IMPORT AND EXPORT OF THE NUCLEAR-PROTEIN IMPORT RECEPTOR TRANSPORTIN BY A MECHANISM INDEPENDENT OF GTP HYDROLYSIS

Background: Nuclear protein import and export are mediated by receptor proteins that recognize nuclear localization sequences (NLSs) or nucl ear export sequences (NESs) and target the NLS-bearing or NES-bearing protein to the nuclear pore complex (NPC). Temperature-dependent trans location of the receptor-cargo complex in both directions through the NPC requires the GTPase Ran, and it has been proposed that the Ran GTP ase cycle mediates translocation. We have addressed the role of GTP hy drolysis in these processes by studying the import receptor transporti n, which mediates the import of a group of abundant heterogeneous nucl ear RNA-binding proteins bearing the Mg NLS. Results: We investigated the transport properties of transportin and found that the carboxy-ter minal region of transportin could, by itself, be imported into the nuc leus. Transportin import and export were inhibited by low temperature in vitro, but were unaffected by the non-hydrolyzable GTP analogue GMP -PNP. Conclusions: Temperature-dependent import and export through the NPC can be uncoupled from the Ran GTPase cycle and can occur without GTP hydrolysis.