-M.Mingot contributed equally to this work Many nuclear transport pathways are mediated by importin b-related transport receptors. Here, we identify human importin (Imp) 4b as well as mouse Imp4a, Imp9a and Imp9b as novel family members. Imp4a mediates import of the ribosomal protein (rp) S3a, while Imp9a and Imp9b import rpS7, rpL18a and apparently numerous other substrates. Ribosomal proteins, histones and many other nuclear import substrates are very basic proteins that aggregate easily with cytoplasmic polyanions such as RNA. Imp9 effectively prevents such precipitation of, for example, rpS7 and rpL18a by covering their basic domains. The same applies to Imp4, Imp5, Imp7 and Impb and their respective basic import substrates. The Impb±Imp7 heterodimer appears specialized for the most basic proteins, such as rpL4, rpL6 and histone H1, and is necessary and suf®cient to keep them soluble in a cytoplasmic environment prior to rRNA or DNA binding, respectively. Thus, just as heat shock proteins function as chaperones for exposed hydrophobic patches, importins act as chaperones for exposed basic domains, and we suggest that this represents a major and general cellular function of importins. Keywords: chaperone/histone/importin/ribosomal protein IntroductionThe nuclear envelope (NE) divides eukaryotic cells into a nuclear and a cytoplasmic compartment, uncouples transcription from translation and thereby necessitates nucleocytoplasmic transport (reviewed in Mattaj and Englmeier, 1998;Nakielny and Dreyfuss, 1999). tRNAs, mRNAs and rRNAs, for example, need to be exported from the nucleus to the cytoplasm, where they mediate translation. Conversely, virtually all nuclear functions are dependent on proteins which are imported from the cytoplasm. The nuclear compartment already represents the ®nal destination for many import substrates, such as histones or polymerases. Numerous other proteins, however, pass through nuclei only transiently. Ribosomal proteins, for example, are ®rst imported, assemble in the nucleoli with rRNAs, and ®nally become exported as ribosomal subunits to the cytoplasm (for a review see Venema and Tollervey, 1999).Macromolecular transport between the nucleus and cytoplasm proceeds through nuclear pore complexes (NPCs) and is normally receptor mediated. Impb-type transport receptors account for most, but not all, nuclear transport pathways (for reviews see Mattaj and Englmeier, 1998;Nakielny and Dreyfuss, 1999;Conti and Izaurralde, 2001). They constitute a diverse protein superfamily and occur in two forms, import mediators (importins) and exportins. They circulate between nucleus and cytoplasm, recognize cargo molecules and transfer them from one side of the NE to the other. Substrate loading and release is guided by a gradient of RanGTP across the NE, whereby a high nuclear RanGTP concentration favours cargo loading onto exportins and substrate displacement from importins, while cytoplasmic conditions with low RanGTP levels release substrates from exportins but allow importin±cargo complexes to fo...

Importin b-related receptors mediate translocation through nuclear pore complexes. Co-operation with the RanGTPase system allows them to bind and subsequently release their substrates on opposite sides of the nuclear envelope, which in turn ensures a directed nucleocytoplasmic transport. Here we identify a novel family member from higher eukaryotes that functions primarily, but not exclusively, in import. It accounts for nuclear accumulation of the SUMO-1/sentrin-conjugating enzyme hUBC9 and mediates import of the RBM8 (Y14) protein, and is therefore referred to as importin 13 (Imp13). Unexpectedly, Imp13 also shows export activity towards the translation initiation factor eIF1A and is thus a case where a single importin b-like receptor transports different substrates in opposite directions. However, Imp13 operates differently from typical exportins in that the binding of eIF1A to Imp13 is only regulated indirectly by RanGTP, and the cytoplasmic release of eIF1A from Imp13 is triggered by the loading of import substrates onto Imp13.

Human immunodeficiency virus type 1 (HIV-1), like other lentiviruses, can infect non-dividing cells. This property depends on the active nuclear import of its intracellular reverse transcription complex (RTC). We have studied nuclear import of purified HIV-1 RTCs in primary macrophages and found that importin 7, an import receptor for ribosomal proteins and histone H1, is involved in the process. Nuclear import of RTCs requires, in addition, energy and the components of the Ran system. Depletion of importin 7 from cultured cells by small interfering RNA inhibits HIV-1 infection. These results provide a new insight into the molecular mechanism for HIV-1 nuclear import and reveal potential targets for therapeutic intervention.

Most transport pathways between cell nucleus and cytoplasm are mediated by nuclear transport receptors of the importin b family. These receptors are in continuous circulation between the two compartments and transfer cargo molecules from one side of the nuclear envelope to the other. RanBP16 is a family member from higher eukaryotes of so far unknown function. We now show that it exports p50RhoGAP from the nucleus and thereby confines this activity to the cytoplasm. It also accounts for nuclear exclusion of 14-3-3r, which in turn is known to anchor, for example, cyclin-dependent kinases in the cytoplasm. Our data further suggest that RanBP16 exports several additional cargoes. It thus appears to be a nuclear export mediator with broad substrate specificity and we will therefore refer to it as exportin 7 (Exp7). Finally, we demonstrate that Exp7-dependent nuclear export signals differ fundamentally from the leucine-rich, CRM1-dependent ones: First, they are not just short linear sequences, but instead include folded motifs. Second, basic residues are critical for Exp7 recruitment.

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