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Robert S. Molday, Stepan Lenevich, Faraz Quazi; ABCA4 Linked To Stargardt Disease is a Novel ABC Transporter of N-retinylidene-phosphatidylethanolamine and Phosphatidylethenolamine. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3358.
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© ARVO (1962-2015); The Authors (2016-present)
ABCA4 is a member of the superfamily of ABC transporters implicated in the clearance of potentially toxic retinoid compounds from rod and cone cells following photoexcitation. Mutations in ABCA4 are responsible for Stargardt disease and other retinal degenerative diseases. Currently there is no direct biochemical evidence for the function of ABCA4 as a retinoid transporter and the direction of transport remains to be determined. The purpose of this study was to identify substrates which are transported by ABCA4 and define the direction of transport.
Retinoid transport activity of ABCA4 was determined by measuring ATP-dependent transfer of radiolabeled retinal from donor proteoliposomes reconstituted with purified ABCA4 or outer segment disc vesicles of WT and abca4 KO mice to acceptor liposomes. Phospholipid transport activity was determined by measuring ATP-dependent flipping of fluorescent-labeled lipids across the bilayer of proteoliposomes reconstituted with ABCA4. The retinoid substrate for ABCA4 was determined by HPLC, spectrophotometry, and competitive inhibition.
ATP-dependent transfer of retinal from donor to acceptor vesicles was observed when the donor vesicles were either phosphatidylethanolamine(PE)-containing proteoliposomes reconstituted with ABCA4 or disc membrane vesicles from WT mice. No ATP-dependent retinal transfer was observed from donor phosphatidylcholine proteoliposomes or disc membranes from abca4 knockout mice. N-retinyl-PE but not retinol inhibited the retinal transfer activity of ABCA4. The nonprotonated form of N-retinylidene-PE was the principal retinoid substrate for ABCA4. In the absence of retinal, ABCA4 actively flipped PE across the lipid bilayer. BothN-retinylidene-PE and PE were transported from the lumen to the cytoplasmic side of membranes. Several Stargardt mutations severely reduced N-retinylidene-PE and PE transport activity of ABCA4.
ABCA4 functions as an active ‘importer’ of N-retinylidene-PE and PE transporting these substrates from the lumen to the cytopolasmic side of the disc membrane bilayer, a direction that is opposite to the ‘export’ direction observed for all other characterized eukaryotic ABC transporters. These studies support the role of ABCA4 in the removal of retinal from the disc membrane and suggest that ABCA4 can also contribute to the asymmetrical distribution of PE across the disc membrane. Finally, these studies provide further insight into the mechanisms underlying Stargardt disease.
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