Structural basis of substrate recognition and translocation by human ABCD1

Human ATP-binding cassette (ABC) transporter ABCD1 transports CoA esters of saturated/monounsaturated very long chain fatty acid from cytosol to the peroxisome for β-oxidation. Dysfunction of human ABCD1 usually causes the severe progressive genetic disorder X-linked adrenoleukodystrophy, which eventually affects the adrenal glands and/or the central nervous system. Here, we report three cryo-EM structures of human ABCD1 in various states. The apo-form ABCD1 at 3.53 Å resolution adopts an inward-facing conformation, harboring a phosphatidyl ethanolamine (PE) molecule at each lateral entry of substrate cavity. In the substrate-bound ABCD1 structure at 3.59 Å resolution, two molecules of C22:0-CoA (one of the physiological substrates of ABCD1) is symmetrically bound to the transmembrane domains (TMDs). Each C22:0-CoA adopts an unpresented L-shape configuration: the CoA portion inserts into a polar pocket at the TMD at a pose parallel to the membrane plane, whereas the acyl chain portion perpendicular to membrane plane is embedded in a hydrophobic pocket at the opposite TMD. Upon binding the two C22:0-CoA molecules, which resemble a pair of hinges crossing the two TMDs, the two nucleotide-binding domains (NBDs) of ABCD1 approach towards each other. Addition ATP to the substrate-bound ABCD1 enabled us to reveal an ATP-bound structure at 2.79 Å, which shows an outward-facing conformation with the dimerized NBDs succeeding substrate release. These three structures combined with biochemical assays exhibit a snapshot of ABCD1-mediated substrate recognition, translocation and release. These findings provide the structural insights into the transport mechanism of ABC transporters that transport amphipathic molecules with long acyl chains.