when gtp , gdp , and tubulin concentrations were varied at a constant mg2+ concentration ( 0.2 mm ) , initiation of assembly required that 35 % of the nucleotide-bearing tubulin be in the form of tubulin-gtp , and incorporation of tubulin-gdp into microtubules during elongation was only 60 % as efficient as would be predicted on the basis of its proportional concentration in the reaction mixtures .
C0041348
tubulin
C0026046
microtubules