Abstract
In this study, we determined the crystal structure of N-terminal importin-β-binding domain (IBB)-truncated human importin-α1 (ΔIBB-h-importin-α1) at 2.63 Å resolution. The crystal structure of ΔIBB-h-importin-α1 reveals a novel closed homodimer. The homodimer exists in an autoinhibited state in which both the major and minor nuclear localization signal (NLS) binding sites are completely buried in the homodimerization interface, an arrangement that restricts NLS binding. Analytical ultracentrifugation studies revealed that ΔIBB-h-importin-α1 is in equilibrium between monomers and dimers and that NLS peptides shifted the equilibrium toward the monomer side. This finding suggests that the NLS binding sites are also involved in the dimer interface in solution. These results show that when the IBB domain dissociates from the internal NLS binding sites, e.g., by binding to importin-β, homodimerization possibly occurs as an autoinhibition state.
Highlights
In eukaryotic cells, the cytoplasm and the nucleus are divided by the nuclear membrane
We investigated the binding of SV40 nuclear localization signal (NLS) and nucleoplasmin NLS to h-importin-α1 and ΔIBB-h-importin-α1
The analytical ultracentrifugation (AUC)-SV for ΔIBB-h-importin-α1 shows that it is in a concentration-dependent monomer–dimer equilibrium (Fig. 1A, B)
Summary
The cytoplasm and the nucleus are divided by the nuclear membrane. The nuclear pore complex (NPC), situated on the nuclear membrane, acts as a gate that allows free diffusion of particles, such as small molecules and ions of less than ~9 nm [1,2,3]. Particles that are larger than 40 kDa and comprise proteins or protein–nucleic acid complexes are transported through the NPC by a variety of active transport mechanisms [4], [5]. Systems consisting of importin α/β have been intensively studied [6,7,8]. Human importin-αs (h-importin-αs) comprise at least seven isoforms: h-importin-α1 (Rch1/KPNA2), h-importin-α3 (Qip1/ KPNA4), h-importin-α4 (KPNA3), h-importin-α5 (NPI-1/KPNA1), h-importin-α6 (KPNA5), h-importin-α7 (KPNA6), and h-importin-α8 (KPNA7) [9], [10].
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