Polymerization and calcium binding of the tubulin-colchicine complex in the GDP state

The tubulin-colchicine complex instead of tubulin was used in an imidazole buffer throughout experiments. The interaction with calcium was examined, especially in the GDP state. The high affinity sites of calcium took part in the polymerization of the complex in the GTP state, while the low ones participated in the depolymerization. The complex had 2 high affinity sites with the dissociation constant of 11.5 x 10(-6) M, and 16 low affinity sites with the dissociation constant of 2.27 x 10(-4) M in the GTP state. In the case of GDP state, the dissociation constant of the high affinity site was 7.2 x 10(-6) M, and the low affinity site was not observed. The ultracentrifugal experiment indicated a little compact structure in the GTP state compared with the GDP state. This agreed with the results of calcium binding.