Functional analysis of genetic mutations in nucleotide binding domain 2 of the human retina specific ABC transporter

The rod outer segment (ROS) ABC transporter (ABCR) plays an important role in the outer segment of retinal rod cells, where it functions as a transporter of all-trans retinal, most probably as the complex lipid, retinylidene-phosphatidyl-ethanolamine. We report here a quantitative analysis of the structural and functional effects of genetic mutations, associated with several macular degenerations, in the second nucleotide-binding domain of ABCR (NBD2). We have analyzed the ATP binding, kinetics of ATP hydrolysis, and structural changes. The results of these multifaceted analyses were correlated with the disease severity and prognosis. Results presented here demonstrated that, in wild type NBD2, distinct conformational changes accompany nucleotide (ATP and ADP) binding. Upon ATP binding, NBD2 protein changed to a relaxed conformation where tryptophans became more solvent-exposed, while ADP binding reverses this process and leads back to a taut conformation that is also observed with the unbound protein. This sequence of conformational change appears to be important in the energetics of the ATP hydrolysis and may have important structural consequences in the ability of the NBD2 domain to act as a regulator of the nucleotide-binding domain 1. Some of the mutant proteins displayed strikingly different patterns of conformational changes upon nucleotide binding that pointed to unique structural consequences of these genetic mutations. The ABCR dysfunctions, associated with various retinopathies, are multifaceted in nature and include alterations in protein structure as well as the attenuation of ATPase activity and nucleotide binding.