Characterization of Fe-S Clusters in Proteins by Mӧssbauer Spectroscopy

TusA influences Fe-S cluster assembly and iron homeostasis in E. coli by reducing the translation efficiency of Fur

All sulfur transfer pathways have generally a l-cysteine desulfurase as an initial sulfur-mobilizing enzyme in common, which serves as a sulfur donor for the biosynthesis of numerous sulfur-containing biomolecules in the cell. In Escherichia coli, the housekeeping l-cysteine desulfurase IscS has several interaction partners, which bind at different sites of the protein. So far, the interaction sites of IscU, Fdx, CyaY, and IscX involved in iron-sulfur (Fe-S) cluster assembly have been mapped, in addition to TusA, which is required for molybdenum cofactor biosynthesis and mnm5s2U34 tRNA modifications, and ThiI, which is involved in thiamine biosynthesis and s4U8 tRNA modifications. Previous studies predicted that the sulfur acceptor proteins bind to IscS one at a time. E. coli TusA has, however, been suggested to be involved in Fe-S cluster assembly, as fewer Fe-S clusters were detected in a ∆tusA mutant. The basis for this reduction in Fe-S cluster content is unknown. In this work, we investigated the role of TusA in iron-sulfur cluster assembly and iron homeostasis. We show that the absence of TusA reduces the translation of fur, thereby leading to pleiotropic cellular effects, which we dissect in detail in this study.IMPORTANCEIron-sulfur clusters are evolutionarily ancient prosthetic groups. The ferric uptake regulator plays a major role in controlling the expression of iron homeostasis genes in bacteria. We show that a ∆tusA mutant is impaired in the assembly of Fe-S clusters and accumulates iron. TusA, therefore, reduces fur mRNA translation leading to pleiotropic cellular effects.

Structure-based insights into the mechanism of [4Fe-4S]-dependent sulfur insertase LarE

Several essential cellular metabolites, such as enzyme cofactors, contain sulfur atoms and their biosynthesis requires specific thiolation enzymes. LarE is an ATP-dependent sulfur insertase, which catalyzes the sequential conversion of the two carboxylate groups of the precursor of the lactate racemase cofactor into thiocarboxylates. Two types of LarE enzymes are known, one that uses a catalytic cysteine as a sacrificial sulfur donor, and the other one that uses a [4Fe-4S] cluster as a cofactor. Only the crystal structure of LarE from Lactobacillus plantarum (LpLarE) from the first class has been solved. We report here the crystal structure of LarE from Methanococcus maripaludis (MmLarE), belonging to the second class, in the cluster-free (apo-) and cluster-bound (holo-) forms. The structure of holo-MmLarE shows that the [4Fe-4S] cluster is chelated by three cysteines only, leaving an open coordination site on one Fe atom. Moreover, the fourth nonprotein-bonded iron atom was able to bind an anionic ligand such as a phosphate group or a chloride ion. Together with the spectroscopic analysis of holo-MmLarE and the previously reported biochemical investigations of holo-LarE from Thermotoga maritima, these crystal structures support the hypothesis of a reaction mechanism, in which the [4Fe-4S] cluster binds a hydrogenosulfide ligand in place of the chloride anion, thus generating a [4Fe-5S] intermediate, and transfers it to the substrate, as in the case of [4Fe-4S]-dependent tRNA thiolation enzymes.

The HBx protein from hepatitis B virus coordinates a redox-active Fe-S cluster

The viral protein HBx is the key regulatory factor of the hepatitis B virus (HBV) and the main etiology for HBV-associated liver diseases, such as cirrhosis and hepatocellular carcinoma. Historically, HBx has defied biochemical and structural characterization, deterring efforts to understand its molecular mechanisms. Here we show that soluble HBx fused to solubility tags copurifies with either a [2Fe-2S] or a [4Fe-4S] cluster, a feature that is shared among five HBV genotypes. We show that the O2-stable [2Fe-2S] cluster form converts to an O2-sensitive [4Fe-4S] state when reacted with chemical reductants, a transformation that is best described by a reductive coupling mechanism reminiscent of Fe-S cluster scaffold proteins. In addition, the Fe-S cluster conversions are partially reversible in successive reduction-oxidation cycles, with cluster loss mainly occurring during (re)oxidation. The considerably negative reduction potential of the [4Fe-4S]2+/1+ couple (-520 mV) suggests that electron transfer may not be likely in the cell. Collectively, our findings identify HBx as an Fe-S protein with striking similarities to Fe-S scaffold proteins both in cluster type and reductive transformation. An Fe-S cluster in HBx offers new insights into its previously unknown molecular properties and sets the stage for deciphering the roles of HBx-associated iron (mis)regulation and reactive oxygen species in the context of liver tumorigenesis.

Metal Dependence and Functional Diversity of Type I Cas3 Nucleases

Type I CRISPR-Cas systems provide prokaryotes with protection from parasitic genetic elements by cleaving foreign DNA. In addition, they impact bacterial physiology by regulating pathogenicity and virulence, making them key players in adaptability and evolution. The signature nuclease Cas3 is a phosphodiesterase belonging to the HD-domain metalloprotein superfamily. By directing specific metal incorporation, we map a promiscuous metal ion cofactor profile for Cas3 from Thermobifida fusca (Tf). Tf Cas3 affords significant ssDNA cleavage with four homo-dimetal centers (Fe²⁺, Co²⁺, Mn²⁺, and Ni²⁺), while the diferrous form is the most active and likely biologically relevant in vivo. Electron paramagnetic resonance (EPR) spectroscopy and Mössbauer spectroscopy show that the diiron cofactor can access three redox forms, while the diferrous form can be readily obtained with mild reductants. We further employ EPR and Mössbauer on Fe-enriched proteins to establish that Cas3″ enzymes harbor a dinuclear cofactor, which was not previously confirmed. We demonstrate that the ancillary His ligand is critical for efficient ssDNA cleavage but not for diiron assembly or small molecule hydrolysis. We further explore the ability of Cas3 to hydrolyze cyclic mononucleotides and show that Tf Cas3 hydrolyzes 2'3'-cAMP with catalytic efficiency comparable to that of the conserved virulence factor A (CvfA), an HD-domain protein hydrolyzing 2'3'-cylic phosphodiester bonds at RNA 3'-termini. Because this CvfA activity is linked to virulence regulation, Cas3 may also utilize 2'3'-cAMP hydrolysis as a possible molecular route to control virulence.

Mössbauer effect using 57Fe-ferrabisdicarbollide ([o-57FESAN]-): a glance into the potential of a low-dose approach for glioblastoma radiotherapy

Although a variety of cancers is initially susceptible to chemotherapy, they eventually develop multi-drug resistance. To overcome this situation, more effective and selective treatments are necessary by using anti-tumour agents...