Linkages between the dissociation of alpha beta tubulin into subunits and ligand binding: the ground state of tubulin is the GDP conformation.

The effects of ligands on the dissociation of the alpha beta tubulin dimer into the two subunits were investigated using calf brain tubulin. Sedimentation equilibrium studies showed a number of linkages. In the absence of magnesium in the medium, tubulin-GTP, tubulin-GDP, and tubulin with the exchangeable site unoccupied associate with essentially the same strength (K alpha beta = 1 x 10(7) M-1). This indicates that the ground state of tubulin (i.e., in the absence of magnesium) is not affected by occupancy of the exchangeable nucleotide binding site (E site). The alpha beta association is enhanced by magnesium ions. The association of tubulin with GDP in the E site is linked to the uptake of twice as many magnesium ions as that of tubulin with GTP in the E site. This suggests that magnesium binding is linked to an E-site-related conformational change. Consideration of the linkages between the binding of magnesium ions, E-site occupancy, and tubulin conformation in terms of the model [Howard, W. D., & Timasheff, S. N. (1986) Biochemistry 25, 8292-8300] in which the tubulin alpha beta dimer exists in an equilibrium between two conformations, a microtubule-forming ("straight") state favored by GTP and a double-ring-forming ("curved") state favored by GDP, leads to the conclusion that the ground state of tubulin is the ring-forming or "curved" conformation. Thus, in the absence of magnesium, the tubulin heterodimer exists in the ring-forming conformation, whether the E site is occupied by GTP or GDP.(ABSTRACT TRUNCATED AT 250 WORDS)