Regulation of microtubule cold stability by calmodulin-dependent and -independent phosphorylation

Cold-labile microtubule protein can be rendered cold-stable by addition of a fraction containing a small number of polypeptides that are derived from cold-stable microtubules. These polypeptides can be obtained from purified cold-stable microtubules by passage through a DEAE-cellulose (DE-52) ion exchange column from which they emerge in the first eluate fraction. The stabilizing activity of these proteins is abolished by phosphorylation catalyzed by two types of protein kinases, one dependent on calmodulin and the other independent of that regulatory protein. The calmodulin-dependent reaction appears to phosphorylate mainly two polypeptides, 56 and 72 kilodaltons; the reaction is blocked by trifluoperazine. The calmodulin-independent reaction appears to phosphorylate different cold-stable microtubule-associated proteins. That reaction is observed only in purified material obtained from vigorously homogenized brain tissue. Gently homogenization yields cold-stable microtubules that are responsive only to the calmodulin-dependent protein kinase. A distinguishing feature of the calmodulin-independent reaction is that it does not occur on polypeptides while they are bound to the microtubules.