Tubulin assembly protein: immunochemical and immunofluorescent studies on its function and distribution in microtubules and cultured cells.

Cytoplasmic microtubule assembly from tubulin monomers requires an accessory protein or proteins present is isolated microtubules. These proteins have been designated "tau" factors. One such factor, tubulin assembly protein (TAP), has been purified to homogeneity from calf brain microtubules. A precipitating, monospecific antibody against the protein has been prepared. The antibody has been used to investigate the mechanism of TAP action in microtubule assembly and the distribution of TAP in cellular microtubules. Immunochemical, immunofluorescent and electron microscopic studies indicate that TAP functions stoichiometrically by binding physically to tubulin to form a complex active in microtubule assembly. TAP is an elongation protein which is required throughout the growth of a microtubule and which is actually present along the entire microtubule. Immunofluorescence microscopy has been used to demonstrate that TAP is distributed throughout the cytoplasmic microtubule network of cultured human, hamster and rat cells-both normal and virally transformed. Immunofluorescence of cells in mitosis shows that TAP is present in the mitotic spindle. These results demonstrate the biological importance of tubulin assembly protein and suggest that it or immunologically related "tau" proteins represent ubiquitous cofactors in cytoplasmic microtubule assembly.