The structural basis of ankyrin-like repeat function as revealed by the solution structure of myotrophin

Background: Myotrophin is a 12.5 kDa protein that appears to have a key role in the initiation of cardiac hypertrophy, a central process in many heart diseases. Myotrophin primarily comprises ankyrin-like (ANK) repeats, the 33 amino acid motifs involved in a wide range of protein-protein interactions. As a first step in the structure-based search for cardiac hypertrophy antagonists and in order to gain insight into the molecular basis of action of the ubiquitous and multifunctional ANK repeat motif, we have determined the solution structure of myotrophin using multidimensional heteronuclear NMR spectroscopy. Results: The myotrophin structure determination was based on 2786 experimental NMR restraints, and the precision of the coordinates for the final 45 simulated-annealing structures is 0.43 Angstrom for the backbone atoms and 0.87 Angstrom for all atoms. The structure of myotrophin is well defined and is ellipsoidal: approximately 46 Angstrom long and 21 Angstrom wide. The ANK repeats, which constitute the main part of the myotrophin structure, are characteristic of a hairpin-like protruding tip followed by a helix-turn-helix motif. The V-shaped helix-turn-helix of the ANK repeats stack sequentially in bundles and are stabilized by compact hydrophobic cores, whereas the protruding tips are less ordered. This arrangement is quite different to the continuous beta-sheet topology observed in the corresponding regions of another ANK protein, 53BP2, the structure of which was determined in complex with p53. Conclusions: The solution structure of myotrophin provides important insights into the structural and dynamic features of the ANK motif, and suggests that the protruding tips with highly variable sequences may be critical to facilitate diverse protein-protein recognition. The present structure also provides a molecular basis for the further functional characterization of myotrophin and the development of therapeutics for hypertrophy-related heart diseases.