Identification of the retinal pigment epithelium protein RET-PE2 as CE- 9/OX-47, a member of the immunoglobulin superfamily

Purpose. To identify the retinal pigment epithelium (RPE) surface antigen recognized by the monoclonal antibody RET-PE2. Methods. A lambda bacteriophage complementary DNA (cDNA) expression library, representing the rat RPE cell line RPE-J, was constructed and screened with the RET-PE2 monoclonal antibody. Transient transfections of the RET-PE2 cDNA, immunofluorescence stainings of tissue sections or cultured cells, and Western blot analyses of tissue and cell detergent extracts served to prove that the protein resulting from expression of the cDNA is the RET-PE2 antigen. Results. Three independent cDNAs were cloned that shared overlapping sequences. Sequence alignment with EMBL database entries revealed identity to the published cDNA of CE-9/OX-47, a member of the immunoglobulin superfamily. One of the clones encoded the entire open reading frame of CE-9. The expression pattern of the RET-PE2 antigen matched that of CE-9, which is widely expressed. Chinese hamster ovary cells transiently transfected with the RET-PE2 cDNA produced a membrane-localized protein that was recognized by RET-PE2 and CE-9 antibodies. Conclusions. The antibody RET-PE2 recognizes the CE-9/OX-47 gene product, a transmembrane protein of the immunoglobulin superfamily. Contrary to results reported earlier, RET-PE2 immunoreactivity is widely distributed among different rat tissues-kidney, liver, and testis. In epithelia other than the adult RPE, it is confined to the basolateral plasma membrane. Its apical polarization in the RPE of adult rats supports earlier findings that some proteins that are basolateral in other epithelia exhibit reversed polarity in the RPE.