Conformational changes induced by binding of bivalent cations to oncomodulin, a paravalbumin-like tumour protein

When Mg2+ was added to rat oncomodulin, a paravalbumin-like tumour protein, changes in the c.d. spectrum and tyrosine fluorescence intensity were observed. The addition of Ca2+ resulted in even greater changes in these spectra. The fluorescence excitation spectra of apo- and Mg-oncomodulin were superimposable, whereas that of Ca-oncomodulin was markedly different. The u.v.-absorption spectrum of the Ca2+ form also showed major differences from those of the other two forms. These observations indicate that Ca2+ induced a significant and specific conformational change in the protein that was not observed on binding Mg2+. In contrast, the conformational change induced by either Mg2+ or Ca2+ was identical in the homologous rat parvalbumin. This Ca2+-specific conformational change may be the basis for oncomodulin's Ca2+-dependent protein/protein interaction.