The L49F mutation in alpha erythroid spectrin induces local disorder in the tetramer association region: Fluorescence and molecular dynamics studies of free and bound alpha spectrin.

The bundling of the N-terminal, partial domain helix (Helix C') of human erythroid alpha-spectrin (alphaI) with the C-terminal, partial domain helices (Helices A' and B') of erythroid beta-spectrin (betaI) to give a spectrin pseudo structural domain (triple helical bundle A'B'C') has long been recognized as a crucial step in forming functional spectrin tetramers in erythrocytes. We have used apparent polarity and Stern-Volmer quenching constants of Helix C' of alphaI bound to Helices A' and B' of betaI, along with previous NMR and EPR results, to propose a model for the triple helical bundle. This model was used as the input structure for molecular dynamics simulations for both wild type (WT) and alphaI mutant L49F. The simulation output structures show a stable helical bundle for WT, but not for L49F. In WT, four critical interactions were identified: two hydrophobic clusters and two salt bridges. However, in L49F, the region downstream of Helix C' was unable to assume a helical conformation and one critical hydrophobic cluster was disrupted. Other molecular interactions critical to the WT helical bundle were also weakened in L49F, possibly leading to the lower tetramer levels observed in patients with this mutation-induced blood disorder.