1
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Lindemeier D, Graubner W, Mehner-Breitfeld D, Malešević M, Brüser T. Positive charges promote the recognition of proteins by the chaperone SlyD from Escherichia coli. PLoS One 2024; 19:e0305823. [PMID: 38917203 PMCID: PMC11198818 DOI: 10.1371/journal.pone.0305823] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2023] [Accepted: 06/04/2024] [Indexed: 06/27/2024] Open
Abstract
SlyD is a widely-occurring prokaryotic FKBP-family prolyl isomerase with an additional chaperone domain. Often, such as in Escherichia coli, a third domain is found at its C-terminus that binds nickel and provides it for nickel-enzyme biogenesis. SlyD has been found to bind signal peptides of proteins that are translocated by the Tat pathway, a system for the transport of folded proteins across membranes. Using peptide arrays to analyze these signal peptide interactions, we found that SlyD interacted only with positively charged peptides, with a preference for arginines over lysines, and large hydrophobic residues enhanced binding. Especially a twin-arginine motif was recognized, a pair of highly conserved arginines adjacent to a stretch of hydrophobic residues. Using isothermal titration calorimetry (ITC) with purified SlyD and a signal peptide-containing model Tat substrate, we could show that the wild type twin-arginine signal peptide was bound with higher affinity than an RR>KK mutated variant, confirming that positive charges are recognized by SlyD, with a preference of arginines over lysines. The specific role of negative charges of the chaperone domain surface and of hydrophobic residues in the chaperone active site was further analyzed by ITC of mutated SlyD variants. Our data show that the supposed key hydrophobic residues of the active site are indeed crucial for binding, and that binding is influenced by negative charges on the chaperone domain. Recognition of positive charges is likely achieved by a large negatively charged surface region of the chaperone domain, which is highly conserved although individual positions are variable.
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Affiliation(s)
- Daniel Lindemeier
- Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany
| | - Wenke Graubner
- Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany
| | | | - Miroslav Malešević
- Institute of Biochemistry and Biotechnology, Martin-Luther-University Halle-Wittenberg, Halle, Germany
| | - Thomas Brüser
- Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany
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2
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Sebastiampillai S, Lacasse MJ, McCusker S, Campbell T, Nitz M, Zamble DB. Using a high-throughput, whole-cell hydrogenase assay to identify potential small molecule inhibitors of [NiFe]-hydrogenase. Metallomics 2022; 14:6747159. [DOI: 10.1093/mtomcs/mfac073] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2021] [Accepted: 08/29/2022] [Indexed: 11/06/2022]
Abstract
Abstract
[NiFe]-hydrogenases are used by several human pathogens to catalyze the reversible conversion between molecular hydrogen and protons and electrons. Hydrogenases provide an increased metabolic flexibility for pathogens, such as Escherichia coli and Helicobacter pylori, by allowing the use of molecular hydrogen as an energy source to promote survival in anaerobic environments. With the rise of antimicrobial resistance and the desire for novel therapeutics, the [NiFe]-hydrogenases are alluring targets. Inhibiting the nickel insertion pathway of [NiFe]-hydrogenases is attractive as this pathway is required for the generation of functional enzymes and is orthogonal to human biochemistry. In this work, nickel availability for the production and function of E. coli [NiFe]-hydrogenase was explored through immunoblot and activity assays. Whole-cell hydrogenase activities were assayed in high throughput against a small molecule library of known bioactives. Iodoquinol was identified as a potential inhibitor of the nickel biosynthetic pathway of [NiFe]-hydrogenase through a two-step screening process, but further studies with immunoblot assays showed confounding effects dependent on the cell growth phase. This study highlights the significance of considering the growth phenotype for whole-cell based assays overall and its effects on various cellular processes influenced by metal trafficking and homeostasis.
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Affiliation(s)
| | - Michael J Lacasse
- Department of Chemistry, University of Toronto , Toronto, Ontario M5S 3H6 , Canada
| | - Susan McCusker
- Centre for Microbial Chemical Biology , MDCL-2330, Hamilton, Ontario L8S 4K1 , Canada
| | - Tracey Campbell
- Centre for Microbial Chemical Biology , MDCL-2330, Hamilton, Ontario L8S 4K1 , Canada
| | - Mark Nitz
- Department of Chemistry, University of Toronto , Toronto, Ontario M5S 3H6 , Canada
| | - Deborah B Zamble
- Department of Chemistry, University of Toronto , Toronto, Ontario M5S 3H6 , Canada
- Department of Biochemistry, University of Toronto , Toronto, Ontario M5S 1A8 , Canada
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3
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Foster AW, Clough SE, Aki Z, Young TR, Clarke AR, Robinson NJ. Metalation calculators for E. coli strain JM109 (DE3): Aerobic, anaerobic and hydrogen peroxide exposed cells cultured in LB media. Metallomics 2022; 14:6657815. [PMID: 35933161 PMCID: PMC9434800 DOI: 10.1093/mtomcs/mfac058] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2022] [Accepted: 07/24/2022] [Indexed: 11/14/2022]
Abstract
Three web-based calculators, and three analogous spreadsheets, have been generated that predict in vivo metal occupancies of proteins based on known metal affinities. The calculations exploit estimates of the availabilities of the labile buffered pools of different metals inside a cell. Here, metal availabilities have been estimated for a strain of E. coli that is commonly used in molecular biology and biochemistry research, for example in the production of recombinant proteins. Metal availabilities have been examined for cells grown in LB medium aerobically, anaerobically and in response to H2O2 by monitoring the abundance of a selected set of metal-responsive transcripts by qPCR. The selected genes are regulated by DNA-binding metal sensors that have been thermodynamically characterised in related bacterial cells enabling gene expression to be read-out as a function of intracellular metal availabilities expressed as free energies for forming metal complexes. The calculators compare these values with the free energies for forming complexes with the protein of interest, derived from metal affinities, to estimate how effectively the protein can compete with exchangeable binding sites in the intracellular milieu. The calculators then inter-compete the different metals, limiting total occupancy of the site to a maximum stoichiometry of 1, to output percentage occupancies with each metal. In addition to making these new and conditional calculators available, an original purpose of this article was to provide a tutorial which discusses constraints of this approach and presents ways in which such calculators might be exploited in basic and applied research, and in next-generation manufacturing.
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Affiliation(s)
- Andrew W Foster
- Department of Biosciences, Durham University, Durham, UK.,Department of Chemistry, Durham University, Durham, UK
| | - Sophie E Clough
- Department of Biosciences, Durham University, Durham, UK.,Department of Chemistry, Durham University, Durham, UK
| | - Zeynep Aki
- Advanced Research Computing, Durham University, Durham, UK
| | - Tessa R Young
- Department of Biosciences, Durham University, Durham, UK.,Department of Chemistry, Durham University, Durham, UK
| | | | - Nigel J Robinson
- Department of Biosciences, Durham University, Durham, UK.,Department of Chemistry, Durham University, Durham, UK
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4
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Hecel A, Kola A, Valensin D, Kozlowski H, Rowinska-Zyrek M. Metal specificity of the Ni(II) and Zn(II) binding sites of the N-terminal and G-domain of E. coli HypB. Dalton Trans 2021; 50:12635-12647. [PMID: 34545874 DOI: 10.1039/d1dt02126e] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
HypB is one of the chaperones required for proper nickel insertion into [NiFe]-hydrogenase. Escherichia coli HypB has two potential Ni(II) and Zn(II) binding sites-the N-terminal one and the so-called GTPase one. The metal-loaded HypB-SlyD metallochaperone complex activates nickel release from the N-terminal HypB site. In this work, we focus on the metal selectivity of the two HypB metal binding sites and show that (i) the N-terminal region binds Zn(II) and Ni(II) ions with higher affinity than the G-domain and (ii) the lower affinity G domain binds Zn(II) more effectively than Ni(II). In addition, the high affinity N-terminal domain, both in water and membrane mimicking SDS solution, has a larger affinity towards Zn(II) than Ni(II), while an opposite situation is observed at basic pH; at pH 7.4, the affinity of this region towards both metals is almost the same. The N-terminal HypB region is also more effective in Ni(II) binding than the previously studied SlyD metal binding regions. Considering that the nickel chaperone SlyD activates the release of nickel and blocks the release of zinc from the N-terminal high-affinity metal site of HypB, we may speculate that such pH-dependent metal affinity might modulate HypB interactions with SlyD, being dependent on both pH and the protein's metal status.
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Affiliation(s)
- Aleksandra Hecel
- Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland.
| | - Arian Kola
- Department of Biotechnology, Chemistry and Pharmacy, University of Siena, Via A. Moro 2, 53100 Siena, Italy
| | - Daniela Valensin
- Department of Biotechnology, Chemistry and Pharmacy, University of Siena, Via A. Moro 2, 53100 Siena, Italy
| | - Henryk Kozlowski
- Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland. .,Institute of Health Sciences, University of Opole, 68 Katowicka St., 45-060 Opole, Poland
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5
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Principles and practice of determining metal-protein affinities. Biochem J 2021; 478:1085-1116. [PMID: 33710331 PMCID: PMC7959690 DOI: 10.1042/bcj20200838] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2020] [Revised: 02/05/2021] [Accepted: 02/11/2021] [Indexed: 01/02/2023]
Abstract
Metal ions play many critical roles in biology, as structural and catalytic cofactors, and as cell regulatory and signalling elements. The metal–protein affinity, expressed conveniently by the metal dissociation constant, KD, describes the thermodynamic strength of a metal–protein interaction and is a key parameter that can be used, for example, to understand how proteins may acquire metals in a cell and to identify dynamic elements (e.g. cofactor binding, changing metal availabilities) which regulate protein metalation in vivo. Here, we outline the fundamental principles and practical considerations that are key to the reliable quantification of metal–protein affinities. We review a selection of spectroscopic probes which can be used to determine protein affinities for essential biological transition metals (including Mn(II), Fe(II), Co(II), Ni(II), Cu(I), Cu(II) and Zn(II)) and, using selected examples, demonstrate how rational probe selection combined with prudent experimental design can be applied to determine accurate KD values.
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6
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Alfano M, Cavazza C. Structure, function, and biosynthesis of nickel-dependent enzymes. Protein Sci 2020; 29:1071-1089. [PMID: 32022353 DOI: 10.1002/pro.3836] [Citation(s) in RCA: 66] [Impact Index Per Article: 16.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2020] [Revised: 01/23/2020] [Accepted: 01/23/2020] [Indexed: 12/17/2022]
Abstract
Nickel enzymes, present in archaea, bacteria, plants, and primitive eukaryotes are divided into redox and nonredox enzymes and play key functions in diverse metabolic processes, such as energy metabolism and virulence. They catalyze various reactions by using active sites of diverse complexities, such as mononuclear nickel in Ni-superoxide dismutase, glyoxylase I and acireductone dioxygenase, dinuclear nickel in urease, heteronuclear metalloclusters in [NiFe]-carbon monoxide dehydrogenase, acetyl-CoA decarbonylase/synthase and [NiFe]-hydrogenase, and even more complex cofactors in methyl-CoM reductase and lactate racemase. The presence of metalloenzymes in a cell necessitates a tight regulation of metal homeostasis, in order to maintain the appropriate intracellular concentration of nickel while avoiding its toxicity. As well, the biosynthesis and insertion of nickel active sites often require specific and elaborated maturation pathways, allowing the correct metal to be delivered and incorporated into the target enzyme. In this review, the phylogenetic distribution of nickel enzymes will be briefly described. Their tridimensional structures as well as the complexity of their active sites will be discussed. In view of the latest findings on these enzymes, a special focus will be put on the biosynthesis of their active sites and nickel activation of apo-enzymes.
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Affiliation(s)
- Marila Alfano
- University of Grenoble Alpes, CEA, CNRS, IRIG, CBM, Grenoble, France
| | - Christine Cavazza
- University of Grenoble Alpes, CEA, CNRS, IRIG, CBM, Grenoble, France
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7
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Anderl A, Kolmar H, Fuchsbauer HL. The metal-binding properties of the long chaplin from Streptomyces mobaraensis: A bioinformatic and biochemical approach. J Inorg Biochem 2019; 202:110878. [PMID: 31698184 DOI: 10.1016/j.jinorgbio.2019.110878] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2019] [Revised: 09/05/2019] [Accepted: 10/02/2019] [Indexed: 02/07/2023]
Abstract
Chaplins are amphiphilic proteins coating the surface of aerial hyphae under formation of amyloid-like rodlet layers in streptomycetes. The long chaplin from Streptomyces mobaraensis, Sm-Chp1, harbors extended histidine-rich stretches allowing protein attachment to metal affinity resins. A comprehensive BLASTP search revealed similarity with many putative metal-binding proteins but the deduced sequence motifs were not shared by histidine-rich domains of well-studied proteins. Biochemical analyses showed affinity of Sm-Chp1 for Ni2+, Cu2+ and Zn2+, a binding capacity of 7-8 metal ions, and dissociation constants in a double digit micromolar range. The occurrence of genes for membrane-bound metal transporters and several intra- and extracellular metalloenzymes in the genome of S. mobaraensis suggests that Sm-Chp1 may be a novel type of translocase shifting metals across the rodlet layer from the environment into the cell wall.
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Affiliation(s)
- Anita Anderl
- Department of Chemical Engineering and Biotechnology, University of Applied Sciences of Darmstadt, Stephanstraße 7, 64295 Darmstadt, Germany; Department of Chemistry, Technische Universität Darmstadt, Alarich-Weiß-Straße 12, 64287 Darmstadt, Germany
| | - Harald Kolmar
- Department of Chemistry, Technische Universität Darmstadt, Alarich-Weiß-Straße 12, 64287 Darmstadt, Germany
| | - Hans-Lothar Fuchsbauer
- Department of Chemical Engineering and Biotechnology, University of Applied Sciences of Darmstadt, Stephanstraße 7, 64295 Darmstadt, Germany.
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8
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Lacasse MJ, Summers KL, Khorasani-Motlagh M, George GN, Zamble DB. Bimodal Nickel-Binding Site on Escherichia coli [NiFe]-Hydrogenase Metallochaperone HypA. Inorg Chem 2019; 58:13604-13618. [PMID: 31273981 DOI: 10.1021/acs.inorgchem.9b00897] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
[NiFe]-hydrogenase enzymes catalyze the reversible oxidation of hydrogen at a bimetallic cluster and are used by bacteria and archaea for anaerobic growth and pathogenesis. Maturation of the [NiFe]-hydrogenase requires several accessory proteins to assemble and insert the components of the active site. The penultimate maturation step is the delivery of nickel to a primed hydrogenase enzyme precursor protein, a process that is accomplished by two nickel metallochaperones, the accessory protein HypA and the GTPase HypB. Recent work demonstrated that nickel is rapidly transferred to HypA from GDP-loaded HypB within the context of a protein complex in a nickel selective and unidirectional process. To investigate the mechanism of metal transfer, we examined the allosteric effects of nucleotide cofactors and partner proteins on the nickel environments of HypA and HypB by using a combination of biochemical, microbiological, computational, and spectroscopic techniques. We observed that loading HypB with either GDP or a nonhydrolyzable GTP analogue resulted in a similar nickel environment. In addition, interaction with a mutant version of HypA with disrupted nickel binding, H2Q-HypA, does not induce substantial changes to the HypB G-domain nickel site. Instead, the results demonstrate that HypB modifies the acceptor site of HypA. Analysis of a peptide maquette derived from the N-terminus of HypA revealed that nickel is predominately coordinated by atoms from the N-terminal Met-His motif. Furthermore, HypA is capable of two nickel-binding modes at the N-terminus, a HypB-induced mode and a binding mode that mirrors the peptide maquette. Collectively, these results reveal that HypB brings about changes in the nickel coordination of HypA, providing a mechanism for the HypB-dependent control of the acquisition and release of nickel by HypA.
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Affiliation(s)
- Michael J Lacasse
- Department of Chemistry , University of Toronto , Toronto , Ontario M5S 3H6 , Canada
| | - Kelly L Summers
- Department of Chemistry , University of Saskatchewan , Saskatoon , Saskatchewan S7N 5C9 , Canada
| | | | - Graham N George
- Department of Geological Sciences , University of Saskatchewan , Saskatoon , Saskatchewan S7N 5E2 , Canada
| | - Deborah B Zamble
- Department of Chemistry , University of Toronto , Toronto , Ontario M5S 3H6 , Canada.,Department of Biochemistry , University of Toronto , Toronto , Ontario M5S 1A8 , Canada
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9
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Alfano M, Pérard J, Carpentier P, Basset C, Zambelli B, Timm J, Crouzy S, Ciurli S, Cavazza C. The carbon monoxide dehydrogenase accessory protein CooJ is a histidine-rich multidomain dimer containing an unexpected Ni(II)-binding site. J Biol Chem 2019; 294:7601-7614. [PMID: 30858174 DOI: 10.1074/jbc.ra119.008011] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2019] [Revised: 03/09/2019] [Indexed: 01/14/2023] Open
Abstract
Activation of nickel enzymes requires specific accessory proteins organized in multiprotein complexes controlling metal transfer to the active site. Histidine-rich clusters are generally present in at least one of the metallochaperones involved in nickel delivery. The maturation of carbon monoxide dehydrogenase in the proteobacterium Rhodospirillum rubrum requires three accessory proteins, CooC, CooT, and CooJ, dedicated to nickel insertion into the active site, a distorted [NiFe3S4] cluster coordinated to an iron site. Previously, CooJ from R. rubrum (RrCooJ) has been described as a nickel chaperone with 16 histidines and 2 cysteines at its C terminus. Here, the X-ray structure of a truncated version of RrCooJ, combined with small-angle X-ray scattering data and a modeling study of the full-length protein, revealed a homodimer comprising a coiled coil with two independent and highly flexible His tails. Using isothermal calorimetry, we characterized several metal-binding sites (four per dimer) involving the His-rich motifs and having similar metal affinity (KD = 1.6 μm). Remarkably, biophysical approaches, site-directed mutagenesis, and X-ray crystallography uncovered an additional nickel-binding site at the dimer interface, which binds Ni(II) with an affinity of 380 nm Although RrCooJ was initially thought to be a unique protein, a proteome database search identified at least 46 bacterial CooJ homologs. These homologs all possess two spatially separated nickel-binding motifs: a variable C-terminal histidine tail and a strictly conserved H(W/F)X 2HX 3H motif, identified in this study, suggesting a dual function for CooJ both as a nickel chaperone and as a nickel storage protein.
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Affiliation(s)
- Marila Alfano
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Julien Pérard
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Philippe Carpentier
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Christian Basset
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Barbara Zambelli
- the Laboratory of Bioinorganic Chemistry, Department of Pharmacy and Biotechnology, University of Bologna, I-40127 Bologna, Italy
| | - Jennifer Timm
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Serge Crouzy
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
| | - Stefano Ciurli
- the Laboratory of Bioinorganic Chemistry, Department of Pharmacy and Biotechnology, University of Bologna, I-40127 Bologna, Italy
| | - Christine Cavazza
- From the Laboratory of Chemistry and Biology of Metals, Université Grenoble Alpes, CEA, CNRS, F-38000 Grenoble, France and
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10
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Complex formation between the Escherichia coli [NiFe]-hydrogenase nickel maturation factors. Biometals 2019; 32:521-532. [DOI: 10.1007/s10534-019-00173-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2018] [Accepted: 01/18/2019] [Indexed: 11/26/2022]
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11
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Senger M, Stripp ST, Soboh B. Proteolytic cleavage orchestrates cofactor insertion and protein assembly in [NiFe]-hydrogenase biosynthesis. J Biol Chem 2017; 292:11670-11681. [PMID: 28539366 DOI: 10.1074/jbc.m117.788125] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2017] [Revised: 05/23/2017] [Indexed: 01/07/2023] Open
Abstract
Metalloenzymes catalyze complex and essential processes, such as photosynthesis, respiration, and nitrogen fixation. For example, bacteria and archaea use [NiFe]-hydrogenases to catalyze the uptake and release of molecular hydrogen (H2). [NiFe]-hydrogenases are redox enzymes composed of a large subunit that harbors a NiFe(CN)2CO metallo-center and a small subunit with three iron-sulfur clusters. The large subunit is synthesized with a C-terminal extension, cleaved off by a specific endopeptidase during maturation. The exact role of the C-terminal extension has remained elusive; however, cleavage takes place exclusively after assembly of the [NiFe]-cofactor and before large and small subunits form the catalytically active heterodimer. To unravel the functional role of the C-terminal extension, we used an enzymatic in vitro maturation assay that allows synthesizing functional [NiFe]-hydrogenase-2 of Escherichia coli from purified components. The maturation process included formation and insertion of the NiFe(CN)2CO cofactor into the large subunit, endoproteolytic cleavage of the C-terminal extension, and dimerization with the small subunit. Biochemical and spectroscopic analysis indicated that the C-terminal extension of the large subunit is essential for recognition by the maturation machinery. Only upon completion of cofactor insertion was removal of the C-terminal extension observed. Our results indicate that endoproteolytic cleavage is a central checkpoint in the maturation process. Here, cleavage temporally orchestrates cofactor insertion and protein assembly and ensures that only cofactor-containing protein can continue along the assembly line toward functional [NiFe]-hydrogenase.
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Affiliation(s)
- Moritz Senger
- Department of Physics, Experimental Molecular Biophysics, Freie Universitaet Berlin, 14195 Berlin, Germany
| | - Sven T Stripp
- Department of Physics, Experimental Molecular Biophysics, Freie Universitaet Berlin, 14195 Berlin, Germany
| | - Basem Soboh
- Department of Physics, Experimental Molecular Biophysics, Freie Universitaet Berlin, 14195 Berlin, Germany.
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