Important region in the beta-spectrin C-terminus for spectrin tetramer formation

Many hereditary hemolytic anemias are due to spectrin mutations at the C-terminal region of beta-spectrin (the betaC region) that destabilize spectrin tetramer formation. However, little is known about the betaC region of spectrin. We have prepared four recombinant beta-peptides of different lengths from human erythrocyte spectrin, all starting at position 1898 of the C-terminal region, but terminating at position 2070, 2071, 2072 or 2073. Native polyacrylamide gel electrophoresis showed that the two peptides terminating at positions 2070 and 2071 did not associate with an N-terminal region alpha-peptide (Spalpha1-156) in the micromolar range. However, the peptides that terminated at positions 2072 and 2073 associated with the alpha-peptide. Circular dichroism results showed that the unassociated helices in both alpha- and beta-peptides became associated, presumably to form a helical bundle, for those beta-peptides that formed an alphabeta complex, but not for those beta-peptides that did not form an alphabeta complex. In addition, upon association, an increase in the alpha-helical content was observed. These results showed that the beta-peptides ending prior to residue 2072 (Thr) would not associate with alpha-peptide, and that no helical bundling of the partial domains was observed. Thus, we suggest that the C-terminal segment of beta-spectrin, starting from residue 2073 (Thr), is not critical to spectrin tetramer formation. However, the C-terminal region ending with residue 2072 is important for its association with alpha-spectrin in forming tetramers.