The spectrin repeat folds into a three-helix bundle in solution.

Spectrin, a major component of the membrane skeleton, is mainly composed of tandemly repeated segments of approx. 106 amino acids. We have undertaken the determination of the three-dimensional structure of a chicken brain alpha-spectrin repeat by heteronuclear multidimensional NMR. Sedimentation equilibrium demonstrates that this repeat is monomeric at the concentration used for NMR (1 mM). Its secondary structure was identified using a collection of sequential and medium range NOEs, chemical shifts, HN-Halpha coupling constants, and relaxation measurements. These data unequivocally demonstrate the presence of three long helices connected by two loops. A set of interhelical NOEs indicates that the helices assemble into a triple helical structure. Our results provide experimental evidence supporting the triple-helical bundle proposed by modelling.