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Kölsch A, Radon C, Golub M, Baumert A, Bürger J, Mielke T, Lisdat F, Feoktystov A, Pieper J, Zouni A, Wendler P. Current limits of structural biology: The transient interaction between cytochrome c 6 and photosystem I. Curr Res Struct Biol 2020; 2:171-179. [PMID: 34235477 PMCID: PMC8244401 DOI: 10.1016/j.crstbi.2020.08.003] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2020] [Revised: 08/11/2020] [Accepted: 08/17/2020] [Indexed: 12/22/2022] Open
Abstract
Trimeric photosystem I from the cyanobacterium Thermosynechococcus elongatus (TePSI) is an intrinsic membrane protein, which converts solar energy into electrical energy by oxidizing the soluble redox mediator cytochrome c 6 (Cyt c 6 ) and reducing ferredoxin. Here, we use cryo-electron microscopy and small angle neutron scattering (SANS) to characterize the transient binding of Cyt c 6 to TePSI. The structure of TePSI cross-linked to Cyt c 6 was solved at a resolution of 2.9 Å and shows additional cofactors as well as side chain density for 84% of the peptide chain of subunit PsaK, revealing a hydrophobic, membrane intrinsic loop that enables binding of associated proteins. Due to the poor binding specificity, Cyt c 6 could not be localized with certainty in our cryo-EM analysis. SANS measurements confirm that Cyt c 6 does not bind to TePSI at protein concentrations comparable to those for cross-linking. However, SANS data indicate a complex formation between TePSI and the non-native mitochondrial cytochrome from horse heart (Cyt c HH ). Our study pinpoints the difficulty of identifying very small binding partners (less than 5% of the overall size) in EM structures when binding affinities are poor. We relate our results to well resolved co-structures with known binding affinities and recommend confirmatory methods for complexes with K M values higher than 20 μM.
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Affiliation(s)
- A. Kölsch
- Department of Biology, Humboldt–Universität zu Berlin, Philippstrasse 13, 10115, Berlin, Germany
| | - C. Radon
- Institute of Biochemistry and Biology, Department of Biochemistry, University of Potsdam, Karl-Liebknecht Strasse 24-25, 14476, Potsdam-Golm, Germany
| | - M. Golub
- Institute of Physics, University of Tartu, Wilhelm Ostwaldi 1, 50411, Tartu, Estonia
| | - A. Baumert
- Institute of Biochemistry and Biology, Department of Biochemistry, University of Potsdam, Karl-Liebknecht Strasse 24-25, 14476, Potsdam-Golm, Germany
| | - J. Bürger
- Max-Planck Institute for Molecular Genetics, Ihnestrasse 63-73, 14195, Berlin, Germany
- Charité, Institut für Medizinische Physik und Biophysik, Charitéplatz 1, 10117, Berlin, Germany
| | - T. Mielke
- Max-Planck Institute for Molecular Genetics, Ihnestrasse 63-73, 14195, Berlin, Germany
| | - F. Lisdat
- Institute of Applied Life Sciences, Technical University of Applied Sciences Wildau, Hochschulring 1, 15745, Wildau, Germany
| | - A. Feoktystov
- Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Lichtenbergstr. 1, 85748, Garching, Germany
| | - J. Pieper
- Institute of Physics, University of Tartu, Wilhelm Ostwaldi 1, 50411, Tartu, Estonia
| | - A. Zouni
- Department of Biology, Humboldt–Universität zu Berlin, Philippstrasse 13, 10115, Berlin, Germany
| | - P. Wendler
- Institute of Biochemistry and Biology, Department of Biochemistry, University of Potsdam, Karl-Liebknecht Strasse 24-25, 14476, Potsdam-Golm, Germany
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