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]. (45)Ca 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.