Expression and characterization of a Rana pipiens amelogenin protein

Amelogenin, the major protein of developing enamel matrix, controls enamel crystal growth via unique supermolecular features. While much has been contributed to our understanding of mammalian amelogenin function, little is known about how amelogenin and its unique physico-chemical features have evolved among vertebrates. Here we report, for the first time, amphibian amelogenin recombinant protein expression and characterization in Rana pipiens. In order to characterize R. pipiens amelogenin, the newly discovered amelogenin coding sequence was amplified, subcloned, and expressed in Eshcerichia coli. Our newly generated R. pipiens amelogenin-specific antisera resolved a major 19-kDa band on western blots of frog tooth extracts and revealed an enamel organ tissue-specific localization pattern using immunohistochemistry. Using mass spectroscopy, a single major compound with a molecular weight of 21.6 kDa was detected, which corresponded to the amino acid sequence-based molecular weight prediction of the His fusion recombinant protein. Dynamic light scattering studies resolved 41-nm radius subunits compared with 14-nm radius subunits from mouse recombinant amelogenin controls. Transmission electron microscopy revealed defined spherical subunits in R. pipiens matrix self-assembly in contrast with a homogeneous 'stippled' matrix in mouse amelogenin matrix self-assembly. Our data suggest that R. pipiens amelogenin is distinguished from mammalian amelogenins by a number of unique physico-chemical properties which may be related to specific modes of crystal formation in frog enamel.