Fig. 3. The structure of the AR hinge region bound to importin-. (A) 2.6 Å resolution Fo-Fc difference electron-density map (contoured at 3) showing the density resulting from the AR hinge region after subtraction of density due to importin- in crystals of the complex. The structure of the underlying importin- is shown in yellow. Electron density resulting from the AR hinge region was only seen over the primary NLS-binding site of importin-. (B) Relationship between the binding sites on importin- for the AR hinge region (blue) and the bipartite NLS from nucleoplasmin (red). Whereas the nucleoplasmin NLS binds to both the major and minor binding sites on importin-, the AR hinge region binds to the major site together with an adjacent region of the importin- surface that is not involved in the interaction with other NLSs. (C) Schematic illustration of the positions of the Armadillo (ARM) repeats of importin- corresponding to the models shown in A and B. (D) Schematic representation of the interacting residues for different molecules (AR, SV40 large-T antigen, nucleoplasmin and the importin- IBB domain) bound to importin-. In all cases, binding at the major site involves insertion of side chains between a series of Trp residues (W142, W184, W273) on importin-, complemented by H-bonds between key Asn (N146, N188, N235) and the NLS main-chain peptides, and also neutralization of negative charges by strategically placed acidic residues on importin-. However, details of the interactions at the major site are different and, significantly, a series of residues (621EAGMTLGA628) immediately upstream of the negative cluster (629RKLKK634) in the AR hinge region bind to importin- in a manner not seen in other NLSs. Only the bipartite nucleoplasmin NLS binds to the secondary site.