Identification and characterization of photomedins: novel olfactomedin-domain-containing proteins with chondroitin sulphate-E-binding activity

We screened more than 60000 RIKEN mouse cDNAs for novel ECM (extracellular matrix) proteins by extensive computational screening followed by recombinant expression and immunohistochemical characterization. We identified two novel olfactomedin-family proteins characterized by the presence of tandem CXCX-CX9C motifs in the N-terminal region, a coiled-coil domain and an olfactomedin domain in the C-terminal region. These proteins, named photomedin-1 and photomedin-2, were secreted as disulphide-bonded dimers (photomedin-1) or oligomers/multimers (photomedin-2) with O-linked carbohydrate chains, although photomedin-1 was proteolytically processed in the middle of the molecule after secretion. In the retina, photomedin-1 was selectively expressed in the outer segment of photoreceptor cells and photomedin-2 was expressed in all retinal neurons. Among a panel of ECM components, including glycosaminoglycans, photomedins preferentially bound to chondroitin sulphate-E and heparin. These results, together, indicate that photomedins are novel olfactomedin-domain-containing extracellular proteins capable of binding to proteoglycans containing these glycosaminoglycan chains.