Binding constants and stoichiometries of glyceraldehyde 3-phosphate dehydrogenase-tubulin complexes

The catalytic activity of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) decreased (almost linearly) as a function of increasing concentrations of tubulin; the total loss in activity was attained at a ratio of 1.2 to 1.8 tubulin dimer to GAPDH tetramer. Based on the inhibition data, a dissociation constant for the tubulin-GAPDH complex was calculated to be about 0.73 nM. The stoichiometry and the dissociation constants of the tubulin-GAPDH complex were found to be dependent upon the ionic strength of the assay media. Qualitatively similar results were obtained (i.e., inhibition and ionic strength effect) when the GAPDH-catalyzed reaction was measured in the presence of Sepharose-immobilized tubulin. The physical interaction between these two proteins, i.e., GAPDH and tubulin, was measured by the ability of one protein (immobilized on a Sepharose matrix) to copellet the other protein. By employing this copelleting technique, we measured the dissociation constant and stoichiometry of the immobilized tubulin-GAPDH complex to be about 6.4 nM and 0.91 tubulin dimer/GAPDH tetramer, respectively. The dissociation constant and stoichiometry thus obtained were found to be remarkably similar to those obtained by the tubulin-dependent GAPDH inhibition data. In contrast to these results, (soluble) tubulin had no effect on the catalytic activity of the immobilized GAPDH, albeit the soluble tubulin copelleted with the immobilized GAPDH. The dissociation constant and stoichiometry of immobilized GAPDH-tubulin complex were calculated to be 0.76 +/- 0.13 microM and 3.23 +/- 0.16 tubulin dimer/GAPDH tetramer, respectively. These data suggest that there are two classes of binding sites for tubulin on a tetrameric GAPDH; high-affinity and low-affinity sites. The enzyme is inhibited when tubulin binds at the high-affinity site while the catalytic function of the enzyme is unaffected when the tubulin binds at the low-affinity site. The latter site is suggested herein to be responsible for the cross-linking (bundling) of microtubules.