Mineral Association Changes the Secondary Structure and Dynamics of Murine Amelogenin

Amelogenin is one of the key protein constituents responsible for the exquisite organization of the calcium phosphate crystals in enamel. Amelogenin forms into nanospheres in solution, while its association with hydroxyapatite is also essential to enamel development. Structural information of full-length amelogenin in either of these physiologically important forms has the potential to provide mechanistic information; however, these data are limited because of the difficulty of determining the structure of large protein complexes and proteins bound to surfaces. To obtain structural insights into amelogenin during these early stages of enamel development, we used a lysine-specific 13C-, 15N-labeled sample of murine amelogenin to provide insight into the structure of the hydroxyapatite (HAP)-binding domains of the protein. A combination of one-and two-dimensional solid-state NMR experiments was used to obtain molecular-level insights into the secondary structure and dynamics of full-length amelogenin within a nanosphere-gel and on the surface of HAP. Regions of amelogenin that appear to be primarily random coil in the nanosphere-gel adopt a β-strand structure and are less mobile with HAP binding, indicative of a structural switch upon binding that may be important in the role of amelogenin in enamel development.