Importin β contains a COOH-terminal nucleoporin binding region important for nuclear transport

Proteins containing a classical NLS are transported into the nucleus by the import receptor importin β, which binds to cargoes via the adaptor importin α. The import complex is translocated through the nuclear pore complex by interactions of importin β with a series of nucleoporins. Previous studies have defined a nucleoporin binding region in the NH₂-terminal half of importin β. Here we report the identification of a second nucleoporin binding region in its COOH-terminal half. Although the affinity of the COOH-terminal region for nucleoporins is dramatically weaker than that of the NH₂-terminal region, sets of mutations that perturb the nucleoporin binding of either region reduce the nuclear import activity of importin β to a similar extent (∼50%). An importin β mutant with a combination of mutations in the NH₂- and COOH-terminal regions is completely inactive for nuclear import. Thus, importin β possesses two nucleoporin binding sites, both of which are important for its nuclear import function.