Otoconin-22, the major protein of aragonitic frog otoconia, is a homolog of phospholipase A2.

Otoconia are composites of proteins and inorganic crystals formed in the peripheral portion of the vestibular system of vertebrates. They add mass to the extracellular otoconial membrane, thereby increasing its deflection during linear acceleration. This added mass increases the sensitivity of the underlying sensory maculae. Otoconia provide a promising system to decipher the interaction of protein and mineral during the growth and maintenance of biominerals. We have purified the major protein of the aragonitic otoconia of Xenopus laevis, which we call otoconin-22, and determined its amino acid sequence and carbohydrate composition. The 127 residues are 37% identical to the phospholipase A2 from Crotalus atrox. We propose that otoconin-22 from X. laevis is homologous to phospholipase A2 and has a similar tertiary structure.