Conformational change of erythroid α‐spectrin at the tetramerization site upon binding β‐spectrin

We previously determined the solution structures of the first 156 residues of human erythroid α‐spectrin (SpαI‐1–156, or simply Spα). Spα consists of the tetramerization site of α‐spectrin and associates with a model β‐spectrin protein (Spβ) with an affinity similar to that of native α‐ and β‐spectrin. Upon αβ−complex formation, our previous results indicate that there is an increase in helicity in the complex, suggesting conformational change in either Spα or Spβ or in both. We have now used isothermal titration calorimetry, circular dichroism, static and dynamic light scattering, and solution NMR methods to investigate properties of the complex as well as the conformation of Spα in the complex. The results reveal a highly asymmetric complex, with a Perrin shape parameter of 1.23, which could correspond to a prolate ellipsoid with a major axis of about five and a minor axis of about one. We identified 12 residues, five prior to and seven following the partial domain helix in Spα that moved freely relative to the structural domain in the absence of Spβ but when in the complex moved with a mobility similar to that of the structural domain. Thus, it appears that the association with Spβ induced an unstructured‐to‐helical conformational transition in these residues to produce a rigid and asymmetric complex. Our findings may provide insight toward understanding different association affinities of αβ−spectrin at the tetramerization site for erythroid and non‐erythroid spectrin and a possible mechanism to understand some of the clinical mutations, such as L49F of α‐spectrin, which occur outside the functional partial domain region.