Nuclear Import of the U1A Splicesome Protein Is Mediated by Importin α/β and Ran in Living Mammalian Cells

Abstract

U1A is a component of the uracil-rich small nuclear ribonucleoprotein. The molecular mechanism of nuclear import of U1A was investigated in vivo and in vitro. When recombinant deletion mutants of U1A are injected into the BHK21 cell cytoplasm, the nuclear localization signal (NLS) of U1A is found in the N-terminal half of the central domain (residues 100-144 in mouse UIA). In an in vitro assay, it was found that the U1A-NLS accumulated in only a portion of the nuclei in the absence of cytosolic extract. In contrast, the addition of importin α/β and Ran induced the uniform nuclear accumulation of U1A-NLS in all cells. Furthermore, UIA was found to bind the C-terminal portion of importin α. In addition, the in vitro nuclear migration of full-length U1A was found to be exclusively dependent on importin α/β and Ran. Moreover, in living cells, the full-length U1A accumulated in the nucleus in a Ran-dependent manner, and nuclear accumulation was inhibited by the importin β binding domain of importin α. These results suggest that the nuclear import of U1A is mediated by at least two distinct pathways, an importin α/β and Ran-dependent and an -independent pathway in permeabilized cells, and that the latter pathway may be suppressed in intact cells.