Role of importin-beta in coupling ran to downstream targets in microtubuleassembly

The guanosine triphosphatase Ran stimulates assembly of microtubule asters and spindles in mitotic Xenopus egg extracts. A carboxyl-terminal region of the nuclear-mitotic apparatus protein (NuMA), a nuclear protein required f or organizing mitotic spindle poles, mimics Ran's ability to induce asters. This NuMA fragment also specifically interacted with the nuclear transport factor, importin-beta. We show that importin-beta is an inhibitor of micro tubule aster assembly in Xenopus egg extracts and that Ran regulates the in teraction between importin-beta and NuMA, Importin-beta therefore Links NUM A to regulation by Ran. This suggests that similar mechanisms regulate nucl ear import during interphase and spindle assembly during mitosis.