In growing neurite of neuronal cells, it is suggested that alpha/beta-tubulin heterodimers assemble to form microtubule, and assembly of microtubule promotes neurite elongation. On the other hand, recent studies reveal importance of intracellular Cl- in regulation of various cellular functions such as cell cycle progression, differentiation, cell migration, and elongation of neurite in neuronal cells. In this study, we investigated effects of Cl- on in vitro tubulin polymerization. We found that efficiency of in vitro tubulin polymerization (the number of microtubule) was higher (3 to 5-fold) in Cl--containing solutions than that in Cl--free solutions containing Br- or NO3-. On the other hand, GTPase activity of tubulin was lower (2/3-fold) in Cl--containing solutions than that in Cl--free solutions containing Br- or NO3- Efficiency of in vitro tubulin polymerization in solutions containing a non-hydrolyzable analogue of GTP (GpCpp) instead of GTP was much higher than that in the presence of GTP. Effects of replacement of GTP with GpCpp on in vitro tubulin polymerization was weaker in Cl- solutions (10-fold increases) than that in Br- or NO3- solutions (20-fold increases), although the efficiency of in vitro tubulin polymerization in Cl- solutions containing GpCpp was still higher than that in Br- or NO3- solutions containing GpCpp. Our results suggest that a part of stimulatory effects of Cl- on in vitro tubulin polymerization is mediated via an inhibitory effect on GTPase activity of tubulin, although Cl- would also regulate in vitro tubulin polymerization by factors other than an inhibitory effect on GTPase activity. (c) 2012 Elsevier Inc. All rights reserved.