Relative domain orientation of the L289K HIV-1 reverse transcriptase monomer.

HIV-1 reverse transcriptase (RT) is a heterodimer comprised p66 and p51 subunits (p66/p51). Several single amino acid substitutions in RT, including L289K, decrease p66/p51 dimer affinity, and reduce enzymatic functioning. Here, small-angle X-ray scattering (SAXS) with proton paramagnetic relaxation enhancement (PRE), 19 F site-specific NMR, and size exclusion chromatography (SEC) were performed for the p66 monomer with the L289K mutation, p66L289K . NMR and SAXS experiments clearly elucidated that the thumb and RNH domains in the monomer do not rigidly interact with each other but are spatially close to the RNH domain. Based on this structural model of the monomer, p66L289K and p51 were predicted to form a heterodimer while p66 and p51L289K not. We tested this hypothesis by SEC analysis of p66 and p51 containing L289K in different combinations and clearly demonstrated that L289K substitution in the p51 subunit, but not in the p66 subunit, reduces p66/p51 formation. Based on the derived monomer model and the importance of the inter-subunit RNH-thumb domain interaction in p66/p51, validated by SEC, the mechanism of p66 homodimer formation was discussed.

The reduced form of the antibody CH2 domain.

Among the immunoglobulin domains, the CH2 domain has the lowest thermal stability, which also depends on amino acid sequence and buffer conditions. To further identify factors that influence CH2 folding and stability, we characterized the domain in the reduced form using differential scanning fluorimetry and nuclear magnetic resonance. We show that the CH2 domain can fold, similarly to the disulfide-bridged form, without forming a disulfide-bridge, even though the protein contains two Cys residues. Although the reduced form exhibits thermal stability more than 15°C lower than the disulfide-bridged form, it does not undergo immediate full oxidization. To explain this phenomenon, we compared CH2 oxidization at different conditions and demonstrate a need for significant fluctuation of the folded conformation to enhance CH2 disulfide-bridge formation. We conclude that, since CH2 can be purified as a folded, semi-stable, reduced protein that can coexist with the oxidized form, verification of the level of oxidization at each step is critical in CH2 engineering studies.

Large Multidomain Protein NMR: HIV-1 Reverse Transcriptase Precursor in Solution.

NMR studies of large proteins, over 100 kDa, in solution are technically challenging and, therefore, of considerable interest in the biophysics field. The challenge arises because the molecular tumbling of a protein in solution considerably slows as molecular mass increases, reducing the ability to detect resonances. In fact, the typical 1H-13C or 1H-15N correlation spectrum of a large protein, using a 13C- or 15N-uniformly labeled protein, shows severe line-broadening and signal overlap. Selective isotope labeling of methyl groups is a useful strategy to reduce these issues, however, the reduction in the number of signals that goes hand-in-hand with such a strategy is, in turn, disadvantageous for characterizing the overall features of the protein. When domain motion exists in large proteins, the domain motion differently affects backbone amide signals and methyl groups. Thus, the use of multiple NMR probes, such as 1H, 19F, 13C, and 15N, is ideal to gain overall structural or dynamical information for large proteins. We discuss the utility of observing different NMR nuclei when characterizing a large protein, namely, the 66 kDa multi-domain HIV-1 reverse transcriptase that forms a homodimer in solution. Importantly, we present a biophysical approach, complemented by biochemical assays, to understand not only the homodimer, p66/p66, but also the conformational changes that contribute to its maturation to a heterodimer, p66/p51, upon HIV-1 protease cleavage.

Determinants of Active-Site Inhibitor Interaction with HIV-1 RNase H.

The ribonuclease H (RNH) activity of HIV-1 reverse transcriptase (RT) is essential for viral replication and can be a target for drug development. Yet, no RNH inhibitor to date has substantial antiviral activity to allow advancement into clinical development. Herein, we describe our characterization of the detailed binding mechanisms of RNH active-site inhibitors, YLC2-155 and ZW566, that bind to the RNH domain through divalent metal ions, using NMR, molecular docking, and quantum mechanical calculations. In the presence of Mg2+, NMR spectra of RNH exhibited split (two) resonances for some residues upon inhibitor binding, suggesting two binding modes, an observation consistent with the docking results. The relative populations of the two binding conformers were independent of inhibitor or Mg2+ concentration, with one conformation consistently more favored. In our docking study, one distinctive pose of ZW566 showed more interactions with surrounding residues of RNH compared to the analogous binding pose of YLC2-155. Inhibitor titration experiments revealed a lower dissociation constant for ZW566 compared to YLC2-155, in agreement with its higher inhibitory activity. Mg2+ titration data also indicated a stronger dependence on Mg2+ for the RNH interaction with ZW566 compared to YLC2-155. Combined docking and quantum mechanical calculation results suggest that stronger metal coordination as well as more protein-inhibitor interactions may account for the higher binding affinity of ZW566. These findings support the idea that strategies for the development of potent competitive active site RNH inhibitors should take into account not only metal-inhibitor coordination but also protein-inhibitor interaction and conformational selectivity.

Conformational Changes in HIV-1 Reverse Transcriptase that Facilitate Its Maturation.

HIV-1 reverse transcriptase (RT) is translated as part of the Gag-Pol polyprotein that is proteolytically processed by HIV-1 protease (PR) to finally become a mature heterodimer, composed of a p66 and a p66-derived 51-kDa subunit, p51. Our previous work suggested that tRNALys3 binding to p66/p66 introduces conformational changes in the ribonuclease (RNH) domain of RT that facilitate efficient cleavage of p66 to p51 by PR. In this study, we characterized the conformational changes in the RNH domain of p66/p66 imparted by tRNALys3 using NMR. Moreover, the importance of tRNALys3 in RT maturation was confirmed in cellulo by modulating the levels of Lys-tRNA synthetase, which affects recruitment of tRNALys3 to the virus. We also employed nonnucleoside RT inhibitors, to modulate the p66 dimer-monomer equilibrium and monitor the resulting structural changes. Taken together, our data provide unique insights into the conformational changes in p66/p66 that drive PR cleavage.

Binding thermodynamics of metal ions to HIV-1 ribonuclease H domain.

Metal-protein interactions are not necessarily tight in many transient biological processes, such as cellular signaling, enzyme regulation, and molecular recognition. Here, we analyzed the binding thermodynamics and characterized the structural effect of divalent metal ions, i.e. Mn2+, Zn2+, and Mg2+, to the isolated ribonuclease H (RNH) of human immunodeficiency virus (HIV) using isothermal titration calorimetry (ITC) and circular dichroism. The binding thermodynamics of Mg2+ to RNH was determined using competition ITC experiments, and the binding affinity of Mg2+ was found to be about 40- and 400-times lower than those of Mn2+ and of Zn2+, respectively. The structural analysis showed that Mg2+ binding had little effect on the thermal stability of RNH, while Zn2+ and Mn2+ binding increased the stability. The thermodynamic characteristics of RNH metal binding, compared to intact HIV reverse transcriptase, and a possible mechanism of conformational change induced upon metal ion binding, in correlation with the structure-function relationship, are discussed.

Cisplatin reacts with histone H1 and the adduct forms a ternary complex with DNA.

Cisplatin is an anticancer drug widely used in clinics; it induces the apoptosis of cancer cells by targeting DNA. However, its interaction with proteins has been found to be crucial in modulating the pre and post-target activity. Nuclear DNA is tightly assembled with histone proteins to form nucleosomes in chromatin; this can impede the drug to access DNA. On the other hand, the linker histone H1 is considered 'the gate to nucleosomal DNA' due to its exposed location and dynamic conformation; therefore, this protein can influence the platination of DNA. In this study, we performed a reaction of cisplatin with histone H1 and investigated the interaction of the H1/cisplatin adduct with DNA. The reactions were conducted on the N-terminal domains of H1.4 (sequence 1-90, H1N90) and H1.0 (sequence 1-7, H1N7). The results show that H1 readily reacts with cisplatin and generates bidentate and tridentate adducts, with methionine and glutamate residues as the preferential binding sites. Chromatographic and NMR analyses show that the platination rate of H1 is slightly higher than that of DNA and the platinated H1 can form H1-cisplatin-DNA ternary complexes. Interestingly, cisplatin is more prone to form H1-Pt-DNA ternary complexes than trans-oriented platinum agents. The formation of H1-cisplatin-DNA ternary complexes and their preference for cis- over trans-oriented platinum agents suggest an important role of histone H1 in the mechanism of action of cisplatin.

The HIV-1 p66 homodimeric RT exhibits different conformations in the binding-competent and -incompetent NNRTI site.

Non-nucleoside inhibitors of human immunodeficiency virus type 1 reverse transcriptase (RT), NNRTIs, which bind to the p66/p51 heterodimeric RT, also interact with the p66/p66 homodimer, whose structure is unknown. 19 F nuclear magnetic resonance of a single 4-trifluoromethylphenylalanine (tfmF) residue, incorporated into the NNRTI binding pocket of the p66/p66 homodimer at position 181, was used to investigate NNRTI binding. In the NNRTI-bound homodimer complex, two different 19 F signals are observed, with the resonance frequencies matching those of the NNRTI-bound p66/p51 heterodimer spectra, in which the individual p66-subunit or p51-subunit were labeled with tfmF at positions 181. These data suggest that the NNRTI-bound p66/p66 homodimer conformation, particularly around residue 181, is very similar to that in the p66/p51 heterodimer, explaining why NNRTI binding to p66/p66 enhances dimer formation.

Copper-finger protein of Sp1: the molecular basis of copper sensing.

The cellular copper level is strictly regulated since excessive copper is harmful to cells. It has been proposed that the expression of copper transport protein hCtr1 is transcriptionally regulated by specificity protein 1 (Sp1) in response to the cellular copper level. However, it is not known how Sp1, a zinc-finger-protein (ZFP), can sense copper ions in cells. Here we found that Sp1 demonstrates high binding affinity to cuprous ions, even stronger than Cu-Atox1 binding. Cu(i) can displace Zn(ii) in Sp1, resulting in a well-folded 'Copper-Finger-Protein' (CFP). Although only very little structural alteration occurs upon copper binding, CFP cannot recognize the promoter of hCtr1, therefore copper binding interrupts the transcription. This result indicates that, in addition to apo-to-holo alteration, metal substitution can also lead to transcriptional switch in metal sensing. This work provides insight into the copper sensing mechanism of Sp1 at the molecular level.

A 2-Hydroxyisoquinoline-1,3-Dione Active-Site RNase H Inhibitor Binds in Multiple Modes to HIV-1 Reverse Transcriptase.

The RNase H (RNH) function of HIV-1 reverse transcriptase (RT) plays an essential part in the viral life cycle. We report the characterization of YLC2-155, a 2-hydroxyisoquinoline-1,3-dione (HID)-based active-site RNH inhibitor. YLC2-155 inhibits both polymerase (50% inhibitory concentration [IC50] = 2.6 µM) and RNH functions (IC50 = 0.65 µM) of RT but is more effective against RNH. X-ray crystallography, nuclear magnetic resonance (NMR) analysis, and molecular modeling were used to show that YLC2-155 binds at the RNH-active site in multiple conformations.

A Combined NMR and SAXS Analysis of the Partially Folded Cataract-Associated V75D γD-Crystallin.

A cataract is a pathological condition characterized by the clouding of the normally clear eye lens brought about by deposition of crystallin proteins in the lens fiber cells. These protein aggregates reduce visual acuity by scattering or blocking incoming light. Chemical damage to proteins of the crystallin family, accumulated over a lifetime, leads to age-related cataract, whereas inherited mutations are associated with congenital or early-onset cataract. The V75D mutant of γD-crystallin is associated with congenital cataract in mice and was previously shown to un/fold via a partially folded intermediate. Here, we structurally characterized the stable equilibrium urea unfolding intermediate of V75D at the ensemble level using solution NMR and small-angle x-ray scattering. Our data show that, in the intermediate, the C-terminal domain retains a folded conformation that is similar to the native wild-type protein, whereas the N-terminal domain is unfolded and comprises an ensemble of random conformers, without any detectable residual structural propensities.

Human ßB2-Crystallin Forms a Face-en-Face Dimer in Solution: An Integrated NMR and SAXS Study.

ßγ-Crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each ßγ-crystallin comprises two homologous domains, which are connected by a short linker. γ-Crystallins are monomeric, while ß-crystallins crystallize as dimers and multimers. In the crystal, human ßB2-crystallin is a domain-swapped dimer while the N-terminally truncated ßB1-crystallin forms a face-en-face dimer. Combining and integrating data from multi-angle light scattering, nuclear magnetic resonance, and small-angle X-ray scattering of full-length and terminally truncated human ßB2-crystallin in solution, we show that both these ßB2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no inter-molecular paramagnetic relaxation enhancement effects compatible with domain swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human ßB2-crystallin is not domain swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated ßB1-crystallin.

Glutathione selectively modulates the binding of platinum drugs to human copper chaperone Cox17.

The copper chaperone Cox17 (cytochrome c oxidase copper chaperone) has been shown to facilitate the delivery of cisplatin to mitochondria, which contributes to the overall cytotoxicity of the drug [Zhao et al. (2014) Chem. Commun. 50: , 2667-2669]. Kinetic data indicate that Cox17 has reactivity similar to glutathione (GSH), the most abundant thiol-rich molecule in the cytoplasm. In the present study, we found that GSH significantly modulates the reaction of platinum complexes with Cox17. GSH enhances the reactivity of three anti-cancer drugs (cisplatin, carboplatin and oxaliplatin) to Cox17, but suppresses the reaction of transplatin. Surprisingly, the pre-formed cisplatin-GSH adducts are highly reactive to Cox17; over 90% platinum transfers from GSH to Cox17. On the other hand, transplatin-GSH adducts are inert to Cox17. These different effects are consistent with the drug activity of these platinum complexes. In addition, GSH attenuates the protein aggregation of Cox17 induced by platination. These results indicate that the platinum-protein interactions could be substantially influenced by the cellular environment.

The reaction of arsenite with proteins relies on solution conditions.

Arsenic is a biologically interesting element with both antitumor and carcinogenic effects. Zinc finger proteins (ZFPs) have been confirmed to be the cellular targets of arsenite; however, arsenite inhibits ZFPs much less efficiently in vitro than in vivo. The molecular mechanism of this difference is unknown. In this work, we found that the reaction of arsenite with ZFPs relies on the presence of small biomolecules such as glutathione (GSH), histidine, and cysteine (Cys). The weak acidity also enhances the reaction. Further study shows that the coordination of zinc is much more susceptible than that of arsenic to these solution conditions, which enhance the competition of arsenic. Notably, different from C3H-type ZFPs, the C2H2-type ZFPs are more significantly influenced by the presence of thiol-containing molecules in the reaction. GSH and Cys can facilitate the reaction by participation of the coordination to As(III) together with C2H2-type ZFPs. Consequently, the reactions are promoted both thermodynamically and kinetically via the formation of ternary complexes GSH-As-ZFP or Cys-As-ZFP. These results indicate that the reactions between arsenite and proteins are considerably modulated by environments such as the small biomolecules and the acidity of the solution. This finding clarifies the discrepancy observed in the reactions of arsenite in vitro versus in cells, and provides an insight into the molecular mechanism of arsenite.

Copper binding modulates the platination of human copper chaperone Atox1 by antitumor trans-platinum complexes.

The transport system of platinum-based anticancer agents is crucial for drug sensitivity. Increasing evidence indicates that the copper transport system is also involved in the cellular influx and efflux of platinum drugs. The copper chaperone Atox1 has been shown to bind to cisplatin in vitro and in cells. Previous results reveal that copper binding promotes the reaction between Atox1 and cisplatin. Here, we have performed detailed solution NMR and ESI-MS experiments to investigate the effect of Cu(i) binding on the reactions of Atox1 with two antitumor active trans-platinum agents, trans-EE and trans-PtTz. Results indicate that, similar to the reaction of cisplatin, copper coordination also enhances the platination of Atox1 by two trans-platinum complexes, and platinum binds to the copper coordinating residues. However, copper binding promotes the trans-platinum transfer from Atox1 to dithiothreitol (DTT). This result is in contrast to the reaction of Atox1 with cisplatin, in which the presence of copper largely suppresses the platination of DTT. Additionally, both apo- and Cu(I)-Atox1 react faster with trans-platinum complexes than with cisplatin, however, less protein aggregation is observed in the reaction of trans-platinum complexes. These results indicate that the roles of Atox1 in the regulation of cellular trafficking of platinum drugs are dependent on the coordination configurations.

Cisplatin binds to human copper chaperone Cox17: the mechanistic implication of drug delivery to mitochondria.

Cox17 facilitates the platinum accumulation in mitochondria, which contributes to the overall cytotoxicity of cisplatin.

Copper binding promotes the interaction of cisplatin with human copper chaperone Atox1.

Cu(I) binding promotes the platination of Atox1, although cisplatin binds to the copper coordination sites. In addition, Cu(I) binding enhances the competition of Atox1 with DTT in the reaction of cisplatin. These results indicate that cuprous ions could regulate the cellular trafficking of cisplatin.

Conserved residue modulates copper-binding properties through structural dynamics in human copper chaperone Atox1.

The human copper chaperone Atox1 plays a central role in the transport of copper in cells. It has been reported that the conserved residue Lys60 contributes to the heterocomplex stability of Atox1 with its target protein ATPase, and that the K60A mutation could diminish the copper transfer. In this work, we carried out the structure determination and dynamic analysis of Atox1 with the K60A mutation in order to elucidate the role of the conserved residue Lys60 in the copper transport. Results show that the K60A mutation results in crucial secondary structure rearrangements and side-chain orientation alteration of the metal-binding residues in Atox1. Protein dynamic studies reveal that the K60A mutation leads to increased overall flexibility, and a significant difference in dynamic properties of the metal-binding sites. The structure and dynamic changes cause a decrease in the copper-binding stability of the K60A mutant. In addition, Cu(i)-mediated hetero-protein interactions with ATP7A are present in the metal transfer of both Atox1 variants, although copper transfer is accompanied with smaller structural alteration in the K60A mutant. These results indicate that Lys60 is crucial in maintaining the structure and dynamic properties of Atox1.

Interaction between platinum complexes and the C-terminal motif of human copper transporter 1.

Human copper transporter 1 (hCTR1) facilitates the cellular uptake of cisplatin, and the extracellular N-terminal domain has been proven to coordinate to platinum drugs. It has been reported that the intracellular C-terminal motif is crucial for the function of hCTR1 in cisplatin influx. In this work, we conduct reactions of the intracellular motif with platinum drugs. The octapeptide from the C-terminal domain of hCTR1 is used, and the reactions are investigated using ultraviolet, high-performance liquid chromatography, electrospray ionization mass spectrometry, and nuclear magnetic resonance spectroscopy. Results show that the C8 peptide is highly reactive to cisplatin and oxaliplatin, and the -HCH sequence is the most favorable binding site of platinum agents. Cisplatin first binds to the cysteine residues in the reaction with the C8 peptide. The ammine ligand, even trans to a thiol ligand, can remain coordinated in platination adducts for a >12 h reaction. Intramolecular platinum migration was observed in the C8 peptide, and the ammine ligands remain coordinated to platinum during this process. This result indicates that hCTR1 can transfer cisplatin in the active form through a trans chelation process. These findings provide insight into the mechanism of the C-terminus of hCTR1 in the transfer of platinum drugs from the trimeric pore of hCTR1 to the cytoplasm.

Discovery of a siderophore export system essential for virulence of Mycobacterium tuberculosis.

Iron is an essential nutrient for most bacterial pathogens, but is restricted by the host immune system. Mycobacterium tuberculosis (Mtb) utilizes two classes of small molecules, mycobactins and carboxymycobactins, to capture iron from the human host. Here, we show that an Mtb mutant lacking the mmpS4 and mmpS5 genes did not grow under low iron conditions. A cytoplasmic iron reporter indicated that the double mutant experienced iron starvation even under high-iron conditions. Loss of mmpS4 and mmpS5 did not change uptake of carboxymycobactin by Mtb. Thin layer chromatography showed that the ΔmmpS4/S5 mutant was strongly impaired in biosynthesis and secretion of siderophores. Pull-down experiments with purified proteins demonstrated that MmpS4 binds to a periplasmic loop of the associated transporter protein MmpL4. This interaction was corroborated by genetic experiments. While MmpS5 interacted only with MmpL5, MmpS4 interacted with both MmpL4 and MmpL5. These results identified MmpS4/MmpL4 and MmpS5/MmpL5 as siderophore export systems in Mtb and revealed that the MmpL proteins transport small molecules other than lipids. MmpS4 and MmpS5 resemble periplasmic adapter proteins of tripartite efflux pumps of Gram-negative bacteria, however, they are not only required for export but also for efficient siderophore synthesis. Membrane association of MbtG suggests a link between siderophore synthesis and transport. The structure of the soluble domain of MmpS4 (residues 52-140) was solved by NMR and indicates that mycobacterial MmpS proteins constitute a novel class of transport accessory proteins. The bacterial burden of the mmpS4/S5 deletion mutant in mouse lungs was lower by 10,000-fold and none of the infected mice died within 180 days compared to wild-type Mtb. This is the strongest attenuation observed so far for Mtb mutants lacking genes involved in iron utilization. In conclusion, this study identified the first components of novel siderophore export systems which are essential for virulence of Mtb.