Comparison of structural models of spectrin di-repeats suggest a binding mechanism aided by a specific bend angle between repeats 14 and 15. (A) Superposition of multiple structures of repeats 14 and 15 of β-spectrin^27,33,37  in the unbound state (gray) onto repeats 14 and 15 in the complex (blue) show marked structural similarities. In total, 5 models from 3 independent crystal structures of unbound β-spectrin repeats 14 and 15 were superimposed. The structures are virtually identical, showing the same relative orientation of the 2 repeats (RMSD values of ∼ 1.5 Å between all of the structures). The similarity among the structures suggests that the presence of a specific bending angle is important in recognition. (B) Alignment of 3 different spectrin di-repeat structures to repeat 14 of the complex demonstrate how the overall bend angle between β-spectrin repeats 14 and 15 (blue; HEβ1415) generates a close-fitting and matching interaction with ZU5-ANK (gold; ZU5-ANK). In contrast, the shallower bend angles seen in human erythroid β-spectrin repeats 8 and 9⁴⁴  (gray; HEβ89) and chicken brain α-spectrin repeats 15 and 16⁴⁵  (purple; CBα1516) would reduce the interacting surface or induce steric clashes, respectively. The dashed blue outline completing repeat 15 was modeled from the structure of brain β-spectrin repeats 14 to 16.³⁷