Interactions of the a-Spectrin N-terminal Region with b-Spectrin IMPLICATIONS FOR THE SPECTRIN TETRAMERIZATION REACTION*

Spectrin of the erythrocyte membrane skeleton is composed of aand b-spectrin, which associate to form heterodimers and tetramers. It has been suggested that a fractional domain (helix C) in the amino-terminal region of a-spectrin (Na region) bundles with another fractional domain in the carboxyl-terminal region of b-spectrin (Cb region) to yield a triple a-helical bundle and that this helical bundling is largely responsible for tetramer formation. However, there are certain objections to assigning a preeminent role to this helical bundling in the tetramerization reactions. We prepared several recombinant peptides of a-spectrin fragments spanning only the Na region (lacking the dimer nucleation site) and quantitatively studied their interaction with b-spectrin. We found that a majority of the interactions were localized, as expected, in the Na-helix C region but that there was also some contribution from the nonhomologous region. More importantly, the temperature and ionic strength dependence of this interaction in our model peptides was different from that in intact spectrin. We suggest that, although the regions involving the putative helical bundling in aand b-spectrin undoubtedly play a significant role in tetramerization, regions distal to the Na-helix C region in spectrin are also involved in tetramer formation. Structural flexibility and lateral interactions may play a role in spectrin tetramerization.