Inhibition of microtubule elongation by GDP.

GDP reduces both the rate and amplitude of GTP-induced assembly of microtubules from tubulin dimer or from microtubule protein, and promotes disassembly from microtubules at the steady state. One interpretation postulates that added GDP modifies microtubule ENDS so that tubulin-GTP, the species involved in steady state elongation of microtubules, cannot bind to a microtubule END containing tubulin-GDP. This concept has been used in subsequent models of assembly which treat the 'dynamic instability' of microtubules. We question this interpretation on the basis of the published experimental data and the results reported here. Using a relatively simple model for microtubule assembly, we show by numerical simulation that the quantitative effects of GDP on the rate and amplitude of microtubule assembly and inhibition of steady state GTPase activity are well accounted for by the nucleotide exchange equilibrium of tubulin-GDP and tubulin-GTP. We therefore conclude that the effect of added GDP on elongation of MAP-containing microtubules and on steady state GTPase activity does not indicate modification of the activity of microtubule ENDs but depends on the tubulin-GTP/tubulin-GDP equilibrium. Additional evidence argues that microtubule ENDS containing GDP can indeed accept elongation by tubulin-GTP.