The hidden thermodynamics of a zinc finger

The Zn finger provides a model for studies of protein structure and stability. Its core contains a conserved phenylalanine residue adjoining three architectural elements: a beta-hairpin, an alpha-helix and a tetrahedral Zn(2+)-binding site. Here, we demonstrate that the consensus Phe is not required for high-affinity Zn(2+) binding but contributes to the specification of a precise DNA-binding surface. Substitution of Phe by leucine in a ZFY peptide permits Zn(2+)-dependent folding. Although a native-like structure is retained, structural fluctuations lead to attenuation of selected nuclear Overhauser enhancements and accelerated amide proton exchange. Surprisingly, wild-type Zn affinity is maintained by entropy-enthalpy compensation (EEC): a hidden entropy penalty (TDeltaDeltaS 7kcal/mol) is balanced by enhanced enthalpy of association (DeltaDeltaH -7kcal/mol) at 25 degrees C. Because the variant is less well ordered than the Phe-anchored domain, the net change in entropy is opposite to the apparent change in configurational entropy. By analogy to the thermodynamics of organometallic complexation, we propose that EEC arises from differences in solvent reorganization. Exclusion of Leu among biological sequences suggests an evolutionary constraint on the dynamics of a Zn finger.     

Solvation and the hidden thermodynamics of a zinc finger probed by nonstandard repair of a protein crevice

The classical Zn finger contains a phenylalanine at the crux of its three architectural elements: a beta-hairpin, an alpha-helix, and a Zn(2+)-binding site. Surprisingly, phenylalanine is not required for high-affinity Zn2+ binding, but instead contributes to the specification of a precise DNA-binding surface. Substitution of phenylalanine by leucine leads to a floppy but native-like structure whose Zn affinity is maintained by marked entropy-enthalpy compensation (DeltaDeltaH -8.3 kcal/mol and -TDeltaDeltaS 7.7 kcal/mol). Phenylalanine and leucine differ in shape, size, and aromaticity. To distinguish which features correlate with dynamic stability, we have investigated a nonstandard finger containing cyclohexanylalanine at this site. The structure of the nonstandard finger is similar to that of the native domain. The cyclohexanyl ring assumes a chair conformation, and conformational fluctuations characteristic of the leucine variant are damped. Although the nonstandard finger exhibits a lower affinity for Zn2+ than does the native domain (DeltaDeltaG -1.2 kcal/mol), leucine-associated perturbations in enthalpy and entropy are almost completely attenuated (DeltaDeltaH -0.7 kcal/mol and -TDeltaDeltaS -0.5 kcal/mol). Strikingly, global changes in entropy (as inferred from calorimetry) are in each case opposite in sign from changes in configurational entropy (as inferred from NMR). This seeming paradox suggests that enthalpy-entropy compensation is dominated by solvent reorganization rather than nominal molecular properties. Together, these results demonstrate that dynamic and thermodynamic perturbations correlate with formation or repair of a solvated packing defect rather than type of physical interaction (aromatic or aliphatic) within the core.     

Emerging role of zinc finger protein A20 as a suppressor of hepatocellular carcinoma

Hepatocellular carcinoma (HCC), the third leading cause of cancer-associated mortality worldwide, is a major public health problem. Zinc finger protein A20 (A20), an acute phase response gene, is a potent inhibitor of NF-κB signaling. A20 serves a critical role in liver protection, including limiting inflammation following hepatic injury, stimulating hepatocyte growth, and preventing hepatic ischemia-reperfusion injury. A20 is also involved in different processes, including tumorigenesis, progression, and metastasis through multiple mechanisms. Accumulated studies have reported the clinical implications and biological relevance of A20 in the development and progression of HCC. The underlying mechanisms of A20 in HCC include inhibition of epithelial–mesenchymal transition, protein tyrosine kinase 2 activation and Rac family GTPase 1 activity. Combining liver protection with tumor inhibition is a unique advantage of A20, which has the potential to be a novel treatment for promoting liver regeneration following liver resection in patients with HCC with liver cirrhosis. This review discusses the hepato-protective effect of A20 on hepatocytes and its potential role in cancer development, particularly its suppressor effect on HCC.     

Overexpression and purification of single zinc finger peptides of human zinc finger protein ZNF191

ZNF191, a new human zinc finger protein, probably relates to some hereditary diseases and cancers. To obtain structural information of zinc finger domain a convenient method for obtaining milligram quantities of each zinc finger peptide of ZNF191 is necessary. Here, we report an Escherichia coli expression system for rapid and high-level expression of zinc finger 3 and zinc finger 4 of ZNF191. The gene of zinc finger 3 or zinc finger 4 was cloned into pET31b vector to allow expression of single zinc finger peptide as a ketosteroid isomerase (KSI) fusion protein. The KSI-single zinc finger fusion protein was overexpressed in the form of inclusion body, which can be purified by washing several times using buffer solutions, and then be cleaved directly by cyanogen bromide to release single zinc finger peptide. The more than 20mg/L yield of single zinc finger peptide was achieved with more than 95% purity by using YM ultrafiltration membranes. Circular dichroism spectra of these two single zinc finger peptides titrated with Zn(2+) ions demonstrate that they have different secondary structures.     

A small zinc finger thylakoid protein plays a role in maintenance of photosystem II in Arabidopsis thaliana

This work identifies LOW QUANTUM YIELD OF PHOTOSYSTEM II1 (LQY1), a Zn finger protein that shows disulfide isomerase activity, interacts with the photosystem II (PSII) core complex, and may act in repair of photodamaged PSII complexes. Two mutants of an unannotated small Zn finger containing a thylakoid membrane protein of Arabidopsis thaliana (At1g75690; LQY1) were found to have a lower quantum yield of PSII photochemistry and reduced PSII electron transport rate following high-light treatment. The mutants dissipate more excess excitation energy via nonphotochemical pathways than wild type, and they also display elevated accumulation of reactive oxygen species under high light. After high-light treatment, the mutants have less PSII-light-harvesting complex II supercomplex than wild-type plants. Analysis of thylakoid membrane protein complexes showed that wild-type LQY1 protein comigrates with the PSII core monomer and the CP43-less PSII monomer (a marker for ongoing PSII repair and reassembly). PSII repair and reassembly involve the breakage and formation of disulfide bonds among PSII proteins. Interestingly, the recombinant LQY1 protein demonstrates a protein disulfide isomerase activity. LQY1 is more abundant in stroma-exposed thylakoids, where key steps of PSII repair and reassembly take place. The absence of the LQY1 protein accelerates turnover and synthesis of PSII reaction center protein D1. These results suggest that the LQY1 protein may be involved in maintaining PSII activity under high light by regulating repair and reassembly of PSII complexes.     

Selectivity of arsenite interaction with zinc finger proteins

Arsenic is a carcinogenic element also used for the treatment of acute promyelocytic leukemia. The reactivity of proteins to arsenic must be associated with the various biological functions of As. Here, we investigated the selectivity of arsenite to zinc finger proteins (ZFPs) with different zinc binding motifs (C2H2, C3H, and C4). Single ZFP domain proteins were used for the direct comparison of the reactivity of different ZFPs. The binding constants and the reaction rates have been studied quantitatively. Results show that both the binding affinity and reaction rates of single-domain ZFPs follow the trend of C4 > C3H ? C2H2. Compared with the C2H2 motif ZFPs, the binding affinities of C3H and C4 motif ZFPs are nearly two orders of magnitude higher and the reaction rates are approximately two-fold higher. The formation of multi-domain ZFPs significantly enhances the reactivity of C2H2 type ZFPs, but has negligible effects on C3H and C4 ZFPs. Consequently, the reactivities of the three types of multi-domain ZFPs are rather similar. The 2D NMR spectra indicate that the As(III)-bound ZFPs are also unfolded, suggesting that arsenic binding interferes with the function of ZFPs.     

The plant zinc finger protein ZPT2-2 has a unique mode of DNA interaction.

ZPT2-2 is a DNA-binding protein of petunia that contains two canonical TFIIIA-type zinc finger motifs separated by a long linker. We previously reported that ZPT2-2 bound to two separate AGT core sites, with each zinc finger making contact with each core site. Here we present our further characterization of ZPT2-2 by using selected and amplified binding sequence imprinting and surface plasmon resonance analyses; together, these assays revealed some unusual features of the interaction between ZPT2-2 and DNA. These experiments allowed us to conclude that 1) the optimal binding sequence for the N-terminal zinc finger is AGC(T), and that of the C-terminal one is CAGT; 2) multiple arrangements of the two core sites accommodate binding; and 3) the spacing between the two core sites affects the binding affinity. In light of these observations, we propose a new model for the DNA-ZPT2-2 interaction. Further, consistent with this model, a high affinity binding site for ZPT2-2 was found in the promoter region of the ZPT2-2 gene. This site may serve as a cis-element for the autoregulation of ZPT2-2 gene expression.