Nuclear import of the Ran exchange factor, RCC1, is mediated by at least two distinct mechanisms

RCC1, the only known guanine-nucleotide exchange factor for the Ran GTPase, is an similar to 45-kD nuclear protein that can bind chromatin, An importa nt question concerns how RCC1 traverses the nuclear envelope. We now show t hat nuclear RCC1 is not exported readily in interphase cells and that the i mport of RCC1 into the nucleoplasm is extremely rapid. Import can proceed b y at least two distinct mechanisms. The first is a classic import pathway m ediated by basic residues within the NH2-terminal domain (NTD) of RCC1. Thi s pathway is dependent upon both a preexisting Ran gradient and energy, and preferentially uses the importin-alpha 3 isoform of importin-alpha. The se cond pathway is not mediated by the NTD of RCC1. This novel pathway does no t require importin-alpha or importin-beta or the addition of any other solu ble factor in vitro; however, this pathway is saturable and sensitive only to a subset of inhibitors of classical import pathways. Furthermore, the nu clear import of RCC1 does not require a preexisting Ran gradient or energy. We speculate that this second import pathway evolved to ensure that RCC1 n ever accumulates in the cytoplasm.