Structural features of the assembled tetramerization site. (A) Crystallographic structure determination of the assembled tetramerization site showed a complex akin to many 3-repeat spectrin structures. As can be shown from the ribbon diagram (α-helices are depicted as coils), the interaction surface is between α-spectrin (blue) partial repeat 0 and β-spectrin (yellow) partial repeat 17. (B) The helical wheel diagram of the interacting helices shows a clustering of hydrophobic residues (yellow) at the core of the 3-helix bundle. Each helical wheel depicts the radial positions of side chains upon projection down the helical axis. The helices are roughly positioned according to their placement in the structure. Residues are labeled according to single-letter code with each being color coded on the basis of properties of the side chains (positively-charged = blue, negatively charged = pink, polar = purple, hydrophobic = yellow, other = white/beige). (C) A cut-away view of the ribbon diagram near the core of the interacting surface; the orientation of the helices is similar to that in panel B.