Otoconin-90, the mammalian otoconial matrix protein, contains two domains of homology to secretory phospholipase A(2)

The ability to sense orientation relative to gravity requires dense particl es, called otoconia, which are localized in the vestibular macular organs. In mammals, otoconia are composed of proteins (otoconins) and calcium carbo nate crystals in a calcite lattice. Little is known about the mechanisms th at regulate otoconial biosynthesis. To begin to elucidate these mechanisms, we have partially sequenced and cloned the major protein component of muri ne otoconia, otoconin-90 (OC90), The amino acid sequence identified an orph an chimeric human cDNA. Because of its similarity to secretory phospholipas e Az (sPLA(2)), this gene was referred to as PLA(2)-like (PLA2L) and enable d the identification of human Oc90. Partial murine cDNA and genomic clones were isolated and shown to be specifically expressed in the developing mous e otocyst. The mature mouse OC90 is composed of 453 residues and contains t wo domains homologous to sPLA2. The cloning of Oc90 will allow an examinati on of the role of this protein in otoconial biosynthesis and in diseases th at affect the vestibular system.