Potency of a small molecule that targets the molluscum contagiosum virus processivity factor increases when conjugated to a tripeptide.

We recently developed compound FC-7269 for targeting the Molluscum contagiosum virus processivity factor (mD4) and demonstrated its ability to inhibit viral processive DNA synthesis in vitro and cellular infection of an mD4-dependent virus (Antiviral Res 211, 2023,105520). However, despite a thorough medicinal chemistry campaign we were unable to generate a potent second analog as a requisite for drug development. We overcame this impasse, by conjugating a short hydrophobic trivaline peptide to FC-7269 to produce FC-TriVal-7269 which significantly increased antiviral potency and reduced cellular toxicity.

A small molecule that targets the processivity factor of molluscum contagiosum virus has therapeutic potential.

Molluscum contagiosum (MC) is an infectious disease that occurs only in humans with a tropism that is narrowly restricted to the outermost epidermal layer of the skin. Molluscum contagiosum virus (MCV) is the causative agent of MC which produces skin lesions that can persist for months to several years. MCV is efficiently transmitted by direct physical contact or by indirect contact with fomites. MC is most prevalent in children and immune compromised patients. The failure to develop a drug that targets MCV replication has been hampered for decades by the inability to propagate MCV in cell culture. To address this dilemma, we recently engineered a surrogate poxvirus expressing the MCV processivity factor (mD4) as the drug target. The mD4 protein is essential for viral replication by keeping the viral polymerase tethered to the DNA template. In this study we have designed and synthesized a lead compound (7269) that is able to prevent mD4 dependent processive DNA synthesis in vitro (IC50 = 6.8 µM) and effectively inhibit propagation of the mD4-VV surrogate virus in BSC-1 cells (EC50 = 13.2 µM) with negligible cytotoxicity. In human liver microsomes, 7269 was shown to be stable for almost 2 h. When tested for penetration into human cadaver skin in a formulated gel, the level of 7269 in the epidermal layer was nearly 100 times the concentration (EC50) needed to inhibit propagation of the mD4-VV surrogate virus in BSC-1 cells. The gel formulated 7269 was scored as a non-irritant on skin and shown to have a shelf-life that was completely stable after several months. In summary, 7269 is a potential Lead for becoming the first MCV anti-viral compound to treat MC and thereby, addresses this unmet medical need that has persisted for many decades.

Discovery of a potent cytotoxic agent that promotes G2/M phase cell cycle arrest and apoptosis in a malignant human pharyngeal squamous carcinoma cell line.

Squamous cell carcinoma is the major form of malignancy that arises in head and neck cancer. The modest improvement in the 5­year survival rate underpins its complex etiology and provides the impetus for the discovery of new therapeutics. The present study describes the discovery of an indole­based small molecule (24a) that was a potent cytotoxic agent with antiproliferative and pro­apoptotic properties against a pharyngeal carcinoma cell line, Detroit 562, effectively killing the cells at a half­maximal inhibitory concentration of 0.03 µM, as demonstrated using cell proliferation studies. The antiproliferative property of 24a was demonstrated by its ability to promote G2/M blockade, as assessed by cell cycle analysis using flow cytometry and the monitoring of real­time cell cycle progression by the fluorescence ubiquitination­based cell cycle indicator. This pro­apoptotic property is supported by the promotion of TUNEL­staining and increase in the activities of caspases­3/7 and ­6, in addition to the expression of death receptors and the cleavage of poly (ADP­ribose) polymerase 1 protein as demonstrated by western blotting. Given that Detroit 562 lacks functional p53, it is suggested that 24a acts independently of the tumor suppressor.

Development, testing and validation of a SARS-CoV-2 multiplex panel for detection of the five major variants of concern on a portable PCR platform.

Many SARS-CoV-2 variants have emerged during the course of the COVID-19 pandemic. These variants have acquired mutations conferring phenotypes such as increased transmissibility or virulence, or causing diagnostic, therapeutic, or immune escape. Detection of Alpha and the majority of Omicron sublineages by PCR relied on the so-called S gene target failure due to the deletion of six nucleotides coding for amino acids 69-70 in the spike (S) protein. Detection of hallmark mutations in other variants present in samples relied on whole genome sequencing. However, whole genome sequencing as a diagnostic tool is still in its infancy due to geographic inequities in sequencing capabilities, higher cost compared to other molecular assays, longer turnaround time from sample to result, and technical challenges associated with producing complete genome sequences from samples that have low viral load and/or high background. Hence, there is a need for rapid genotyping assays. In order to rapidly generate information on the presence of a variant in a given sample, we have created a panel of four triplex RT-qPCR assays targeting 12 mutations to detect and differentiate all five variants of concern: Alpha, Beta, Gamma, Delta, and Omicron. We also developed an expanded pentaplex assay that can reliably distinguish among the major sublineages (BA.1-BA.5) of Omicron. In silico, analytical and clinical testing of the variant panel indicate that the assays exhibit high sensitivity and specificity. This panel can help fulfill the need for rapid identification of variants in samples, leading to quick decision making with respect to public health measures, as well as treatment options for individuals. Compared to sequencing, these genotyping PCR assays allow much faster turn-around time from sample to results-just a couple hours instead of days or weeks.

Debulking SARS-CoV-2 in saliva using angiotensin converting enzyme 2 in chewing gum to decrease oral virus transmission and infection.

To advance a novel concept of debulking virus in the oral cavity, the primary site of viral replication, virus-trapping proteins CTB-ACE2 were expressed in chloroplasts and clinical-grade plant material was developed to meet FDA requirements. Chewing gum (2 g) containing plant cells expressed CTB-ACE2 up to 17.2 mg ACE2/g dry weight (11.7% leaf protein), have physical characteristics and taste/flavor like conventional gums, and no protein was lost during gum compression. CTB-ACE2 gum efficiently (>95%) inhibited entry of lentivirus spike or VSV-spike pseudovirus into Vero/CHO cells when quantified by luciferase or red fluorescence. Incubation of CTB-ACE2 microparticles reduced SARS-CoV-2 virus count in COVID-19 swab/saliva samples by >95% when evaluated by microbubbles (femtomolar concentration) or qPCR, demonstrating both virus trapping and blocking of cellular entry. COVID-19 saliva samples showed low or undetectable ACE2 activity when compared with healthy individuals (2,582 versus 50,126 ΔRFU; 27 versus 225 enzyme units), confirming greater susceptibility of infected patients for viral entry. CTB-ACE2 activity was completely inhibited by pre-incubation with SARS-CoV-2 receptor-binding domain, offering an explanation for reduced saliva ACE2 activity among COVID-19 patients. Chewing gum with virus-trapping proteins offers a general affordable strategy to protect patients from most oral virus re-infections through debulking or minimizing transmission to others.

Herpes Simplex Virus-1 infection in human primary corneal epithelial cells is blocked by a stapled peptide that targets processive DNA synthesis.

PURPOSE: Acyclovir is most commonly used for treating ocular Herpes Keratitis, a leading cause of infectious blindness. However, emerging resistance to Acyclovir resulting from mutations in the thymidine kinase gene of Herpes Simplex Virus -1 (HSV-1), has prompted the need for new therapeutics directed against a different viral protein. One novel target is the HSV-1 Processivity Factor which is essential for tethering HSV-1 Polymerase to the viral genome to enable long-chain DNA synthesis. METHODS: A series of peptides, based on the crystal structure of the C-terminus of HSV-1 Polymerase, were constructed with hydrocarbon staples to retain their alpha-helical conformation. The stapled peptides were tested for blocking both HSV-1 DNA synthesis and infection. The most effective peptide was further optimized by replacing its negative N-terminus with two hydrophobic valine residues. This di-valine stapled peptide was tested for inhibiting HSV-1 infection of human primary corneal epithelial cells. RESULTS: The stapled peptides blocked HSV-1 DNA synthesis and HSV-1 infection. The unstapled control peptide had no inhibitory effects. Specificity of the stapled peptides was confirmed by their inabilities to block infection by an unrelated virus. Significantly, the optimized di-valine stapled peptide effectively blocked HSV-1 infection in human primary corneal epithelial cells with selectivity index of 11.6. CONCLUSIONS: Hydrocarbon stapled peptides that simulate the α-helix from the C-terminus of HSV-1 DNA polymerase can specifically block DNA synthesis and infection of HSV-1 in human primary corneal epithelial cells. These stapled peptides provide a foundation for developing a topical therapeutic for treating human ocular Herpes Keratitis.

Unique roles of iron and zinc binding to the yeast Fe-S cluster scaffold assembly protein "Isu1".

Mitochondrial Fe-S cluster biosynthesis is accomplished within yeast utilizing the biophysical attributes of the "Isu1" scaffold assembly protein. As a member of a highly homologous protein family, Isu1 has sequence conservation between orthologs and a conserved ability to assemble [2Fe-2S] clusters. Regardless of species, scaffold orthologs have been shown to exist in both "disordered" and "structured" conformations, a structural architecture that is directly related to conformations utilized during Fe-S cluster assembly. During assembly, the scaffold helps direct the delivery and utilization of Fe(ii) and persulfide substrates to produce [2Fe-2S] clusters, however Zn(ii) binding alters the activity of the scaffold while at the same time stabilizes the protein in its structured state. Additional studies confirm Zn binds to the scaffold's Cys rich active site, and has an impact on the protein's ability to make Fe-S clusters. Understanding the interplay between Fe(ii) and Zn(ii) binding to Isu1 in vitro may help clarify metal loading events that occur during Fe-S cluster assembly in vivo. Here we determine the metal : protein stoichiometry for Isu1 Zn and Fe binding to be 1 : 1 and 2 : 1, respectively. As expected, while Zn binding shifts the Isu1 to its structured state, folding is not influenced by Fe(ii) binding. X-ray absorption spectroscopy (XAS) confirms Zn(ii) binds to the scaffold's cysteine rich active site but Fe(ii) binds at a location distinct from the active site. XAS results show Isu1 binding initially of either Fe(ii) or Zn(ii) does not significantly perturb the metal site structure of alternate metal. XAS confirmed that four scaffold orthologs bind iron as high-spin Fe(ii) at a site composed of ca. 6 oxygen and nitrogen nearest neighbor ligands. Finally, in our report Zn binding dramatically reduces the Fe-S cluster assembly activity of Isu1 even in the presence of frataxin. Given the Fe-binding activity we report for Isu1 and its orthologs here, a possible mechanism involving Fe(ii) transport to the scaffold's active site during cluster assembly has been considered.

Mutation and structure guided discovery of an antiviral small molecule that mimics an essential C-Terminal tripeptide of the vaccinia D4 processivity factor.

The smallpox virus (variola) remains a bioterrorism threat since a majority of the human population has never been vaccinated. In the event of an outbreak, at least two drugs against different targets of variola are critical to circumvent potential viral mutants that acquire resistance. Vaccinia virus (VACV) is the model virus used in the laboratory for studying smallpox. The VACV processivity factor D4 is an ideal therapeutic target since it is both essential and specific for poxvirus replication. Recently, we identified a tripeptide (Gly-Phe-Ile) motif at the C-terminus of D4 that is conserved among poxviruses and is necessary for maintaining protein function. In the current work, a virtual screening for small molecule mimics of the tripeptide identified a thiophene lead that effectively inhibited VACV, cowpox virus, and rabbitpox virus in cell culture (EC50 = 8.4-19.7 µM) and blocked in vitro processive DNA synthesis (IC50 = 13.4 µM). Compound-binding to D4 was demonstrated through various biophysical methods and a dose-dependent retardation of the proteolysis of D4 proteins. This study highlights an inhibitor design strategy that exploits a susceptible region of the protein and identifies a novel scaffold for a broad-spectrum poxvirus inhibitor.

A Conserved Tripeptide Sequence at the C Terminus of the Poxvirus DNA Processivity Factor D4 Is Essential for Protein Integrity and Function.

Vaccinia virus (VACV) is a poxvirus, and the VACV D4 protein serves both as a uracil-DNA glycosylase and as an essential component required for processive DNA synthesis. The VACV A20 protein has no known catalytic function itself but associates with D4 to form the D4-A20 heterodimer that functions as the poxvirus DNA processivity factor. The heterodimer enables the DNA polymerase to efficiently synthesize extended strands of DNA. Upon characterizing the interaction between D4 and A20, we observed that the C terminus of D4 is susceptible to perturbation. Further analysis demonstrated that a conserved hexapeptide stretch at the extreme C terminus of D4 is essential for maintaining protein integrity, as assessed by its requirement for the production of soluble recombinant protein that is functional in processive DNA synthesis. From the known crystal structures of D4, the C-terminal hexapeptide is shown to make intramolecular contact with residues spanning the inner core of the protein. Our mutational analysis revealed that a tripeptide motif (215GFI217) within the hexapeptide comprises apparent residues necessary for the contact. Prediction of protein disorder identified the hexapeptide and several regions upstream of Gly215 that comprise residues of the interface surfaces of the D4-A20 heterodimer. Our study suggests that 215GFI217 anchors these potentially dynamic upstream regions of the protein to maintain protein integrity. Unlike uracil-DNA glycosylases from diverse sources, where the C termini are disordered and do not form comparable intramolecular contacts, this feature may be unique to orthopoxviruses.

Poxvirus uracil-DNA glycosylase-An unusual member of the family I uracil-DNA glycosylases.

Uracil-DNA glycosylases are ubiquitous enzymes, which play a key role repairing damages in DNA and in maintaining genomic integrity by catalyzing the first step in the base excision repair pathway. Within the superfamily of uracil-DNA glycosylases family I enzymes or UNGs are specific for recognizing and removing uracil from DNA. These enzymes feature conserved structural folds, active site residues and use common motifs for DNA binding, uracil recognition and catalysis. Within this family the enzymes of poxviruses are unique and most remarkable in terms of amino acid sequences, characteristic motifs and more importantly for their novel non-enzymatic function in DNA replication. UNG of vaccinia virus, also known as D4, is the most extensively characterized UNG of the poxvirus family. D4 forms an unusual heterodimeric processivity factor by attaching to a poxvirus-specific protein A20, which also binds to the DNA polymerase E9 and recruits other proteins necessary for replication. D4 is thus integrated in the DNA polymerase complex, and its DNA-binding and DNA scanning abilities couple DNA processivity and DNA base excision repair at the replication fork. The adaptations necessary for taking on the new function are reflected in the amino acid sequence and the three-dimensional structure of D4. An overview of the current state of the knowledge on the structure-function relationship of D4 is provided here.

The processivity factor complex of feline herpes virus-1 is a new drug target.

Feline herpes virus-1 (FHV-1) is ubiquitous in the cat population and is a major cause of blindness for which antiviral drugs, including acyclovir, are not completely effective. Recurrent infections, due to reactivation of latent FHV-1 residing in the trigeminal ganglia, can lead to epithelial keratitis and stromal keratitis and eventually loss of sight. This has prompted the medical need for an antiviral drug that will specifically inhibit FHV-1 infection. A new antiviral target is the DNA polymerase and its associated processivity factor, which forms a complex that is essential for extended DNA strand synthesis. In this study we have cloned and expressed the FHV-1 DNA polymerase (f-UL30) and processivity factor (f-UL42) and demonstrated that both proteins are required to completely synthesize the 7249 nucleotide full-length DNA from the M13 primed-DNA template in vitro. Significantly, a known inhibitor of human herpes simplex virus-1 (HSV-1) processivity complex was shown to inhibit FHV-1 processive DNA synthesis in vitro and block infection of cells. This validates using f-UL42/f-UL30 as a new antiviral drug target to treat feline ocular herpes infection.

A novel target and approach for identifying antivirals against molluscum contagiosum virus.

The dermatological disease molluscum contagiosum (MC) presents as lesions restricted solely to the skin. The poxvirus molluscum contagiosum virus (MCV) is responsible for this skin disease that is easily transmitted through casual contact among all populations, with greater frequency in children and immunosuppressed individuals. In addition, sexual transmission of MCV in adolescents and adults is a health concern. Although the skin lesions ultimately resolve in immunocompetent individuals, they can persist for extended periods, be painful, and result in scarring. Treatment is problematic, and there is no drug that specifically targets MCV. The inability of MCV to propagate in cell culture has impeded drug development. To overcome these barriers, we integrated three new developments. First, we identified a new MCV drug target (mD4) that is essential for processive DNA synthesis in vitro. Second, we discovered a small chemical compound that binds to mD4 and prevents DNA synthesis in vitro. Third, and most significant, we engineered a hybrid vaccinia virus (mD4-VV) in which the natural vaccinia D4 (vD4) gene is replaced by the mD4 target gene. This hybrid virus is dependent on mD4 for viral growth in culture and is inhibited by the small compound. This target system provides, for the first time, a platform and approach for the discovery and evaluation of new therapeutics that can be used to treat MC.

Mitigating effects of L-selenomethionine on low-dose iron ion radiation-induced changes in gene expression associated with cellular stress.

Ionizing radiation associated with highly energetic and charged heavy (HZE) particles poses a danger to astronauts during space travel. The aim of the present study was to evaluate the patterns of gene expression associated with cellular exposure to low-dose iron ion irradiation, in the presence and absence of L-selenomethionine (SeM). Human thyroid epithelial cells (HTori-3) were exposed to low-dose iron ion (1 GeV/n) irradiation at 10 or 20 cGy with or without SeM pretreatment. The cells were harvested 6 and 16 h post-irradiation and analyzed by the Affymetrix U133Av2 gene chip arrays. Genes exhibiting a 1.5-fold expression cut-off and 5% false discovery rate (FDR) were considered statistically significant and subsequently analyzed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) for pathway analysis. Representative genes were further validated by real-time RT-PCR. Even at low doses of radiation from iron ions, global genome profiling of the irradiated cells revealed the upregulation of genes associated with the activation of stress-related signaling pathways (ubiquitin-mediated proteolysis, p53 signaling, cell cycle and apoptosis), which occurred in a dose-dependent manner. A 24-h pretreatment with SeM was shown to reduce the radiation effects by mitigating stress-related signaling pathways and downregulating certain genes associated with cell adhesion. The mechanism by which SeM prevents radiation-induced transformation in vitro may involve the suppression of the expression of genes associated with stress-related signaling and certain cell adhesion events.

Design of potent poxvirus inhibitors of the heterodimeric processivity factor required for viral replication.

Smallpox constitutes a major bioterrorism threat, which underscores the need to develop antiviral drugs for rapid response in the event of an attack. Viral processivity factors are attractive drug targets in being both specific and essential for their cognate DNA polymerases to synthesize extended strands of DNA. An in silico model of the vacinnia virus processivity factor, comprised of the A20 and D4 heterocomplex, was constructed and used for lead optimization of an indole-based scaffold identified earlier from a high-throughput screening. On the basis of this model, a new class of potent antivirals against vaccinia virus was designed and synthesized, of which two (24a and 24b) exhibited superior improvement over the parent scaffold (IC50 = 42 and 46 vs 82000 nM, respectively). The ability of 24a to suppress vaccinia DNA synthesis is supported by the inhibition of late viral gene expression, as well as by the diminished incorporation of bromodeoxyuridine into viral replication factories.

Crystallization and preliminary X-ray diffraction analysis of three recombinant mutants of Vaccinia virus uracil DNA glycosylase.

Amino-acid residues located at a highly flexible area in the uracil DNA glycosylase of Vaccinia virus were mutated. In the crystal structure of wild-type D4 these residues lie at the dimer interface. Specifically, three mutants were generated: (i) residue Arg167 was replaced with an alanine (R167AD4), (ii) residues Glu171, Ser172 and Pro173 were substituted with three glycine residues (3GD4) and (iii) residues Glu171 and Ser172 were deleted (Δ171-172D4). Mutant proteins were expressed, purified and crystallized in order to investigate the effects of these mutations on the structure of the protein.

Identification of protein-protein interaction inhibitors targeting vaccinia virus processivity factor for development of antiviral agents.

Poxvirus uracil DNA glycosylase D4 in association with A20 and the catalytic subunit of DNA polymerase forms the processive polymerase complex. The binding of D4 and A20 is essential for processive polymerase activity. Using an AlphaScreen assay, we identified compounds that inhibit protein-protein interactions between D4 and A20. Effective interaction inhibitors exhibited both antiviral activity and binding to D4. These results suggest that novel antiviral agents that target the protein-protein interactions between D4 and A20 can be developed for the treatment of infections with poxviruses, including smallpox.

Identification of inhibitors that block vaccinia virus infection by targeting the DNA synthesis processivity factor D4.

Smallpox was globally eradicated 30 years ago by vaccination. The recent threat of bioterrorism demands the development of improved vaccines and novel therapeutics to effectively preclude a reemergence of smallpox. One new therapeutic target is the vaccinia poxvirus processivity complex, comprising D4 and A20 proteins that enable the viral E9 DNA polymerase to synthesize extended strands. Five compounds identified from an AlphaScreen assay designed to disrupt A20:D4 binding were shown to be effective in: (i) blocking vaccinia processive DNA synthesis in vitro, (ii) preventing cellular infection with minimal cytotoxicity, and (iii) binding to D4, as evidenced by ThermoFluor. The EC(50) values for inhibition of viral infectivity ranged from 9.6 to 23 µM with corresponding selectivity indices (cytotoxicity CC(50)/viral infectivity EC(50)) of 3.9 to 17.8. The five compounds are thus potential therapeutics capable of halting smallpox DNA synthesis and infectivity through disruptive action against a component of the vaccinia processivity complex.

Induction of cytokine gene expression in human thyroid epithelial cells irradiated with HZE particles (iron ions).

Gene expression profiles were examined using cDNA microarray technology in human thyroid epithelial (Htori-3) cells exposed to a low, non-toxic dose (10 cGy) of radiation from HZE particles in the form of iron ions in the absence or presence of selenomethionine (SeM). A total of 215 genes were differentially regulated 2 h after exposure to a 10-cGy dose of iron-ion radiation. In the microarray analysis, SeM had profound effects on the radiation-induced expression of several specific genes, which includes PLAU, IGFBP3, FOLR1, B4GALT1 and COL1A1. Of particular interest to us was a gene cluster, "secreted proteins", that was up-regulated after radiation exposure. Seven up-regulated genes of this gene cluster fall within the chemokine/cytokine gene cluster, namely, CXCL1, CXCL2, IL6, IL11, IL8, IL24 and TGFbeta2. In microarray studies, the radiation-induced up-regulated expression of some these genes encoding cytokine/chemokine proteins was significantly decreased by SeM treatment. For IL8, TGFbeta2, CXCL1 and CXCL2, these observations were validated by qPCR techniques. It is concluded that SeM can regulate ionizing radiation-induced gene expression and may serve as an effective countermeasure for some of the acute inflammatory/immune responses induced by low-dose HZE-particle radiation.

Protective effects of dietary antioxidants on proton total-body irradiation-mediated hematopoietic cell and animal survival.

Abstract Dietary antioxidants have radioprotective effects after gamma-radiation exposure that limit hematopoietic cell depletion and improve animal survival. The purpose of this study was to determine whether a dietary supplement consisting of l-selenomethionine, vitamin C, vitamin E succinate, alpha-lipoic acid and N-acetyl cysteine could improve survival of mice after proton total-body irradiation (TBI). Antioxidants significantly increased 30-day survival of mice only when given after irradiation at a dose less than the calculated LD(50/30); for these data, the dose-modifying factor (DMF) was 1.6. Pretreatment of animals with antioxidants resulted in significantly higher serum total white blood cell, polymorphonuclear cell and lymphocyte cell counts at 4 h after 1 Gy but not 7.2 Gy proton TBI. Antioxidants significantly modulated plasma levels of the hematopoietic cytokines Flt-3L and TGFbeta1 and increased bone marrow cell counts and spleen mass after TBI. Maintenance of the antioxidant diet resulted in improved recovery of peripheral leukocytes and platelets after sublethal and potentially lethal TBI. Taken together, oral supplementation with antioxidants appears to be an effective approach for radioprotection of hematopoietic cells and improvement of animal survival after proton TBI.

Iron-sulfur cluster biosynthesis: characterization of IscU-IscS complex formation and a structural model for sulfide delivery to the [2Fe-2S] assembly site.

Recent work on the bacterial iron-sulfur cluster (isc) family of gene products, and eukaryotic homologs, has advanced the molecular understanding of cellular mechanisms of iron-sulfur cluster biosynthesis. Members of the IscS family are pyridoxyl-5'-phosophate dependent proteins that deliver inorganic sulfide during assembly of the [2Fe-2S] cluster on the IscU scaffold protein. Herein it is demonstrated through calorimetry, fluorescence, and protein stability measurements that Thermotoga maritima IscS forms a 1:1 complex with IscU in a concentration-dependent manner (K(D) varying from 6 to 34 microM, over an IscS concentration range of approximately 2-50 microM). Docking simulations of representative IscU and IscS proteins reveal critical contact surfaces at the N-terminal helix of IscU and a C-terminal loop comprising a chaperone binding domain. Consistent with the isothermal titration calorimetry results described here, an overall dominant contribution of charged surfaces with a change in the molar heat capacity of binding, DeltaC(p) approximately 199.8 kcal K(-1) mol(-1), is observed that accounts for approximately 10% of the total accessible surface area at the binding interface. Both apo and holo IscUs and homologs were found to bind to IscS in an enthalpically driven reaction with comparable K(D) values. Both helix and loop regions are highly conserved among phylogenetically diverse organisms from a pool of archael, bacterial, fungal, and mammalian representatives.