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.