Molecular Dynamics Simulations of the Apo and Holo States of the Copper Binding Protein CueR Reveal Principal Bending and Twisting Motions

The Dynamic Plasticity of P. aeruginosa CueR Copper Transcription Factor upon Cofactor and DNA Binding

Bacteria use specialized proteins, like transcription factors, to rapidly control metal ion balance. CueR is a Gram-negative bacterial copper regulator. The structure of E. coli CueR complexed with Cu(I) and DNA was published, since then many studies have shed light on its function. However, P. aeruginosa CueR, which shows high sequence similarity to E. coli CueR, has been less studied. Here, we applied room-temperature electron paramagnetic resonance (EPR) measurements to explore changes in dynamics of P. aeruginosa CueR in dependency of copper concentrations and interaction with two different DNA promoter regions. We showed that P. aeruginosa CueR is less dynamic than the E. coli CueR protein and exhibits much higher sensitivity to DNA binding as compared to its E. coli CueR homolog. Moreover, a difference in dynamical behavior was observed when P. aeruginosa CueR binds to the copZ2 DNA promoter sequence compared to the mexPQ-opmE promoter sequence. Such dynamical differences may affect the expression levels of CopZ2 and MexPQ-OpmE proteins in P. aeruginosa. Overall, such comparative measurements of protein-DNA complexes derived from different bacterial systems reveal insights about how structural and dynamical differences between two highly homologous proteins lead to quite different DNA sequence-recognition and mechanistic properties.

An in-cell spin-labelling methodology provides structural information on cytoplasmic proteins in bacteria

EPR in-cell spin-labeling was applied to CueR in E. coli. The methodology employed a Cu(II)-NTA complexed with dHis. High resolved in-cell distance distributions were obtained revealing minor differences between in vitro and in-cell data. This methodology allows study of structural changes of any protein in-cell, independent of size or cellular system.

Copper coordination states affect the flexibility of copper Metallochaperone Atox1: Insights from molecular dynamics simulations

Copper is an essential element in nature but in excess, it is toxic to the living cell. The human metallochaperone Atox1 participates in copper homeostasis and is responsible for copper transmission. In a previous multiscale simulation study, we noticed a change in the coordination state of the Cu(I) ion, from 4 bound cysteine residues to 3, in agreement with earlier studies. Here, we perform and analyze classical molecular dynamic simulations of various coordination states: 2, 3, and 4. The main observation is an increase in protein flexibility as a result of a decrease in the coordination state. In addition, we identified several populated conformations that correlate well with double electron-electron resonance distance distributions or an X-ray structure of Cu(I)-bound Atox1. We suggest that the increased flexibility might benefit the process of ion transmission between interacting proteins. Further experiments can scrutinize this hypothesis and shed additional light on the mechanism of action of Atox1.

EPR Spectroscopy Provides New Insights into Complex Biological Reaction Mechanisms

In the last 20 years, the use of electron paramagnetic resonance (EPR) has made a pronounced and lasting impact in the field of structural biology. The advantage of EPR spectroscopy over other structural techniques is its ability to target even minor conformational changes in any biomolecule or macromolecular complex, independent of its size or complexity, or whether it is in solution or in the cell during a biological or chemical reaction. Here, we focus on the use of EPR spectroscopy to study transmembrane transport and transcription mechanisms. We discuss experimental and analytical concerns when referring to studies of two biological reaction mechanisms, namely, transfer of copper ions by the human copper transporter hCtr1 and the mechanism of action of the Escherichia coli copper-dependent transcription factor CueR. Last, we elaborate on future avenues in the field of EPR structural biology.

Tying up a loose end: On the role of the C-terminal CCHHRAG fragment of the metalloregulator CueR

The transcriptional regulator CueR is activated by the binding of Cu^I, Ag^I, or Au^I to two cysteinates in a near‐linear fashion. The C‐terminal CCHHRAG sequence in Escherichia coli CueR present potential additional metal binding ligands and here we explore the effect of deleting this fragment on the binding of Ag^I to CueR. CD spectroscopic and ESI‐MS data indicate that the high Ag^I‐binding affinity of WT‐CueR is significantly reduced in Δ7C‐CueR.^[111 Ag PAC spectroscopy demonstrates that the WT‐CueR metal site structure (AgS₂) is conserved, but less populated in the truncated variant. Thus, the function of the C‐terminal fragment may be to stabilize the two‐coordinate metal site for cognate monovalent metal ions. In a broader perspective this is an example of residues beyond the second coordination sphere affecting metal site physicochemical properties while leaving the structure unperturbed.

Allostery‐driven changes in dynamics regulate the activation of bacterial copper transcription factor

Metalloregulators bind and respond to metal ions by regulating the transcription of metal homeostasis genes. Copper efflux regulator (CueR) is a copper‐responsive metalloregulator that is found in numerous Gram‐negative bacteria. Upon Cu(I) coordination, CueR initiates transcription by bending the bound DNA promoter regions facilitating interaction with RNA polymerase. The structure of Escherichia coli CueR in presence of DNA and metal ion has been reported using X‐ray crystallography and cryo‐EM, providing information about the mechanism of action. However, the specific role of copper in controlling this transcription mechanism remains elusive. Herein, we use room temperature electron paramagnetic resonance (EPR) experiments to follow allosterically driven dynamical changes in E. coli CueR induced by Cu(I) binding. We suggest that more than one Cu(I) ion binds per CueR monomer, leading to changes in site‐specific dynamics at the Cu(I) binding domain and at the distant DNA binding site. Interestingly, Cu(I) binding leads to an increase in dynamics about 27 Å away at the DNA binding domain. These changes in the dynamics of the DNA binding domain are important for exact coordination with the DNA. Thus, Cu(I) binding is critical to initiate a series of conformational changes that regulate and initiate gene transcription.