Figure 1.

(a) Schematic illustration of the regulation of nucleo-cytoplasmic transport. As an example for nuclear export, the CRM1-dependent pathway is depicted. (b) Heat-repeat map of CRM1, which is predicted to contain 19 repeats separated by a loop within repeat 8. (c) Kinetic classification of NES sequences established by microinjection experiments of recombinant GST-NES-GFP substrates into the nucleus. Amino acids essential for function are highlighted in grey, putative consensus sequence for leucine-rich NES is shown below.

(a) NLS-bearing cargoes are imported via binding to importins. In the nucleus, the importin-cargo complex dissociates upon Ran_GTP binding. CAS exports the respective importin back into the cytoplasm. RanGAP hydrolyses Ran_GTP to Ran_GDP resulting in release of the importin from CAS. Leucine-rich NES-bearing cargoes bind to CRM1 in the presence of Ran_GTP, and the complex translocates into the cytoplasm, where the cargo is released following hydrolysis of Ran_GTP to Ran_GDP. Ran_GDP is reimported by NTF2 and reconverted into Ran_GTP by RanGEF. Leptomycin B (LMB) inactivates CRM1 by covalent binding to Cys₅₂₈, thereby irreversibly inhibiting NES-mediated nuclear export. For the selective export of cargoes, NES-specific cofactors (X) have been suggested. RanGEF - Ran guanine nucleotide exchange factor, RanGAP - Ran GTPase activating protein 1, NTF2 - Nuclear Transport Factor 2, CAS - cellular apoptosis susceptibility, X – putative NES-specific cofactor. (b) Heat repeat 10 contains the cysteine residue 528, which is covalently modified by LMB. The C-terminal region (CTR) comprises heat repeats 14 to 19, colored from yellow to black. The CTRs of two proteolytic fragments of CRM1 (residues 707–1027, PDB 1W9C) are displayed as a dimeric solid ribbon representation of the backbone superposition (heat repeats of one fragment colored as above).