Br-labeled importin-beta-binding domain of importin alpha incorporating the unnatural amino acid diBrK (N-ε-3,5-dibromobenzyloxycarbonyl-L-lysine). The positions of the diBrK (*) in the protein sequence are: SGSG*TNENANTPAARLHRFKNKGKDSTEMRRRRIEVNVELRKAKKDDQMLKRRNVSSFPDDATSPLQENRNNQ*G