Spectrins comprise α- and β-subunits made up predominantly of a series of homologous repeating units of about 106 amino acids; the α- and β-chains form antiparallel dimers by lateral association, and tetramers through head-to-head contacts between the dimers. Here we consider the first of these interactions. (1) We confirm earlier observations, showing that the first two paired repeats (βIR1 with αIR21, and βIR2 with αRI20) at one end of the erythroid spectrin (αIβI) dimer are necessary and sufficient to unite the chains; (2) we resolve a conflict in published reports by showing that the strength of the interaction is considerably increased on adding the adjoining pair of repeats (βIR3-αIR19); (3) in brain (αIIβII) spectrin the first two pairs of repeats are similarly essential and sufficient for heterodimer formation; (4) this interaction is ~60-fold stronger than that in the erythroid counterpart, but no enhancement can be detected on addition of three further pairs of repeats; (5) formation of a tight αIβI dimer probably depends on structural coupling of the first two repeats in each chain; (6) an analysis of the sequences of the strongly interacting repeats, βIR1, βIIR1, αIR21 and αIIR20 and repeats in α-actinin, which also interact very strongly in forming an antiparallel dimer, affords a possible explanation for the different properties of the two spectrin isoforms in respect of the stability of the inter-chain interactions, and also suggests the evolutionary path by which the erythroid and non-erythroid sequences diverged.
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