Relationship between the human red cell spectrin dimer-tetramer equilibrium and tetramer site univalent recombinant peptides. (A) A model depicting the 2 equilibria in the overall dimer-tetramer equilibrium of human red cell spectrin. The first step in tetramer formation is opening of a closed dimer (top panel), followed by head-to-head association of 2 open dimers to form a tetramer. Dimer and tetramer models schematically illustrate the repeats that comprise the α and β monomers as follows: rectangles represent the many tandem homologous “spectrin type” repeats; the loop attached to the α9 repeat depicts the SH3 domain, which is inserted in the loop between the B and C helices of repeat 9 (this SH3 domain is designated α10 for historical reasons); the hexagons at the α chain C-terminus represent EF-hand regions (calmodulin-like domains); the elongated rectangle (ABD) at the N-terminus of β-spectrin represents the actin-binding domain (calponin homology domain); the squiggly “tail” represents the nonhomologous phosphorylated C-terminal end of β-spectrin. An enlarged view of the tetramerization site schematically illustrates the α0-1 and β16-17 recombinant peptides using cylinders to represent the 3 helix bundles. In this model, the tetramer binding site is composed of a C helix from the partial α0 repeat and a B and C helix from the β17 partial repeat. The amino acid residues and residue numbers in the α0 C helix that are mutated in HE/HPP patients are shown in the black bar immediately below the tetramer site model. (B) A 1-D 12% Bis-Tris SDS gel stained with Coomassie Brilliant Blue of the purified recombinant proteins (2 μg). Molecular weights of standard proteins are indicated on the left (in kilodaltons).