The N-terminal domains of Bacillus subtilis CopA do not form a stable complex in the absence of their inter-domain linker

Sensing mechanisms of iron-sulfur cluster regulatory proteins elucidated using native mass spectrometry

The ability to sense and respond to various key environmental cues is important for the survival and adaptability of many bacteria, including pathogens. The particular sensitivity of iron-sulfur (Fe-S) clusters is exploited in nature, such that multiple sensor-regulator proteins, which coordinate the detection of analytes with a (in many cases) global transcriptional response, are Fe-S cluster proteins. The fragility and sensitivity of these Fe-S clusters make studying such proteins difficult, and gaining insight of what they sense, and how they sense it and transduce the signal to affect transcription, is a major challenge. While mass spectrometry is very widely used in biological research, it is normally employed under denaturing conditions where non-covalently attached cofactors are lost. However, mass spectrometry under conditions where the protein retains its native structure and, thus, cofactors, is now itself a flourishing field, and the application of such 'native' mass spectrometry to study metalloproteins is now relatively widespread. Here we describe recent advances in using native MS to study Fe-S cluster proteins. Through its ability to accurately measure mass changes that reflect chemistry occurring at the cluster, this approach has yielded a remarkable richness of information that is not accessible by other, more traditional techniques.

Native Mass Spectrometry of Iron-Sulfur Proteins

Iron-sulfur clusters constitute a large and widely distributed group of protein cofactors that play key roles in a wide range of metabolic processes. The inherent reactivity of iron-sulfur clusters toward small molecules, for example, O₂, NO, or free Fe, makes them ideal for sensing changes in the cellular environment. Nondenaturing, or native, MS is unique in its ability to preserve the noncovalent interactions of many (if not all) species, including stable intermediates, while providing accurate mass measurements in both thermodynamic and kinetic experimental regimes. Here, we provide practical guidance for the study of iron-sulfur proteins by native MS, illustrated by examples where it has been used to unambiguously determine the type of cluster coordinated to the protein framework. We also describe the use of time-resolved native MS to follow the kinetics of cluster conversion, allowing the elucidation of the precise series of molecular events for all species involved. Finally, we provide advice on a unique approach to a typical thermodynamic titration, uncovering early, quasi-stable, intermediates in the reaction of a cluster with nitric oxide, resulting in cluster nitrosylation.

Mass spectrometric studies of Cu(I)-binding to the N-terminal domains of B. subtilis CopA and influence of bacillithiol

CopA is a Cu(I)-exporting transmembrane P_1B-type ATPase from Bacillus subtilis. It contains two N-terminal cytoplasmic domains, CopAab, which bind Cu(I) with high affinity and to form higher-order complexes with multiple Cu(I) ions. To determine the precise nature of these species, electrospray ionisation mass spectrometry (ESI-MS) under non-denaturing conditions was employed. Up to 1 Cu per CopAab resulted in Cu coordination to one or both CopAab domains. At >1 Cu/CopAab, two distinct dimeric charge state envelopes were observed, corresponding to distinct conformations, each with Cu₆(CopAab)₂ as its major form. The influence of the physiologically relevant low molecular weight thiol bacillithiol (BSH) on Cu(I)-binding to CopAab was assessed. Dimeric CopAab persisted in the presence of BSH, with previously undetected Cu₇(CopAab)₂ and Cu₆(CopAab)₂(BSH) forms apparent.