Letter | Published:

Structural mechanism for sterol sensing and transport by OSBP-related proteins

Abstract

The oxysterol-binding-protein (OSBP)-related proteins (ORPs) are conserved from yeast to humans1,2, and are implicated in the regulation of sterol homeostasis3,4 and in signal transduction pathways5. Here we report the structure of the full-length yeast ORP Osh4 (also known as Kes1) at 1.5–1.9 Å resolution in complexes with ergosterol, cholesterol, and 7-, 20- and 25-hydroxycholesterol. We find that a single sterol molecule binds within a hydrophobic tunnel in a manner consistent with a transport function for ORPs. The entrance is blocked by a flexible amino-terminal lid and surrounded by basic residues that are critical for Osh4 function. The structure of the open state of a lid-truncated form of Osh4 was determined at 2.5 Å resolution. Structural analysis and limited proteolysis show that sterol binding closes the lid and stabilizes a conformation favouring transport across aqueous barriers and signal transmission. The structure of Osh4 in the absence of ligand exposes potential phospholipid-binding sites that are positioned for membrane docking and sterol exchange. On the basis of these observations, we propose a model in which sterol and membrane binding promote reciprocal conformational changes that facilitate a sterol transfer and signalling cycle.

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Acknowledgements

We thank T. Levine, V. Bankaitis and M. Brown for discussions and sharing unpublished data, N. DeAngelis for technical assistance, R. Craigie for assistance with protein sequencing, J. Kim for advice on the early stages of this project, G. Miller and H. Shi for collecting synchrotron data, C. Beh and R. Scheckman for yeast strains, F. Dyda for maintaining the home X-ray facility, and the staff of beamline X25, National Synchrotron Light Source, Brookhaven National Laboratory and of SER-CAT, Advanced Photon Source, Argonne National Laboratory for assistance with data collection. This research was supported by the intramural program of the NIDDK. Y.J.I. thanks S. H. Eom for mentoring and support. Y.J.I. was partly supported by the Korea Science and Engineering Foundation. Research carried out at the National Synchrotron Light Source is supported by the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Science.

Author information

Correspondence to James H. Hurley.

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Competing interests

Coordinates have been deposited with the Protein Data Bank with accession numbers 1ZHT, 1ZHW, 1ZHX, 1ZHY, 1ZHZ and 1ZI7. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Notes

Word file containing Supplementary Methods, additional references, and Supplementary Figure Legends. (DOC 39 kb)

Supplementary Table S1

Statistics of data collection, MIR phasing, and crystallographic refinement. (DOC 40 kb)

Supplementary Figure S1

Experimental electron density. (PDF 453 kb)

Supplementary Figure S2*

Topology of the ORD fold. *Supplementary Fig. 2 of this Letter was substituted on 07/09/05 for an inadvertently duplicated version of Fig. S3. (PDF 132 kb)

Supplementary Figure S3

Comparison to bacterial outer membrane transporters. (PDF 267 kb)

Supplementary Figure S4a

Structure-based alignment of ORDs. (PDF 659 kb)

Supplementary Figure S4b

Structure-based alignment of ORDs. (PDF 388 kb)

Supplementary Figure S5

25-hydroxycholesterol binding to Osh4. (PDF 47 kb)

Supplementary Figure S6

Quantitative immunoblotting. (PDF 71 kb)

Supplementary Figure S7

Sterol-dependent changes in lid conformation. (PDF 73 kb)

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Further reading

Figure 1: Structure of Osh4.
Figure 2: Mutational analysis of Osh4 binding and function.
Figure 3: Conformational changes in Osh4.
Figure 4: Mechanism of sterol transfer.

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