Binding surfaces of the spectrin-ankyrin interaction reveal a bimodal interaction. In all panels the molecules of the complex are opened and rotated such that the interacting regions of both face the reader. (A) The spectrin residues involved in ankyrin binding are found principally along repeat 14. The interacting surfaces present many charged residues along the N-terminal portion of repeat 14, whereas more hydrophobic residues are localized near the spectrin B/C loop. (B) Surface footprints of spectrin (green/blue) and ankyrin (red/gold) illustrate the shape complementarity. The interacting residues in spectrin and ankyrin are shown in green and red, respectively. (C) The electrostatic surface of the molecules show significant charge interactions involving a negatively charged region along repeat 14 of spectrin and in a positively charged patch on the ankyrin fragment. The molecular surface of the molecules is shown with the equipotential electrostatic surface mapped onto them at ±15 k_bT/e_c with red corresponding to negative and blue corresponding to positive charges.