even though mg2+ enhances the binding of gtp to tubulin ( the equilibrium constant for the exchange of gtp for gdp was 0.2 in the absence of exogenous mg2+ , 3 with 0.2 mm mg2+ , 5 with 0.5 mm mg2+ , and 11 with 2 and 4 mm mg2+ ) , as mg2+ was increased the proportion of tubulin-gtp required for the initiation of microtubule assembly rose greatly , and the direct incorporation of tubulin-gdp into microtubules during elongation became progressively more efficient .
C0041348
tubulin
C0026046
microtubules