The nuclear autoantigen La, a transcription/termination factor of RNA polymerase III, was recently shown to translocalize to the cell surface of growth-stimulated cells during transition from G0- to G1-phase. Here we describe the staining of living mitotic cells with the anti-La mab La11G7. Moreover, La protein added to cell culture medium was able to enter into synchronized mitotic cells. Uptake was inhibited by the anti-La mab. La protein taken up into prophase cells assembled into a fibrillar network. Taken up by ana/telophase cells, La protein was preferentially transported into the newly forming or formed nuclei. This import allowed us to study directly the intranuclear localization of La protein in living cells by the use of confocal laser scanning microscopy (cLSM). Adsorbed La protein was found in the nucleoplasm but also assembled into nuclear speckles. Some of these speckles surrounded the nucleolus like a ring. © 1992.
