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Conformational change of erythroid -spectrin at the tetramerization site upon binding -spectrin

¹ Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, USA
² Research Core Facilities, University of Missouri—Columbia, Columbia, Missouri 65211, USA
³ Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, Illinois 60612, USA
⁴ Department of Biochemistry, College of Medicine, Inha University, Incheon 400-712, Korea

(RECEIVED July 10, 2007; FINAL REVISION August 15, 2007; ACCEPTED August 15, 2007)

We previously determined the solution structures of the first 156 residues of human erythroid -spectrin (SpI-1–156, or simply Sp). Sp consists of the tetramerization site of -spectrin and associates with a model -spectrin protein (Sp) with an affinity similar to that of native - and -spectrin. Upon –complex formation, our previous results indicate that there is an increase in helicity in the complex, suggesting conformational change in either Sp or Sp 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 Sp 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 Sp that moved freely relative to the structural domain in the absence of Sp but when in the complex moved with a mobility similar to that of the structural domain. Thus, it appears that the association with Sp 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 –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 -spectrin, which occur outside the functional partial domain region.

Keywords: erythroid spectrin; -complex; tetramer; prolate ellipsoid

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