A Proteomic Dissection of the Hg(II) Toxicity Paradox

EMSL Project ID

19827

Award DOI

https://dx.doi.org/10.46936/sthm.proj.2006.19827/60002623

Abstract

Mercury (Hg) in all its forms is a contaminant at most DOE sites, especially at Oak Ridge National Laboratory (ORNL) where the spread of spilled Hg and its effects on microbial populations have been monitored for decades. Hg is also released from burning fossil fuels and reduction of this source of human and general ecosytem exposure costs tens of millions of dollars annually. Surprisingly, the exact target, the weakest link, in Hg(II) toxicity is not known in any cellular system. It is assumed that Hg(II) binds protein thiols and eliminates their functions. The paradox is that Hg(II) is toxic at micromolar levels to prokayrotes and at nanomolar levels to eukaryotic cells despite the fact that most bacteria and all eukaryotes have millimolar levels of small thiols such as glutathione as a sink for thiophilic toxic metals. Summers' and Miller's groups study bacterial mercury resistance (mer), the widespread and only naturally occurring, dedicated biotic metal detoxification system. The mer operon gene products reduce ionic Hg(II) to volatile, monoatomic Hg(0), dissipating it from the microbes' environment. We know much about how mer works; what no one yet knows is exactly how Hg(II) is toxic and why Nature selects to volatilize it rather than precipitating it with H2S which many facultative bacteria make. These questions are central for understanding the ubiquity of the mer operon and its role in bioremediation, as well as understanding the actual risks to humans, animals and plants that environmental Hg poses. Thus, we propose to test these hypotheses:

1. The proteins most vulnerable to Hg(II) are those of the electron transport system (ETS); thus, low level Hg(II)-exposure in air provokes formation of reactive oxygen species (ROS);
2. At higher levels, in addition to ROS, Hg(II) also forms stable RS-Hg-SR' ligands with cellular proteins that are not effectively removed by low molecular weight monothiol buffers such as glutathione;
3. One or more proteins of the plasmid-encoded mer operon are essential for restoring the function of these unusually vulnerable high affinity Hg(II)-binding cellular proteins.
This novel application of accurate mass tag FT-ICR mass spectrometry emphasizes that bioremediation involves the whole cell, not just a single reductive process. In addition to identifying the most vulnerable targets of Hg exposure, the techniques we work out for quantifying toxic metal impact on this model bacterial proteome will apply to assessing the impacts of other toxic and radioactive substrates on bacteria such as Geobacter, Shewanella and Desulfovibrio that are naturally involved in metal/radionuclide sequestration. Studies of the metal-response proteome can reveal shock responses that might interfere with the reductive pathway of interest as well as metabolic pathways which may play a previously unrecognized supportive role. They will also set the stage for assessment of the Hg shock response in eukaryotic model systems including whole animals (e.g. zebrafish) and tissue culture, i.e. for understanding the molecular basis for the risks and possible remedies of Hg exposure in humans.

Project Details

Project type

Large-Scale EMSL Research

Start Date

2006-09-01

End Date

2009-09-30

Status

Closed

Released Data Link

https://search.emsl.pnnl.gov/?project_id_search=19827

Team

Principal Investigator

Anne Summers

Institution

University of Georgia

Team Members

Susan Miller

Institution

University of California, San Francisco

Related Publications

Benjamin J. Polacco, Samuel O. Purvine, Erika M. Zink, Stephen P. LaVoie, Mary S. Lipton, Anne O. Summers, Susan M. Miller (2011) Discovering mercury protein modifications in whole proteomes using natural isotope distributions observed in liquid chromatography-tandem mass spectrometry. Molecular & Cellular Proteomics, 30 April 2011 (online)

https://dx.doi.org/