Mammalian brain microtubules are sensitive to cyclic AMP in vitro

Microtubules assembled in vitro with ATP were depolymerized by the addition of cyclic AMP, which correlates with a stimulation of the endogeneous phosphorylation reaction. When assembled with GTP, however, microtubules were only sensitive to cyclic AMP when ATP was present. This nucleoside triphosphate induced the disassembly of microtubules in a concentration-dependent, cyclic nucleotide-stimulated manner. Since UTP, CTP and the nonhydrolyzable ATP analog adenosine-5'-(beta, gamma-methylene)triphosphate were without comparable effect, it was assumed that phosphorylation of the microtubule-associated proteins may represent a physiological mechanism by which microtubules in the living cell respond to external stimuli.