Ravindra_1990_Biochem.Pharmacol_40_457

To understand the role of tubulin, an integral component of neural membranes, in signal transduction processes, the influence of anti-tubulin antibodies on the low Km GTPase activity associated with transducer G proteins was examined in rat striatum. Membranes were prepared from striatum by conventional procedures, and the low Km GTPase activity (EC 3.6.1.-) was determined using [gamma-32P]GTP at 37 degrees in an ATP-regenerating buffer containing 0.2 to 2.0 microM unlabeled GTP. GTPase activity was linear for up to 30 min and was directly proportional to protein concentration. Polyclonal anti-tubulin antibodies, anti-alpha-tubulin antibodies, and anti-beta-tubulin antibodies (10 micrograms) stimulated G protein GTPase activity. Anti-beta-tubulin antibody (10 micrograms) stimulated GTPase activity by about 60% at each time point, while 10 micrograms of either anti-alpha-tubulin or polyclonal anti-tubulin antibodies stimulated GTPase activity by only 20-30% at each time point. The Vmax/Km ratio, an index of the enzyme-substrate interaction, increased by only 26% with the anti-alpha-tubulin antibody and by 52% with anti-beta-tubulin antibody; polyclonal anti-tubulin antibodies did not affect this ratio. GTPase activity was stimulated by acetylcholine in an atropine-sensitive manner. At 100 microM, acetylcholine stimulated GTPase activity by about 50%. Polyclonal anti-tubulin, anti-alpha-tubulin, or anti-beta-tubulin antibodies (10 micrograms) potentiated acetylcholine stimulation of GTPase activity. Two possible mechanisms by which anti-tubulin antibodies could stimulate low Km GTPase activity and potentiate the stimulatory effects of acetylcholine are: (1) by inhibiting GTP binding to beta-tubulin, and (2) by eliminating a chronic inhibitory effect of tubulin on G protein or receptor-G protein interaction.