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

Hu NJ, Iwata S, Cameron AD, Drew D - Nature (2011)

Bottom Line: The ASBT(NM) 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 ASBT.The position of the taurocholate molecule, together with the molecular architecture, suggests the rudiments of a possible transport mechanism.

View Article: PubMed Central - PubMed

Affiliation: Division of Molecular Biosciences, Imperial College London, London SW7 2AZ, UK.

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 ASBT, and ASBT is thus a target for hypercholesterolaemia drugs. Here we report the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBT(NM)) at 2.2 Å. ASBT(NM) 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 NhaA, despite having no detectable sequence homology. The ASBT(NM) 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 ASBT. The position of the taurocholate molecule, together with the molecular architecture, suggests the rudiments of a possible transport mechanism.

Show MeSH

Related in: MedlinePlus

Putative mechanism for ASBTNM transporta, Superposition of ASBTNM (red Panel, blue Core) and the outward-facing model as described in the text (light grey). The superposition has been optimized on the Core domains. Loops have been removed for clarity. In the image on the right the Panel of the model has been rotated 25° relative to the Core domain, around the axis shown in the left image, to superimpose the Panels. Significant kinks in the helices are represented as breaks. The area of the cavity is depicted by a salmon trapezoid. b, NhaA shown in the same view as ASBTNM in a. The Core domain is shown in light blue and the Panel in brown. The two additional TMs and β-strands that are not present in ASBTNM are shown in grey. The position that sodium is thought to bind3 is shown with a black ring. c, Schematic of the proposed mechanism that illustrates the movement of the Panel against the Core domain to transport sodium and bile acid.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC3198845&req=5

Figure 4: Putative mechanism for ASBTNM transporta, Superposition of ASBTNM (red Panel, blue Core) and the outward-facing model as described in the text (light grey). The superposition has been optimized on the Core domains. Loops have been removed for clarity. In the image on the right the Panel of the model has been rotated 25° relative to the Core domain, around the axis shown in the left image, to superimpose the Panels. Significant kinks in the helices are represented as breaks. The area of the cavity is depicted by a salmon trapezoid. b, NhaA shown in the same view as ASBTNM in a. The Core domain is shown in light blue and the Panel in brown. The two additional TMs and β-strands that are not present in ASBTNM are shown in grey. The position that sodium is thought to bind3 is shown with a black ring. c, Schematic of the proposed mechanism that illustrates the movement of the Panel against the Core domain to transport sodium and bile acid.

Mentions: For transport to take place the protein must switch between outward and inward facing states25. The architecture of ASBTNM provides a clue to understanding how this might occur. The sodium ions are located in the Core domain close to the crossover points of the discontinuous helices and occluded from the bulk solvent. In NhaA sodium binding causes a rearrangement of these helices26,27. In ASBTNM similar rearrangements in the Core domain are therefore likely. Since NhaA only translocates ions26 these TM movements might be sufficient for transport. However, because ASBTNM transports much larger substrates, structural movements in more than the Core domain are needed. For the sodium-coupled transporter LeuT, Forrest et al used the internal asymmetry of the repeating motifs to predict global movements from a single structure28; which have been substantiated by crystallographic studies29. In an analogous manner to LeuT, an outward-facing model of ASBTNM was generated by superimposing TMs 1-5 on TMs 6-10 and vice versa (Fig. 4a and see Methods). Comparing the inward-facing ASBTNM structure with the outward-facing model, the largest difference is the position of the Panel relative to the Core domain (Fig 4c). A route through the protein between these domains is in agreement with experimental data, that suggest that the last helix of ASBT and TM9 of NhaA line the transport pathway3,24,26,30. Interestingly, the NhaA domain equivalent to the Panel is placed between that of the outward-facing and inward-facing ASBTNM states (Fig. 4b). This may either be because NhaA translocates a much smaller substrate, or it could represent another conformation of the transporter, likely an occluded state.


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

Hu NJ, Iwata S, Cameron AD, Drew D - Nature (2011)

Putative mechanism for ASBTNM transporta, Superposition of ASBTNM (red Panel, blue Core) and the outward-facing model as described in the text (light grey). The superposition has been optimized on the Core domains. Loops have been removed for clarity. In the image on the right the Panel of the model has been rotated 25° relative to the Core domain, around the axis shown in the left image, to superimpose the Panels. Significant kinks in the helices are represented as breaks. The area of the cavity is depicted by a salmon trapezoid. b, NhaA shown in the same view as ASBTNM in a. The Core domain is shown in light blue and the Panel in brown. The two additional TMs and β-strands that are not present in ASBTNM are shown in grey. The position that sodium is thought to bind3 is shown with a black ring. c, Schematic of the proposed mechanism that illustrates the movement of the Panel against the Core domain to transport sodium and bile acid.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3198845&req=5

Figure 4: Putative mechanism for ASBTNM transporta, Superposition of ASBTNM (red Panel, blue Core) and the outward-facing model as described in the text (light grey). The superposition has been optimized on the Core domains. Loops have been removed for clarity. In the image on the right the Panel of the model has been rotated 25° relative to the Core domain, around the axis shown in the left image, to superimpose the Panels. Significant kinks in the helices are represented as breaks. The area of the cavity is depicted by a salmon trapezoid. b, NhaA shown in the same view as ASBTNM in a. The Core domain is shown in light blue and the Panel in brown. The two additional TMs and β-strands that are not present in ASBTNM are shown in grey. The position that sodium is thought to bind3 is shown with a black ring. c, Schematic of the proposed mechanism that illustrates the movement of the Panel against the Core domain to transport sodium and bile acid.
Mentions: For transport to take place the protein must switch between outward and inward facing states25. The architecture of ASBTNM provides a clue to understanding how this might occur. The sodium ions are located in the Core domain close to the crossover points of the discontinuous helices and occluded from the bulk solvent. In NhaA sodium binding causes a rearrangement of these helices26,27. In ASBTNM similar rearrangements in the Core domain are therefore likely. Since NhaA only translocates ions26 these TM movements might be sufficient for transport. However, because ASBTNM transports much larger substrates, structural movements in more than the Core domain are needed. For the sodium-coupled transporter LeuT, Forrest et al used the internal asymmetry of the repeating motifs to predict global movements from a single structure28; which have been substantiated by crystallographic studies29. In an analogous manner to LeuT, an outward-facing model of ASBTNM was generated by superimposing TMs 1-5 on TMs 6-10 and vice versa (Fig. 4a and see Methods). Comparing the inward-facing ASBTNM structure with the outward-facing model, the largest difference is the position of the Panel relative to the Core domain (Fig 4c). A route through the protein between these domains is in agreement with experimental data, that suggest that the last helix of ASBT and TM9 of NhaA line the transport pathway3,24,26,30. Interestingly, the NhaA domain equivalent to the Panel is placed between that of the outward-facing and inward-facing ASBTNM states (Fig. 4b). This may either be because NhaA translocates a much smaller substrate, or it could represent another conformation of the transporter, likely an occluded state.

Bottom Line: The ASBT(NM) 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 ASBT.The position of the taurocholate molecule, together with the molecular architecture, suggests the rudiments of a possible transport mechanism.

View Article: PubMed Central - PubMed

Affiliation: Division of Molecular Biosciences, Imperial College London, London SW7 2AZ, UK.

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 ASBT, and ASBT is thus a target for hypercholesterolaemia drugs. Here we report the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBT(NM)) at 2.2 Å. ASBT(NM) 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 NhaA, despite having no detectable sequence homology. The ASBT(NM) 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 ASBT. The position of the taurocholate molecule, together with the molecular architecture, suggests the rudiments of a possible transport mechanism.

Show MeSH
Related in: MedlinePlus