Immunoflourescent staining of cytoplasmic and spindle microtubules in mouse fibroblasts with antibody to tau protein.

tau protein isolated from porcine brain microtubules was further purified by electrophoretic elution from polyacrylamide gels and used to prepare antisera in rabbits. The antiserum to tau specifically stains mitotic spindles and a filamentous network within mouse fibroblasts when the indirect immunofluorescence technique is used. The staining of the filamentous network and mitotic spindles is identical to that observed when cells are treated with antiserum prepared against electrophoretically purified tubulin. The filamentous network observed with either serum is sensitive to Colcemid. Absorption of anti-tau serum with electrophoretically purified tubulin does not remove the immunofluorescent staining of the mitotic spindle, whereas absorption with electrophoretically purified tau protein does. Conversely, absorption of antitubulin serum with tubulin eliminates its ability to stain the mitotic spindle, whereas absorption with tau has no effect. We conclude that tau protein and tubulin are antigenically distinct proteins and that tau is an integral part of microtubules in vivo. These results also provide evidence that tau protein, or an antigenically related protein, is associated with microtubules not only in brain but also in other cell types.