Presence of the ?II isotype of tubulin in the nuclei of cultured mesangial cells from rat kidney

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Abstract
Tubulin has generally been considered to be a cytosolic protein whose only function is to form microtubules. This assumption is supported by a great deal of evidence derived from immunohistochemical studies using antibodies directed against whole tubulin or its component polypeptides α- and β-tubulin. We have re-examined the intracellular distribution of tubulin using monoclonal antibodies specific for the βI, βII, βIII, and βIV isotypes of β-tubulin. Our test system is the cultured rat kidney mesangial cell. We have found that βIII is absent from these cells and that βI and βIV are present in microtubules throughout the cytosol. In contrast, βII is present largely in the nuclei. Immunoblotting of purified nuclear extracts shows that the βII-reactive antigen co-migrates with β-tubulin. Extraction of the cytosol and chromatin suggests that βII is concentrated in the nucleoli and also in a reticulated network in the rest of the nucleoplasm. An antibody to tyrosinated α-tubulin shows that α is also present in the nucleoli. Treatment of the cells with fluorescent colchicine shows an accumulation of colchicine in the nucleoli. Finally, fluorescently labeled αβII-tubulin dimers, when microinjected into the cells, enter the nuclei and are concentrated in the nucleoli. These results suggest that the βII isotype of tubulin is present as an αβII dimer in the nuclei of cultured mesangial cells and suggest the possibility that different tubulin isotypes may have specific functions within the cell. Cell Motil. Cytoskeleton 42:274–284, 1999.