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Molecular basis of transport and regulation in the Na+/betaine symporter BetP

Abstract

Osmoregulated transporters sense intracellular osmotic pressure and respond to hyperosmotic stress by accumulation of osmolytes to restore normal hydration levels. Here we report the determination of the X-ray structure of a member of the family of betaine/choline/carnitine transporters, the Na+-coupled symporter BetP from Corynebacterium glutamicum, which is a highly effective osmoregulated uptake system for glycine betaine. Glycine betaine is bound in a tryptophan box occluded from both sides of the membrane with aromatic side chains lining the transport pathway. BetP has the same overall fold as three unrelated Na+-coupled symporters. Whereas these are crystallized in either the outward-facing or the inward-facing conformation, the BetP structure reveals a unique intermediate conformation in the Na+-coupled transport cycle. The trimeric architecture of BetP and the break in three-fold symmetry by the osmosensing C-terminal helices suggest a regulatory mechanism of Na+-coupled osmolyte transport to counteract osmotic stress.

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Figure 1: Structure of an N-terminally truncated, surface-engineered BetP mutant.
Figure 2: The betaine-binding site.
Figure 3: The proposed sodium binding in BetP.
Figure 4: Conformational changes in Na + -coupled transport.
Figure 5: Trimer architecture of BetP.
Figure 6: Regulatory interactions mediated by the C-terminal domains.

Accession codes

Primary accessions

Protein Data Bank

Data deposits

The coordinates for the structure reported in this work have been deposited in the Protein Data Bank under accession number 2W8A.

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Acknowledgements

The authors thank W. Kühlbrandt and R. Krämer for support and comments on the manuscript; J. Standfuß for contributions in the early stages of the project; Ö. Yildiz, T. Barros and R. Wouts for computational support; S. Schulze, S. Morbach, S. Nicklisch and L. Forrest for discussions; J. Hakulinen and J. Carrera for cloning, C. Perez for the reconstitution and freeze fracture experiments; and H. Volk for help with the figures. Special thanks are due to E. Pohl and the X10SA beamline staff at the Swiss Light Source, as well as the European Synchrotron Radiation Facility. This work is supported by the German Research Foundation, Collaborative Research Centre 807 ‘Transport and Communication across Biological Membranes’.

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Correspondence to Christine Ziegler.

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Ressl, S., Terwisscha van Scheltinga, A., Vonrhein, C. et al. Molecular basis of transport and regulation in the Na+/betaine symporter BetP. Nature 458, 47–52 (2009). https://doi.org/10.1038/nature07819

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