The ankyrin binding domain of human β-spectrin (repeats 14-15) displays a conserved, negatively charged surface patch. (A) A group of acidic residues in repeat 14 form a negatively charged patch. The ribbon diagram of repeats 14 and 15 (left) shows the acidic residues involved in forming this patch (circled). Only the side chains of the negatively charged residues are shown (ball-and-stick). The electrostatic surface of the protein shows the presence of the patch (second diagram from left with circle) in the same orientation as the ribbon diagram. The third diagram from the left shows the electrostatic surface of repeats 14 and 15 rotated approximately 180° with respect to the ribbon diagram. As can be seen, this side of the protein does not show any large charged regions. The structures of other known spectrin repeats do not display such a localized charge distribution; the electrostatic map of β-spectrin repeats 8 and 9 (right) is shown as a representative. The molecular surface of the molecule is shown with the equipotential electrostatic surface mapped onto it at ± 15 K_bT/e_c. (B) Sequence alignment of β-spectrin from several isoforms and species. The alignment shows that the acidic residues on helices A and C are well-conserved, although, in general, the entirety of β-spectrin repeats 14 and 15 are well-conserved. The negatively charged amino acids forming the patch are boxed in red and the position of the helices is shown by cylinders above the sequence.