Karyopherin β2B participates in mRNA export from the nucleus

Transport of macromolecules between the cell nucleus and cytoplasm occurs through the nuclear pores and is mediated by soluble carriers known as karyopherins (Kaps), transportins, importins, or exportins. We report that Kap β2B (transportin-2) forms complexes with the mRNA export factor TAP in the presence of RanGTP, as shown by coimmunoprecipitation from HeLa cells. The interaction strictly depends on the presence of RanGTP. In digitonin-permeabilized cells, Kap β2B mediates TAP-GFP export from the nuclei in the presence of RanGTP. A TAP mutant that does not coimmunoprecipitate with Kap β2B is also not exported by Kap β2B. In the permeabilized cells assay, TAP is also exported independently of Kap β2B by direct interaction with nucleoporins, in agreement with previous reports. The export rate is, however, significantly lower than the Kap β2B-mediated pathway. Both Kap β2B and TAP are present and enriched in the poly(A)⁺ RNA complexes isolated from HeLa cell nuclear lysates. Poly(A)⁺ RNA strongly accumulates in the nuclei of HeLa cells treated with Kap β2B short interfering RNA, indicating that Kap β2B is involved in the export of at least a large proportion of the mRNA species. The export of β-actin and GAPDH mRNA is also inhibited, whereas 28S RNA is not affected. The data support the conclusion that Kap β2B participates directly in the export of a large proportion of cellular mRNAs, and TAP connects Kap β2B to the mRNAs to be exported.