Fish otolith matrix macromolecule-64 (OMM-64) and its role in calcium carbonate biomineralization

The inner ear of the rainbow trout contains biominerals called otoliths that are composed of calcium carbonate crystals and a protein organic matrix. The organic matrix macromolecule-64 (OMM-64) is an intrinsically disordered protein associated with high-molecular-weight aggregates. In this study, through a series of bioinspired mineralization experiments, we have determined that OMM-64 affects the size and shape of CaCO3 crystals and stiffens the crystals’ surfaces. OMM-64 serves as a template for precipitating the inorganic phase, which causes greater flexibility of the crystals, promotes crystal nucleation, and in the presence of Mg2+ ions, partially inhibits the formation of aragonite. This latter function appears to be a crucial factor in phase selection. The accumulation of fluorescence in the central part of the crystal suggests the participation of OMM-64 in the initial stages of crystallization and its accumulation in the crystallization nucleus.