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Crystal structure of a bacterial homologue of the bile acid sodium symporter ASBT

Abstract

High cholesterol levels greatly increase the risk of cardiovascular disease. About 50 per cent of cholesterol is eliminated from the body by its conversion into bile acids. However, bile acids released from the bile duct are constantly recycled, being reabsorbed in the intestine by the apical sodium-dependent bile acid transporter (ASBT, also known as SLC10A2). It has been shown in animal models that plasma cholesterol levels are considerably lowered by specific inhibitors of ASBT1,2, and ASBT is thus a target for hypercholesterolaemia drugs. Here we report the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBTNM) at 2.2 Å. ASBTNM contains two inverted structural repeats of five transmembrane helices. A core domain of six helices harbours two sodium ions, and the remaining four helices pack in a row to form a flat, ‘panel’-like domain. Overall, the architecture of the protein is remarkably similar to the sodium/proton antiporter NhaA3, despite having no detectable sequence homology. The ASBTNM structure was captured with the substrate taurocholate present, bound between the core and panel domains in a large, inward-facing, hydrophobic cavity. Residues near this cavity have been shown to affect the binding of specific inhibitors of human ASBT4. The position of the taurocholate molecule, together with the molecular architecture, suggests the rudiments of a possible transport mechanism.

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Figure 1: Sodium-dependent transport of bile acid by ASBTNM.
Figure 2: ASBT NM structure.
Figure 3: ASBT NM structure is inward facing and contains bound sodium and bile acid.
Figure 4: Putative mechanism for ASBT NM transport.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

The coordinates and the structure factors for ASBTNM and ASBTNM_1 have been deposited in the Protein Data Bank under accession numbers 3ZUY and 3ZUX, respectively.

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Acknowledgements

We are grateful to C. Lee and Y. Sekiguchi for assistance with cloning and expression screening of ASBTNM mutants, and to S. van de Graaf for donating fluorescently labelled bile acid, which was used in the initial functional characterization of ASBTNM. Data were collected at the European Synchrotron Radiation Facility, France, and the Diamond Light Source, UK, with assistance from beamline scientists; in particular, we would like to thank J. Sanchez-Weatherby for help with the HCl. We are also grateful to K. Beis and G. von Heijne for reading the manuscript. This work was funded by the Medical Research Council (MRC_G0900990(91997), to A.D.C. and D.D.), the European Union (EMeP grant LSHG-CT-2004-504601, to S.I.) and the Biotechnology and Biological Sciences Research Council (BB/G023425/1, to S.I.). Part of this work was also supported by a grant from the Targeted Proteins Research Program of MEXT, Japan, and the ERATO Iwata Human Receptor Crystallography Project, Japan Science and Technology Agency. The authors are grateful for the use of the Membrane Protein Laboratory funded by the Wellcome Trust (WT089809) at the Diamond Light Source. D.D. acknowledges personal support from The Royal Society through the University Research Fellow scheme.

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Contributions

N.-J.H., S.I., A.D.C. and D.D. contributed to the design of the project. N.-J.H. and D.D. screened homologues, expressed and purified the protein, and carried out functional characterization. N.-J.H., S.I., A.D.C. and D.D. were involved in crystallographic experiments and analysis of data. A.C. and D.D. were responsible for overall project management and wrote the manuscript with assistance from N.-J.H. and S.I.

Corresponding authors

Correspondence to So Iwata, Alexander D. Cameron or David Drew.

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Hu, NJ., Iwata, S., Cameron, A. et al. Crystal structure of a bacterial homologue of the bile acid sodium symporter ASBT. Nature 478, 408–411 (2011). https://doi.org/10.1038/nature10450

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