Importin-beta-like nuclear transport receptors

Abstract

In recent years, our understanding of macromolecular transport processes across the nuclear envelope has grown dramatically, and a large number of soluble transport receptors mediating either nuclear import or nuclear export have been identified. Most of these receptors belong to one large family of proteins, all of which share homology with the protein import receptor importin beta (also named karyopherin beta). Members of this family have been classified as importins or exportins on the basis of the direction they carry their cargo. To date, the family includes 14 members in the yeast Saccharomyces cerevisiae and at least 22 members in humans. Importins and exportins are regulated by the small GTPase Ran, which is thought to be highly enriched in the nucleus in its GTP-bound form. Importins recognize their substrates in the cytoplasm and transport them through nuclear pores into the nucleus. In the nucleoplasm, RanGTP binds to importins, inducing the release of import cargoes. In contrast, exportins interact with their substrates only in the nucleus in the presence of RanGTP and release them after GTP hydrolysis in the cytoplasm, causing disassembly of the export complex. Thus, common features of all importin-beta-like transport factors are their ability to shuttle between the nucleus and the cytoplasm, their interaction with RanGTP as well as their ability to recognize specific transport substrates.

Figure 1

Structure of importin β. (a) Importin β is composed of 19 helical-repeat motifs (HEAT repeats). Each consists of an A and a B helix connected by a short turn, which in HEAT-8 is replaced by an acidic loop critical for the regulation of substrate binding and release. The HEAT repeats 1-8 are required for high-affinity binding to RanGTP [4,5]. The importin-β-binding (IBB) domain of importin α interacts mainly with residues located in repeats 7-19 of importin β [6]. The binding site for nucleoporins of the NPC is located between residues 152 and 352, corresponding to repeats 4-8 [4,5]. On the basis of the crystal structure, the A helices of HEAT repeats 5 and 6 and a region between HEAT repeats 6 and 7 are thought to be critical for recognition of the FxFG motif [9]. N, amino terminus; C, carboxyl terminus. (b) Structure of importin β bound to the IBB domain of importin α (adapted from [6]). Importin β (yellow) forms a superhelical structure that wraps like a snail around the IBB domain (blue). The 19 HEAT repeats share a common core of 21 residues, comprising the A helix with about three turns and the B helix with about four turns. The helices critical for the interaction with FxFG-repeat nucleoporins [9] are in green. Important residues for interaction with RanGTP [8] are in red. Note the acidic loop, which contacts both RanGTP and the IBB domain (white arrow).

Figure 2

A schematic representation of nuclear import and export cycles through the NPC. Typically, an import cargo is first recognized by its importin in the cytoplasm. The cargo-loaded importin translocates through the NPC into the nucleus, where the cargo is dissociated from the importin by binding of importin to RanGTP. The importin-RanGTP complex recycles back to the cytoplasm, where RanGTP hydrolysis is stimulated by RanGAP and RanBP1; this frees the importin for the next round of import. Binding of cargoes to exportins is regulated in a converse manner. Exportins bind their export substrates in the nucleus, forming a trimeric cargo-exportin-RanGTP complex. This complex is exported from the nucleus and dissociated in the cytoplasm by hydrolysis of RanGTP to RanGDP and inorganic phosphate (Pi). This releases the export substrate, and the exportin is recycled back into the nucleus. For details, see text.