Molecular cloning of a novel membrane glycoprotein, pal, specifically expressed in photoreceptor cells of the retina and containing leucine-rich repeat

We have isolated a novel retina-specific gene in a screen for genes of which expression is not apparent neonatally in rat retina but is abundant postnatally on day 14 (P14). This gene, named Pal, encodes a putative type I transmembrane protein containing five leucine-rich repeats (LRRs), a single C2-type Ig-like domain, and a single fibronectin type III domain and is considered to be a new member of the LRR and Ig superfamily. No expression of Pal was found in rat retina at P1, but it was detected at P7 and markedly increased with subsequent development. These expression patterns of Pal appeared to be correlated with the development of the photoreceptor outer segments, because in the adult rat retina it was specifically localized in these segments. Ultrastructually, Pal immunoreactivity was distributed diffusely on the disk membrane in the lamellar regions. On the basis of its structural features and localization pattern, Pal may act as a receptor for a certain trophic factor or for an adhesion molecule participating in morphogenesis. The human homolog of Pal was mapped to chromosome 10q23.2-23.3 using fluorescence in situ hybridization.