Spectrin αII and βII isoforms interact with high affinity at the tetramerization site

Spectrin tetramers form by the interaction of two alpha-beta dinners through two helices close to the C-terminus of a beta subunit and a single helix at the N-terminus of an alpha subunit. Early work on spectrin from solid tissues (typified by alphaII and betaII polypeptides) indicated that it forms a more stable tetramer than erythroid spectrin (alphaI-betaI). In the present study, we have probed the molecular basis of this phenomenon. We have quantified the interactions of N-terminal regions of two human alpha polypeptides (alphaI and alphaII) with the C-terminal regions of three beta isoforms (beta1Sigma1, betaIISigma1 and betaIISigma2). alphaII binds either betaII form with a much higher affinity than alphaI binds betaISigma1 (K-d values of 5-9 nM and 840 nM respectively at 25 degreesC). betaIISigma1 and betaIISigma2 are splice variants with different C-terminal extensions outside the tetramerization site: these extensions affect the rate rather than the affinity of a subunit interaction. all spectrin interacts with each beta subunit with higher affinity than alpha1, and the betaII polypeptides have higher affinities for both alpha chains than betaISigma1. The first full repeat of the a subunit has a major role in determining affinity. Enthalpy changes in the alphaII-betaIISigma2 interaction are large, but the entropy change is comparatively small. The interaction is substantially reduced, but not eliminated, by concentrated salt solutions. The high affinity and slow overall kinetics of association and dissociation of alphaII-betaII spectrin may suit it well to a role in strengthening cell junctions and providing stable anchor points for transmembrane proteins at points specified by cell-adhesion molecules.