration of calmodulin is 1.0 microM (or one-tenth the tubulin concentration). Calmodulin is without effect in the absence of calcium. Troponin C is approximately one-fifth as effective as calmodulin, and parvalbumin is totally ineffective. Troponin I partially inhibits the calcium/calmodulin-induced disassembly of microtubules in the crude extract and blocks the calcium/calmodulin effect on purified cold-stable microtubules. A 5-fold excess of trifluoperazine does not inhibit the calcium/calmodulin-induced disassembly.