Identification of a novel matrix protein contained in a protein aggregate associated with collagen in fish otoliths

In the biomineralization processes, proteins are thought to control the polymorphism and morphology of the crystals by forming complexes of structural and mineral‐associated proteins. To identify such proteins, we have searched for proteins that may form high‐molecular‐weight (HMW) aggregates in the matrix of fish otoliths that have aragonite and vaterite as their crystal polymorphs. By screening a cDNA library of the trout inner ear using an antiserum raised against whole otolith matrix, a novel protein, named otolith matrix macromolecule‐64 (OMM‐64), was identified. The protein was found to have a molecular mass of 64 kDa, and to contain two tandem repeats and a Glu‐rich region. The structure of the protein and that of its DNA are similar to those of starmaker, a protein involved in the polymorphism control in the zebrafish otoliths [Söllner C, Burghammer M, Busch‐Nentwich E, Berger J, Schwarz H, Riekel C & Nicolson T (2003) Science302, 282–286]. 45Ca overlay analysis revealed that the Glu‐rich region has calcium‐binding activity. Combined analysis by western blotting and deglycosylation suggested that OMM‐64 is present in an HMW aggregate with heparan sulfate chains. Histological observations revealed that OMM‐64 is expressed specifically in otolith matrix‐producing cells and deposited onto the otolith. Moreover, the HMW aggregate binds to the inner ear‐specific short‐chain collagen otolin‐1, and the resulting complex forms ring‐like structures in the otolith matrix. Overall, OMM‐64, by forming a calcium‐binding aggregate that binds to otolin‐1 and forming matrix protein architectures, may be involved in the control of crystal morphology during otolith biomineralization.