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CysG structure reveals tetrapyrrole-binding features and novel regulation of siroheme biosynthesis

Nature Structural & Molecular Biology volume 10, pages 1064–1073 (2003)Cite this article

Abstract

Sulfur metabolism depends on the iron-containing porphinoid siroheme. In Salmonella enterica, the S-adenosyl-L-methionine (SAM)-dependent bismethyltransferase, dehydrogenase and ferrochelatase, CysG, synthesizes siroheme from uroporphyrinogen III (uro'gen III). The reactions mediated by CysG encompass two branchpoint intermediates in tetrapyrrole biosynthesis, diverting flux first from protoporphyrin IX biosynthesis and then from cobalamin (vitamin B12) biosynthesis. We determined the first structure of this multifunctional siroheme synthase by X-ray crystallography. CysG is a homodimeric gene fusion product containing two structurally independent modules: a bismethyltransferase and a dual-function dehydrogenase-chelatase. The methyltransferase active site is a deep groove with a hydrophobic patch surrounded by hydrogen bond donors. This asymmetric arrangement of amino acids may be important in directing substrate binding. Notably, our structure shows that CysG is a phosphoprotein. From mutational analysis of the post-translationally modified serine, we suggest a conserved role for phosphorylation in inhibiting dehydrogenase activity and modulating metabolic flux between siroheme and cobalamin pathways.

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Figure 1
Figure 2: Sequence alignments for CysGA and CysGB with homologous proteins.
Figure 3: Overall protein fold and global asymmetry arising from linker differences.
Figure 4: CysGB and Met8p.
Figure 5: A stereo view of the methyltransferase active site from the native CysG structure, which sits deep within a cleft between the domains IA and IIA.
Figure 6: Serine phosphorylation in the dehydrogenase-ferrochelatase active site.

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Acknowledgements

M.E.S. was supported by a predoctoral US National Science Foundation fellowship and the Skaggs Institute for Chemical Biology. This research was funded by a grant from the US National Institutes of Health (NIH) to E.D.G. M.J.W. acknowledges funding from the UK Biotechnology and Biological Research Council and the Wellcome Trust. We thank D. Barondeau, M. Didonato, J. Huffman and C. Kassmann for useful discussions, A. Arvai and E. Garcin for assistance with data collection and M. Pique for help generating Figure 6b,c. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the US Department of Energy (DOE), Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the DOE, Office of Biological and Environmental Research, and by the NIH, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences.

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  1. Department of Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, 92037, California, USA

    M Elizabeth Stroupe, Douglas S Daniels & Elizabeth D Getzoff

  2. School of Biological Sciences, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK

    Helen K Leech & Martin J Warren

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Correspondence to Elizabeth D Getzoff.

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Stroupe, M., Leech, H., Daniels, D. et al. CysG structure reveals tetrapyrrole-binding features and novel regulation of siroheme biosynthesis. Nat Struct Mol Biol 10, 1064–1073 (2003). https://doi.org/10.1038/nsb1007

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