Spectres of spectrin: molecular modeling and hemolytic disease

Abstract

Spectrins are elongated proteins containing a series of triple-helical, coiled-coil bundles. α- and β-Spectrin heterodimerize in an end-to-end fashion and are the major mass constituents of the red blood cell membrane skeleton, a 2D meshwork on the cytoplasmic face of the plasma membrane. The membrane skeleton acts as an elastic semi-solid, allowing brief periods of deformation, followed by re-establishment of the original cell shape. Given that there are >1013 red blood cells in the human blood stream, and that these cells must squeeze through blood vessels of a smaller diameter than themselves hundreds of times a day for ∼100 days without rupturing, a functional erythrocyte membrane skeleton is crucial to cardiovascular health. Supporting this importance, many hemolytic diseases are caused by mutations in α- or β-spectrin. As most other cells also contain a membrane skeleton, and as seven spectrin isoforms exist (two α and five β), the roles of spectrin extend far beyond erythrocytes.Formation of α/β dimers involves the N terminus of α-spectrin and the C terminus of β-spectrin. These dimerization regions resemble incomplete triple-helical motifs and are postulated to self-associate into a triple helix upon dimerization, with β-spectrin providing two helices and α-spectrin one. Over 20 point mutations in the dimerization regions have been identified from patients with hemolytic diseases. Several of these mutations result in seemingly conservative substitutions, raising questions as to how they might perturb dimerization.An elegant study by Zhang et al. [1xSee all References[1] might answer these questions. The authors model the α/β self-association domain based on the crystal structure of the 14th triple-helical region from Drosophila α-spectrin. Using the model, the authors identify 15 interactions between α- and β-spectrin, including eight hydrophobic interactions, four hydrogen bonds and three salt bridges. Analysis of 17 point mutations identified from patients with hemolytic diseases demonstrates structural abnormalities in each of the resulting triple helices. Perhaps most interestingly, the relative clinical severities of these mutations correlate well with the degree to which they perturb the structural model, based on root mean square deviation from the Drosophila crystal structure. Arg28 in α-spectrin is particularly sensitive, as mutation of this residue to Ser, His or Leu results in intermediate to severe hemolytic disease and structural perturbation. The fact that Arg28 hydrogen bonds to two residues in β-spectrin explains this sensitivity.The apparent robust correlation between molecular and clinical manifestations of these mutations provides a structural framework for future analysis of spectrin mutations. Mutations have been identified that affect eight of the 11 amino acids in α-spectrin thought to be crucial for dimerization. The authors boldly predict that mutations affecting the remaining three residues will eventually be identified in patients suffering from hemolytic disease. Based on the strength of the model, I would not bet against them.