The cell-surface-expressed nucleolin is associated with the actin cytoskeleton

Nucleolin is a RNA- and protein-binding multifunctional protein. Mainly cha racterized as a nucleolar protein, nucleolin is continuously expressed on t he surface of different types of cells along with its intracellular pool wi thin the nucleus and cytoplasm. By confocal and electron microscopy using s pecific antibodies against nucleolin, we show that cytoplasmic nucleolin is found in small vesicles that appear to translocate nucleolin to the cell s urface. Translocation of nucleolin is markedly reduced at low temperature o r in serum-free medium, where as conventional inhibitors of intracellular g lycoprotein transport have no effect. Thus, translocation of nucleolin is t he consequence of an active transport by a pathway which is independent of the endoplasmic reticulum-Golgi complex. The cell-surface-expressed nucleol in becomes clustered at the external side of the plasma membrane when cross -linked by the nucleolin-specific monoclonal antibody mAb D3. This clusteri ng, occurring at 20 degreesC and in a well-organized pattern, is dependent on the existence of an intact actin cytoskeleton. At 37 degreesC, mAb D3 be comes internalized, thus illustrating that surface nucleolin can mediate in tracellular import of specific ligands. Our results point out that nucleoli n should also be considered a component of the cell surface where it could be functional as a cell surface receptor for various Ligands reported befor e. (C) 2000 Academic Press.