Possible role of dicalcin, a Ca²⁺-binding protein in frog olfactory cells

Abstract

Ca²⁺ has various cellular functions. In olfactory cells, Ca²⁺ concentrations increase after odorant stimulation, which leads to desensitization of cAMP-gated channels and activation of Ca²⁺-activated chloride channel. In addition to these, we recently found a novel Ca²⁺-sensitive pathway mediated by a Ca²⁺-binding protein, dicalcin (formerly named p26olf), of which function is still unknown. In the present study we tried to find out the role of this protein. Dicalcin was originally found in frog olfactory epithelium and is now known to be present also in frog respiratory epithelium. Dicalcin is composed of two S100 domains aligned sequentially. S100 proteins are Ca²⁺-binding proteins, and functional in a dimer form. So far, dicalcin is found only in frogs, and therefore, this protein would be a functional form of S100 proteins in frogs. To investigate the role of this protein, we tried to identify target proteins of dicalcin, and found several candidate proteins. Among them, 35-38 kDa proteins were most abundant. We found that these proteins are annexin A1, A2 and A5, Ca²⁺-binding proteins found in many tissues. Because annexins are suggested to regulate membrane organization, we measured the effect of dicalcin on the membrane aggregation activity of annexins. Dicalcin enhanced the annexin activity in a Ca²⁺-dependent manner. We currently postulate that dicalcin facilitates reseal of the cilia membrane which could be broken by damages induced mechanically or chemically. [J Physiol Sci. 2007;57 Suppl:S17]