Retinoid Binding Properties of Nucleotide Binding Domain 1 of the Stargardt Disease-associated ATP Binding Cassette (ABC) Transporter, ABCA4

The retina-specific ATP binding cassette transporter, ABCA4 protein, is associated with a broad range of inherited macular degenerations, including Stargardt disease, autosomal recessive cone rod dystrophy, and fundus flavimaculatus. In order to understand its role in retinal transport in rod out segment discs, we have investigated the interactions of the soluble domains of ABCA4 with both 11-cis- and all-trans-retinal. Using fluorescence anisotropy-based binding analysis and recombinant polypeptides derived from the amino acid sequences of the four soluble domains of ABCA4, we demonstrated that the nucleotide binding domain 1 (NBD1) specifically bound 11-cis-retinal. Its affinity for all-trans-retinal was markedly reduced. Stargardt disease-associated mutations in this domain resulted in attenuation of 11-cis-retinal binding. Significant differences in 11-cis-retinal binding affinities were observed between NBD1 and other cytoplasmic and lumenal domains of ABCA4. The results suggest a possible role of ABCA4 and, in particular, the NBD1 domain in 11-cis-retinal binding. These results also correlate well with a recent report on the in vivo role of ABCA4 in 11-cis-retinal transport. The ABCA4 protein is proposed to transport all-trans-retinal from the outer segment discs of retinal rod and cone photoreceptors. 11-cis-Retinal bound specifically and with high affinity to the NBD1 domain of hABCA4. The NBD1 domain plays further roles in addition to nucleotide hydrolysis. ABCA4 may play a novel role in the visual transduction cycle involving 11-cis-retinal.