TAU AS A NUCLEOLAR PROTEIN IN HUMAN NONNEURAL CELLS IN-VITRO AND IN-VIVO

The Tau-1 monoclonal antibody was localized to the nucleolus of interp hase cells and the nucleolar organizing regions (NORs) of acrocentric chromosomes in cultured human cells. Putative nucleolar and NOR tau wa s found in HeLa cells and lymphoblasts as well as in nontransformed fi broblasts and lymphocytes. To confirm the presence of tau in the nucle i of these nonneural cells, immunoblotting analysis was performed on i solated nuclei from lymphoblasts, Several tau bands were noted on the blot of the nuclear extract suggesting the presence of multiple tau is oforms. Tau-1 immunostaining demonstrated variable staining intensitie s between individual acrocentric chromosomes in all cells tested. In c ultured peripheral lymphocytes, these staining patterns were the same from one chromosome spread to the next within an individual. This cons istency of Tau-1 staining and its variability among NORs was reminisce nt of staining patterns obtained using the silver-NOR procedure. Compa risons of Tau-1 immunostaining with silver staining of chromosome spre ads from human lymphocytes demonstrated that Tau-1 did not immunostain all of the NORs that were silver stained. The intensity of Tau-1 fluo rescence in nucleoli was further shown to be increased in phytohemaggl utinin-stimulated lymphocytes, indicating an upregulation of nuclear t au when cells reentered the cell cycle. These results contribute to a growing body of evidence defining tau as a multifunctional protein tha t may be involved in ribosomal biogenesis and/or rRNA transcription in the nucleus of all cells as well as microtubule-stabilizing functions in the neuronal cytoplasm.