A nuclear export signal is essential for the cytosolic localization of the ran binding protein, RanBP1

RanBP1 is a Ran/TC4 binding protein that can promote the interaction between Ran and beta-importin/beta-karyopherin, a component of the docking complex for nuclear protein cargo. This interaction occurs through a Ran binding domain (RED). Here we show that RanBP1 is primarily cytoplasmic, but the isolated RED accumulates in the nucleus. A region COOH-terminal to the RED is responsible for this cytoplasmic localization. This domain acts heterologously, localizing a nuclear cyclin B1 mutant to the cytoplasm. The domain contains a nuclear export signal that is necessary but not sufficient for the nuclear export of a functional RED. In transiently transfected cells, epitope-tagged RanBP1 promotes dexamethasone-dependent nuclear accumulation of a glucocorticoid receptor-green fluorescent protein fusion, but the isolated RED potently inhibits this accumulation, The cytosolic location of RanBP1 may therefore be important for nuclear protein import. RanBP1 may provide a key link between the nuclear import and export pathways.