Abstract
Rim protein (RmP) is an ABC transporter of unknown function in rod outer segment discs. The human gene for RmP ( ABCR) is affected in several recessive retinal degenerations. Here, we characterize the ocular phenotype in abcr knockout mice. Mice lacking RmP show delayed dark adaptation, increased all- trans-retinaldehyde (all- trans-RAL) following light exposure, elevated phosphatidylethanolamine (PE) in outer segments, accumulation of the protonated Schiff base complex of all- trans-RAL and PE ( N-retinylidene-PE), and striking deposition of a major lipofuscin fluorophore (A2-E) in retinal pigment epithelium (RPE). These data suggest that RmP functions as an outwardly directed flippase for N-retinylidene-PE. Delayed dark adaptation is likely due to accumulation in discs of the noncovalent complex between opsin and all- trans-RAL. Finally, ABCR-mediated retinal degeneration may result from “poisoning” of the RPE due to A2-E accumulation, with secondary photoreceptor degeneration due to loss of the RPE support role.
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