Molecular mechanism of substrate transport by human peroxisomal ABCD3
Peroxisomes are eukaryotic oxidative organelles involved in numerous metabolic functions that include fatty acid oxidation, bile acid synthesis, and detoxification of reactive oxygen species. ATP-binding cassette transporters of the D subfamily (ABCD1-3) mediate the import of CoA thioesters of fatty acids into the peroxisome. ABCD3, the most abundant of these transporters in the peroxisomal membrane, facilitates the transport of a broad spectrum of substrates including branched-chain fatty acids, very long-chain fatty acids, bile salt intermediates, and dicarboxylic acids. Mutations in ABCD3 are associated with defects in congenital bile acid synthesis and variants of Zellweger syndrome. The structural and functional details of the human ABCD3 transporter remain unclear, despite its significance. In this study, we report the cryogenic sample electron microscopy (cryo-EM) structures of full-length human ABCD3 in its apo state and bound to one of the physiological substrates (phytanoyl-CoA) at resolutions of 3.33 [A] and 3.13 [A], respectively. Our biochemical assays reveal that substrate binding induces ATPase activity in ABCD3, suggesting a substrate-dependent conformational change. Structural comparison of the apo and substrate bound states demonstrate that the substrate interaction brings nucleotide-binding domains closer, providing a mechanistic basis of substrate induced ATPase activity. These findings offer critical insights into the transport mechanism of ABCD3 and lay a structural foundation for understanding its role in peroxisomal metabolite import and related diseases. Significance StatementPeroxisomes are involved in essential cellular metabolic processes that include fatty acid oxidation, bile acid synthesis, and detoxification. The ABCD subfamily of membrane transporter proteins transport fatty acyl-CoA molecules from the cytosol into peroxisomes. Among the ABCD subfamily members, ABCD3 transports branched chain fatty acids and a range of other important metabolites. Dysfunction in ABCD3 has been associated with several inherited and acquired diseases. Using cryo-EM, we determined high-resolution structures of full-length human ABCD3 in both unbound and phytanoyl-CoA-bound forms. We show that substrate binding stimulates ATP hydrolysis activity by apposition of the two nucleotide binding domains of ABCD3, adding structural insight into the mechanism of action of ABCD3 and its role in peroxisomal function and disease.