Fish otolith contains a unique structural protein, otolin-1.

A collagen-like protein was identified from the otoliths of the chum salmon, Oncorhynchus keta. The otolith, composed mainly of calcium carbonate with small amount of organic matrices, is formed in the inner ear and serves as a part of the hearing and balance systems. Although the organic matrices may play important roles in the growth of otolith, little is known about their chemical nature and physiological function. In this study, a major organic component of the otolith, designated otolin-1, which may serve as a template for calcification, was purified. The sequences of two tryptic peptides from otolin-1 revealed high homology with parts of a saccular collagen which had been described previously [Davis, J.G., Oberholtzer, J.C., Burns, F.R. & Greene, M.I. (1995) Science 267, 1031-1034]. Cloning of a cDNA coding for otolin-1 revealed that the deduced amino-acid sequence contained a collagenous domain in the central part of the protein. Although collagen is the most abundant structural protein in the animal body, otolin-1 mRNA was expressed specifically in the sacculus. Immunohistochemical studies showed that otolin-1 is synthesized in the transitional epithelium and transferred to the otolith and otolithic membrane. This is the first report concerning characterization of a structural protein containing many tandem repeats of the sequence, Gly-Xaa-Yaa, typical for collagen from the biomineral composed of calcium carbonate.