THE 220-KDA RIM PROTEIN OF RETINAL ROD OUTER SEGMENTS IS A MEMBER OF THE ABC TRANSPORTER SUPERFAMILY

Outer segments of mammalian rod photoreceptor cells contain an abundan tly expressed membrane protein that migrates with an apparent molecula r mass of 220 kDa by SDS gel electrophoresis. We have purified the bov ine protein by immunoaffinity chromatography, determined its primary s tructure by cDNA cloning and direct peptide sequence analysis, and map ped its distribution in photoreceptors by immunocytochemical and bioch emical methods, The full-length cDNA encodes a 2280-amino acid protein (calculated molecular mass of 257 kDa) consisting of two structurally related, tandem arranged halves, Each half consists of a hydrophobic domain containing six putative transmembrane segments followed by an A TP binding cassette, A data base homology search showed that the rod o uter segment 220 kDa protein is 40-50% identical in amino acid sequenc e to the ABC1 and ABC2 proteins cloned from a mouse macrophage cell li ne, Photoaffinity labeling with 8-azido-ATP and nucleotide inhibition studies confirmed that both ATP and GTP bind to this protein with simi lar affinities, Concanavalin A labeling and endoglycosidase H digestio n indicated that the rod outer segment protein contains at least one c arbohydrate chain, Immunocytochemical and biochemical studies have rev ealed that the 220 kDa glycoprotein is distributed along the rim regio n and incisures of rod outer segment disc membranes, From these studie s we conclude that the 220-kDa glycoprotein of bovine rod outer segmen t disc membranes or Rim ABC protein is a new member of the superfamily of ABC transporters and is the mammalian homolog of the frog photorec eptor rim protein.