Cytochromes are heme proteins essential for aerobic and anaerobic growth of most organisms, including human pathogens. Recently it has become clear that dedicated assembly factors are crucial for cytochrome biogenesis. The biogenesis of c-type cytochromes occurs by one of three pathways, systems I, II, or III. System I has eight (CcmABCDEFGH) and system II has two (CcsBA) dedicated assembly factors (membrane proteins), while system III of mitochondria uses a single enzyme called holocytochrome c synthase (HCCS). Because only prokaryotes use systems I and II and they function outside the cytoplasmic membrane, like the targets of penicillin, these pathways represent potential targets for new antimicrobial compounds. Moreover, human HCCS variants are responsible for the genetic disease MLS, and mitochondrial pathologies are implicated in cancer, myopathies, neurological, and other diseases. The c-type cytochromes possess heme that is covalently attached to the apocytochrome at two cysteines (at a CXXCH motif), a reaction carried out by the synthetase of each system. This study examines how proteins in systems I, II, and III attach heme to apocytochrome c, including where the CXXCH motif interacts on the synthetases (CcmF/H, CcsBA, HCCS), and how these synthetases function. Location of the CXXCH binding site on all synthetases represent major voids in our knowledge of the pathways, and in vitro reconstitution of synthetase functions are grand challenges for the field. The proposal takes advantage of recent success in purifying all proteins of systems I, II, and III from recombinant Escherichia coli. For most of these purified components, endogenous heme is co-purified, facilitating analyses of heme transport, red-ox control, and attachment mechanisms.
Three aims are proposed, analyzing systems I (Aim 1), system II (Aim 2), and system III (Aim 3). System I is described in two steps. Step 1 is the CcmABCD-mediated synthesis and release of periplasmic holoCcmE (ie with heme).
Aim 1 A analyzes this step, establishing residues in CcmC that directly interact with heme for trafficking and testing our hypotheses on mechanisms of holoCcmE formation. In step 2, holoCcmE chaperones heme to the CcmF/H synthetase for attachment to apocytochrome c. Common goals are described for each of the three synthetases: establish the CXXCH binding site on CcmF/H (Aim 1B), CcsBA (Aim 2), and HCCS (Aim 3). This will be accomplished using in vivo and in vitro crosslinking approaches with purified synthetases. For each purified synthetase (Aim 1B, CcmF/H), CcsBA (Aim 2), HCCS (Aim 3), an in vitro attachment assay will be developed (ie attachment of heme to apocytochrome c). Additionally, Aim 3 will test and further elucidate our hypothesized four-step model of biogenesis by HCCS. Results here will unravel molecular mechanisms of biogenesis for all c-type cytochromes.

Public Health Relevance

Organisms have common mechanisms to convert chemical foodstuffs into energy, for example the mitochondrial aerobic respiratory chain. However, to assemble a ubiquitous component of these chains called cytochrome c, bacteria use quite different pathways (called systems I and II) than humans (system III). Understanding systems I, II, and III at the deep level proposed here will enable the discovery of chemicals (antibiotics) tht specifically target the bacterial systems, thus impacting infectious diseases like tuberculosis, meningitis, and other respiratory and inflammatory illnesses.

Agency
National Institute of Health (NIH)
Institute
National Institute of General Medical Sciences (NIGMS)
Type
Research Project (R01)
Project #
5R01GM047909-22
Application #
9330858
Study Section
Prokaryotic Cell and Molecular Biology Study Section (PCMB)
Program Officer
Anderson, Vernon
Project Start
1994-09-01
Project End
2019-08-31
Budget Start
2017-09-01
Budget End
2018-08-31
Support Year
22
Fiscal Year
2017
Total Cost
Indirect Cost
Name
Washington University
Department
Biology
Type
Schools of Arts and Sciences
DUNS #
068552207
City
Saint Louis
State
MO
Country
United States
Zip Code
63130
Sutherland, Molly C; Tran, Nathan L; Tillman, Dustin E et al. (2018) Structure-Function Analysis of the Bifunctional CcsBA Heme Exporter and Cytochrome c Synthetase. MBio 9:
Sutherland, Molly C; Jarodsky, Joshua M; Ovchinnikov, Sergey et al. (2018) Structurally Mapping Endogenous Heme in the CcmCDE Membrane Complex for Cytochrome c Biogenesis. J Mol Biol 430:1065-1080
Babbitt, Shalon E; Hsu, Jennifer; Mendez, Deanna L et al. (2017) Biosynthesis of Single Thioether c-Type Cytochromes Provides Insight into Mechanisms Intrinsic to Holocytochrome c Synthase (HCCS). Biochemistry 56:3337-3346
Mendez, Deanna L; Babbitt, Shalon E; King, Jeremy D et al. (2017) Engineered holocytochrome c synthases that biosynthesize new cytochromes c. Proc Natl Acad Sci U S A 114:2235-2240
Mendez, Deanna L; Akey, Ildikó V; Akey, Christopher W et al. (2017) Oxidized or Reduced Cytochrome c and Axial Ligand Variants All Form the Apoptosome in Vitro. Biochemistry 56:2766-2769
Babbitt, Shalon E; Hsu, Jennifer; Kranz, Robert G (2016) Molecular Basis Behind Inability of Mitochondrial Holocytochrome c Synthase to Mature Bacterial Cytochromes: DEFINING A CRITICAL ROLE FOR CYTOCHROME c ? HELIX-1. J Biol Chem 291:17523-34
Sutherland, Molly C; Rankin, Joel A; Kranz, Robert G (2016) Heme Trafficking and Modifications during System I Cytochrome c Biogenesis: Insights from Heme Redox Potentials of Ccm Proteins. Biochemistry 55:3150-6
Babbitt, Shalon E; Sutherland, Molly C; San Francisco, Brian et al. (2015) Mitochondrial cytochrome c biogenesis: no longer an enigma. Trends Biochem Sci 40:446-55
Babbitt, Shalon E; San Francisco, Brian; Bretsnyder, Eric C et al. (2014) Conserved residues of the human mitochondrial holocytochrome c synthase mediate interactions with heme. Biochemistry 53:5261-71
San Francisco, Brian; Kranz, Robert G (2014) Interaction of holoCcmE with CcmF in heme trafficking and cytochrome c biosynthesis. J Mol Biol 426:570-85

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