Schematic representation of erythrocyte spectrin equilibria and distal mutations. (A) Full-length red cell spectrin illustrating the closed dimer ↔ open dimer ↔ tetramer equilibria and component domains. The homologous ∼106 residue spectrin type “repeat units” that comprise most of the spectrin molecule are represented by rounded rectangles. The gray rectangles are domains required to initiate antiparallel lateral heterodimer assembly.50 The α-spectrin EF hands are represented by small yellow hexagons and the laterally associated actin binding domain at the N terminus of β-spectrin is represented by larger blue ovals. The SH3 domain (α10) inserted with the α9 domain is indicated by a small white triangle. The domains used in the construction of the recombinant mini-spectrin are highlighted in light blue and wheat and orange and blue. (B) Mini-spectrin in its tetramer form and the corresponding dissociation to open dimer and conversion to closed dimers. The α0-5 is connected to β16-17 using a short glycine linker (gray arc). The approximate locations of the L260P (asterisk) and Q471P (triangle) mutations are shown. (C) The relationship between the secondary structure of the 3-helix bundle motif and the α0-5 spectrin sequence. The black bars above the sequences indicate the locations of the A, B, and C helices of the 3-helix bundle that comprises each repeat, the gray-shaded squiggles indicate locations of loop regions, and the blue-shaded bar and sequence indicate the helical linker region that connects the end of the C helix to the beginning of the A helix in the next repeat. The locations of the L260P and Q471P mutations in the sequence are highlighted in red.
