Rapid disassembly of cold-stable microtubules by calmodulin.

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Abstract
Purified cold-stable microtubules from the rat brain are insensitive to podophyllotoxin and to millimolar concentrations of free Ca. In the presence of calmodulin at concentrations substoichiometric to that of tubulin, Ca causes rapid microtubule disassembly. The half-maximal effective Ca concentration in the presence of calmodulin is 100 .mu.M. With 800 .mu.M free Ca, the half-maximal effective concentration of calmodulin is 1.0 .mu.M (or 1/10 the tubulin concentration). Calmodulin is without effect in the absence of Ca. Troponin C is approximately 1/5 as effective as calmodulin, and parvalbumin is totally ineffective. Troponin I partially inhibits the Ca/calmodulin-induced disassembly of microtubules in the crude extract, and blocks the Ca/calmodulin effect on purfied cold-stable microtubules. A 5-fold excess of trifluoperazine does not inhibit the Ca/calmodulin-induced disassembly.