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Molecular mechanism of vectorial proton translocation by bacteriorhodopsin

Nature volume 406, pages 653–657 (2000)Cite this article

Abstract

Bacteriorhodopsin, a membrane protein with a relative molecular mass of 27,000, is a light driven pump which transports protons across the cell membrane of the halophilic organism Halobacterium salinarum. The chromophore retinal is covalently attached to the protein via a protonated Schiff base. Upon illumination, retinal is isomerized. The Schiff base then releases a proton to the extracellular medium, and is subsequently reprotonated from the cytoplasm. An atomic model for bacteriorhodopsin was first determined by Henderson et al1, and has been confirmed and extended by work in a number of laboratories in the last few years2. Here we present an atomic model for structural changes involved in the vectorial, light-driven transport of protons by bacteriorhodopsin. A ‘switch’ mechanism ensures the vectorial nature of pumping. First, retinal unbends, triggered by loss of the Schiff base proton, and second, a protein conformational change occurs. This conformational change, which we have determined by electron crystallography at atomic (3.2 Å in-plane and 3.6 Å vertical) resolution, is largely localized to helices F and G, and provides an ‘opening’ of the protein to protons on the cytoplasmic side of the membrane.

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Figure 1: Three-dimensional difference density map showing structural changes in the D96G, F171C, F219L triple mutant compared with the unilluminated native wild-type bacteriorhodopsin.
Figure 2: Comparison of atomic models for wild-type bacteriorhodopsin and the D96G, F171C, F219L triple mutant.
Figure 3: Representation of van der Waals surfaces at the entrance of the cytoplasmic channel in unilluminated wild-type bacteriorhodopsin and the D96G, F171C, F219L triple mutant.
Figure 4: Analysis of the nature of the protein conformational change using projection Fourier maps.
Figure 5: Observed conformations of retinal derivatives and proposed displacement of Schiff base on deprotonation.

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Authors and Affiliations

  1. MRC Laboratory of Molecular Biology , Cambridge, CB2 2QH, UK

    Sriram Subramaniam & Richard Henderson

  2. Laboratory of Biochemistry, National Cancer Institute, Bethesda, 20892 , Maryland, USA

    Sriram Subramaniam

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  1. Sriram Subramaniam
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  2. Richard Henderson
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Correspondence to Sriram Subramaniam.

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Subramaniam, S., Henderson, R. Molecular mechanism of vectorial proton translocation by bacteriorhodopsin . Nature 406, 653–657 (2000). https://doi.org/10.1038/35020614

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