Natural osmolyte trimethylamine N-oxide stimulates tubulin polymerization and reverses urea inhibition.

The natural osmolyte trimethylamine N-oxide (TMAO) is one of the methylamine compounds often accumulated by diverse organisms in response to osmotic stress and/or to compensate for the deleterious effects of urea. Tubulin polymerization is promoted by TMAO. At 1 M TMAO, tubulin polymers are produced with properties expected of normal steady-state microtubules (MT): polymerization is reversed by exposure to cold or the antimitotic drug podophyllotoxin, a critical concentration for polymerization at 30 degrees C of 1.5 microM is found, and the morphology of the polymers in electron micrographs is typical of MT and ribbons, or open MT. At 2 M TMAO, polymerization is very rapid and hyperstable polymers are formed. These are resistant to cold-induced depolymerization although still sensitive to podophyllotoxin inhibition. A lower critical concentration of 0.7 microM is observed, and electron micrographs reveal MT, ribbons, and other polymer forms not usually stable, such as splayed protofilaments. Inhibition of tubulin polymerization by low concentrations of urea (Sackett, D. L., B. Bhattacharyya, and J. wolff. Biochemistry 33: 12868-12878, 1994) is largely reversed by the presence of TMAO at one-half the molarity of urea, the physiological ratio observed in cartilaginous fishes. Other methylamines, including betaine, dimethylglycine, glycine, and sarcosine, failed to stimulate MT polymerization or protect against urea inhibition. Trimethylamine, taurine and glycylglycine inhibit polymerization. TMAO did not interfere with binding of MT-associated proteins (MAP) and protected both tubulin assembly and MAP binding from urea.