Diversification of the human nuclear import adaptor importin α. (A) A phylogenetic tree showing the evolutionary divergence of the six different human importin α isoforms. The branch lengths are proportional to the predicted evolutionary time between sequences. Three subfamilies of importin α (shaded in green, yellow and magenta) are identified. Both sequence alignment and phylogenetic tree were generated using the program ClustalW . Ribbon diagram of the mammalian importin α1 (B) (pdb 1EJL) and importin α5 (C) (pdb 2JDQ) (in green and violet, respectively). Both structures consist of 10 tandemly repeated Armadillo repeats (ARM), each formed by three α-helices. Significant differences can be seen between importin α1 and α5 C-terminal ARM-10, which is partially extended in α5 . Both crystal structures in panel (B-C) lack the N-terminal Importin β binding (IBB) domain, which promotes binding to the receptor importin β.