Frog p26olf, a molecule with two S100-like regions in a single peptide

An S100-like calcium-binding protein, p26olf, was originally isolated from the frog (Rana Catesbeiana) olfactory epithelium with four chromatographical steps. The primary structure of p26olf contains two S100-like regions aligned in tandem with four functional EF-hands. At 100 mM K(+), wild-type p26olf binds Ca(2+) with a Kd value of 22 microM and a Hill coefficient of 2.0. Each EF-hand seems to have different affinity for Ca(2+): it is high in EF-A and -B and low in EF-C and -D. In our Ca(2+)-binding model, the order of Ca(2+)-binding to p26olf is EF-B, EF-A, EF-C, and EF-D. Expression of mRNA of p26olf is detected in various frog tissues: it is high in the olfactory epithelium, lung, and spleen, moderate in brain, retina, heart, and kidney, and low in liver and muscle. Immunohistochemical studies revealed that p26olf is prominently localized in the cilia of both olfactory and lung respiratory epithelium and especially enriched in the distal segment of the olfactory cilia. Several proteins in the olfactory cilia bind to p26olf in the presence of Ca(2+), suggesting that they are possible target proteins of p26olf. One of these target proteins is immunologically identified as a beta-adrenergic receptor kinase-like protein. In the olfactory cilia, p26olf may have some roles in the olfactory transduction or adaptation through interaction with this beta-adrenergic receptor kinase-like protein.