Recent evidence has suggested that the monomer-dimer equilibrium of tubulin under conditions of relative low concentration may have a substantial effect on the assembly of microtubules. To this effect we have discovered that the unchanged dye, nile red, binds to the surfaces of proteins with known hydrophobic domains with marked enhancement of fluorescence intensity and blue shifts that are presumed to indicate the polarity of the binding domain. The dye is sensitive to denaturation of the protein, conformational changes produced by other ligands and changes in polymerization. Tubulin fluorescence shows hypsochromic and quantum shift changes that can identify the components of this monomer/dimer equilibrium. The equilibrium seen by fluorescence agrees with that seen by the more cumbersome hydrodynamic analysis. Agents that shift the equilibrium also shift the fluorescence behavior. Our current efforts are directed toward confirming these findings by other physical parameters.
