Functional roles of prolines at amelogenin C terminal during tooth enamel formation

Amelogenins, the chief proteins in enamel matrix, undergo progressive degradation by matrix metalloproteinase-20 (MMP-20) to facilitate crystal growth. Proline is the most abundant residue in amelogenin and is located upstream to all MMP-20 cleavage sites in the amelogenin sequence. Pro(41) is critical for amelogenin N-terminal processing, while the role of prolines at the amelogenin C terminus have not been determined. This study sought to elucidate the effect of the C-terminal prolines on apatite binding and MMP-20 hydrolysis. To compare apatite affinity, recombinant full-length human amelogenin (rh174) and mutated variants (P156T and P164T) were incubated with hydroxyapatite (HAP), and the unbound proteins were quantified by the Bradford assay. rh174 and mutants, as well as 3 oligopeptides, including wild-type peptide and peptides containing 2 mutations, were incubated with MMP-20 both in solution and on HAP. The digested products were analyzed by SDS-PAGE, reverse-phase high-performance liquid chromatography and mass spectrometry. Mutated amelogenins displayed a significantly lower affinity to HAP than the wild type (P156T < P164T < rH174). The proline mutation at amino acid location 164 significantly reduced the initial hydrolysis of either the amelogenins in solution or the proteins bound on HAP, which was confirmed by amelogenin oligopeptide assays. It was concluded that prolines at the amelogenin C terminus are essential for the initial processing of amelogenin and amelogenin-mineral interactions.