Many macromolecules constantly move between the nucleus and cytoplasm of eukaryotic cells. Nuclear proteins and cytoplasmic RNAs, for example, must be transported from where they are made to where they act. Transit occurs via nuclear-pore complexes ? large assemblies which span the nuclear envelope membranes, that separate the nucleus from the cytoplasm. Transport is mediated by receptor proteins, which shuttle between the nucleus and cytoplasm carrying cargo in one direction (Fig. 1). These receptors—known as importins and exportins—mostly belong to the importin-β protein family, and have also been called karyopherins. On pages 221 and 230 of this issue, Cingolani et al. and Chook and Blobel describe the structures of two different import receptors.
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