SPECTRIN CAGLIARI - AN ALA -] GLY SUBSTITUTION IN HELIX-1 OF BETA-SPECTRIN REPEAT-17 THAT SEVERELY DISRUPTS THE STRUCTURE AND SELF-ASSOCIATION OF THE ERYTHROCYTE SPECTRIN HETERODIMER

The spectrin tetramer, the principal structural element of the red cel l membrane skeleton, is formed by stable head-to-head self-association of two spectrin heterodimers. The self-association site appears to be formed by interactions between helices 1 and 2 of beta spectrin repea t 17 of one dimer with helix 3 of alpha spectrin repeat 1 of the other dimer to form two combined alpha-beta triple-helical segments. The he ad of the heterodimer appears to involve similar intradimer interactio ns. We describe the first example of an amino acid substitution in hel ix 1 of this combined alpha-beta triple-helical segment, which, althou gh relatively minor, profoundly impairs tetramer formation. Strikingly , low angle rotary shadowing electron microscopy of isolated spectrin dimers reveals that this mutation also severely disrupts the head of t he heterodimer causing it to be open. Following linkage studies which were most consistent with a beta spectrin gene mutation, a nucleotide change was identified in codon 2018, resulting in an Ala --> Gly subst itution in the first helical domain of beta spectrin repeat 17. Becaus e glycine is a strong helix breaker, this change is predicted to disru pt the conformation of this helical domain. Our results indicate that this helical domain must play direct roles in the alpha-beta interdime r interactions that form the self-association site of the tetramer and in the alpha-beta intradimer interactions at the head of the heterodi mer.