Abstract
ABCB6 plays a crucial role in energy-dependent porphyrin transport, drug resistance, toxic metal resistance, porphyrin biosynthesis, protection against stress, and encoding a blood group system Langereis antigen. However, the mechanism underlying porphyrin transport is still unclear. Here, we determined the cryo-electron microscopy (cryo-EM) structures of nanodisc-reconstituted human ABCB6 trapped in an apo-state and an ATP-bound state at resolutions of 3.6 and 3.5 Å, respectively. Our structures reveal a unique loop in the transmembrane domain (TMD) of ABCB6, which divides the TMD into two cavities. It restrains the access of substrates in the inward-facing state and is removed by ATP-driven conformational change. No ligand cavities were observed in the nucleotide-bound state, indicating a state following substrate release but prior to ATP hydrolysis. Structural analyses and functional characterizations suggest an “ATP-switch” model and further reveal the conformational changes of the substrate-binding pockets triggered by the ATP-driven regulation.
Highlights
ATP-binding cassette (ABC) transporters, which transport a variety of substrates such as nutrients, drugs, and ions by hydrolyzing ATP, are a large class of transmembrane proteins existing extensively in various organisms, ranging from bacteria to human[1,2,3]
The ATP assay of human ABCB6 (hABCB6) in a nanodisc shows a low affinity to ATP (Vmax = 27–32 nmol/min/mg) (Fig. 1b), compared with the reported affinity of ABCB6 reconstructed in liposomes (Vmax = 492 nmol/min/mg)[25]
These results indicate that the hABCB6 nanodisc maintains a relatively stable conformation, suggesting that the lipid environment is important for the ATP activity of hABCB6, and that our recombined nanodisc hABCB6 is suitable for structural studies
Summary
ATP-binding cassette (ABC) transporters, which transport a variety of substrates such as nutrients, drugs, and ions by hydrolyzing ATP, are a large class of transmembrane proteins existing extensively in various organisms, ranging from bacteria to human[1,2,3]. ABC transporters usually mediate the influx of essential nutrients and ions for growth, and the efflux of toxins for self-protection[4,5]. ABC transporters share a canonical structure that contains two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs)[8]. All NBDs contain the characteristic motifs, Walker A and Walker B, which could utilize the energy of ATP to drive transmembrane transportation, while TMDs. ABCB6 is an 842-amino acid protein that belongs to the.
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