Human nucleotide excision repair protein XPA: extended X-ray absorption fine-structure evidence for a metal-binding domain

From zinc homeostasis to disease progression: Unveiling the neurodegenerative puzzle

Zinc is a crucial trace element in the human body, playing a role in various physiological processes such as oxidative stress, neurotransmission, protein synthesis, and DNA repair. The zinc transporters (ZnTs) family members are responsible for exporting intracellular zinc, while Zrt- and Irt-like proteins (ZIPs) are involved in importing extracellular zinc. These processes are essential for maintaining cellular zinc homeostasis. Imbalances in zinc metabolism have been linked to the development of neurodegenerative diseases. Disruptions in zinc levels can impact the survival and activity of neurons, thereby contributing to the progression of neurodegenerative diseases through mechanisms like cell apoptosis regulation, protein phase separation, ferroptosis, oxidative stress, and neuroinflammation. Therefore, conducting a systematic review of the regulatory network of zinc and investigating the relationship between zinc dysmetabolism and neurodegenerative diseases can enhance our understanding of the pathogenesis of these diseases. Additionally, it may offer new insights and approaches for the treatment of neurodegenerative diseases.

Structures of Silver Fingers and a Pathway to Their Genotoxicity

Recently, we demonstrated that Ag^I can directly replace Zn^II in zinc fingers (ZFs). The cooperative binding of Ag^I to ZFs leads to a thermodynamically irreversible formation of silver clusters destroying the native ZF structure. Thus, a reported loss of biological function of ZF proteins is a likely consequence of such replacement. Here, we report an X-ray absorption spectroscopy (XAS) study of Ag_n S_n clusters formed in ZFs to probe their structural features. Selective probing of the local environment around Ag^I by XAS showed the predominance of digonal Ag^I coordination to two sulfur donors, coordinated with an average Ag-S distance at 2.41 Å. No Ag-N bonds were present. A mixed AgS₂ /AgS₃ geometry was found solely in the CCCH Ag^I -ZF. We also show that cooperative replacement of Zn^II ions with the studied Ag₂ S₂ clusters occurred in a three-ZF transcription factor protein 1MEY#, leading to a dissociation of 1MEY# from the complex with its cognate DNA.

Molecular mechanisms of xeroderma pigmentosum (XP) proteins

Nucleotide excision repair (NER) is a highly versatile and efficient DNA repair process, which is responsible for the removal of a large number of structurally diverse DNA lesions. Its extreme broad substrate specificity ranges from DNA damages formed upon exposure to ultraviolet radiation to numerous bulky DNA adducts induced by mutagenic environmental chemicals and cytotoxic drugs used in chemotherapy. Defective NER leads to serious diseases, such as xeroderma pigmentosum (XP). Eight XP complementation groups are known of which seven (XPA-XPG) are caused by mutations in genes involved in the NER process. The eighth gene, XPV, codes for the DNA polymerase ɳ, which replicates through DNA lesions in a process called translesion synthesis (TLS). Over the past decade, detailed structural information of these DNA repair proteins involved in eukaryotic NER and TLS have emerged. These structures allow us now to understand the molecular mechanism of the NER and TLS processes in quite some detail and we have begun to understand the broad substrate specificity of NER. In this review, we aim to highlight recent advances in the process of damage recognition and repair as well as damage tolerance by the XP proteins.

Zinc finger proteins as templates for metal ion exchange and ligand reactivity. Chemical and biological consequences

Zinc finger reactions with inorganic ions and coordination compounds are as diverse as the zinc fingers themselves. Use of metal ions such as Co(2+) and Cd(2+) has given structural, thermodynamic and kinetic information on zinc fingers and zinc-finger-DNA/RNA interactions. It is a general truism that alteration of the coordination sphere in the finger environment will disrupt the recognition with DNA/RNA and this has implications for mechanism of toxicity and carcinogenesis of metal ions. Structural zinc fingers are susceptible to electrophilic attack and the recognition that the coordination sphere of inorganic compounds may be modulated for control of electrophilic attack on zinc fingers raises the possibility of systematic studies of zinc fingers as drug targets using inorganic chemistry. Some inorganic compounds such as those of As(III) and Au(I) may exert their biological effects through inactivation of zinc fingers and novel approaches to specifically attack the zinc-bound ligands using Co(III)-Schiff bases and Platinum(II)-Nucleobase compounds have been proposed. The genomic importance of zinc fingers suggests that the "coordination chemistry" of zinc fingers themselves is ripe for exploration to design new targets for medicinal inorganic chemistry.

Identification of novel small molecule inhibitors of the XPA protein using in silico based screening

The nucleotide excision repair pathway catalyzes the removal of bulky adduct damage from DNA and requires the activity of more than 30 individual proteins and complexes. A diverse array of damage can be recognized and removed by the NER pathway including UV-induced adducts and intrastrand adducts induced by the chemotherapeutic compound cisplatin. The recognition of DNA damage is complex and involves a series of proteins including the xeroderma pigmentosum group A and C proteins and the UV-damage DNA binding protein. The xeroderma pigmentosum group A protein is unique in the sense that it is required for both transcription coupled and global genomic nucleotide excision repair. In addition, xeroderma pigmentosum group A protein is required for the removal of all types of DNA lesions repaired by nucleotide excision repair. Considering its importance in the damage recognition process, the minimal information available on the mechanism of DNA binding, and the potential that inhibition of xeroderma pigmentosum group A protein could enhance the therapeutic efficacy of platinum based anticancer drugs, we sought to identify and characterize small molecule inhibitors of the DNA binding activity of the xeroderma pigmentosum group A protein. In silico screening of a virtual small molecule library resulted in the identification of a class of molecules confirmed to inhibit the xeroderma pigmentosum group A protein-DNA interaction. Biochemical analysis of inhibition with varying DNA substrates revealed a common mechanism of xeroderma pigmentosum group A protein DNA binding to single-stranded DNA and cisplatin-damaged DNA.

XPA gene, its product and biological roles

The 31 kDa XPA protein is part of the core incision complex of the mammalian nucleotide excision repair (NER) system and interacts with DNA as well as with many other NER subunits. In the absence of XPA, no incision complex can form and no excision of damaged DNA damage occurs. A comparative analysis of the DNA-binding properties in the presence of different substrate conformations indicated that XPA protein interacts preferentially with kinked DNA backbones. The DNA-binding domain of XPA protein displays a positively charged deft that is involved in an indirect readout mechanism, presumably by detecting the increased negative potential encountered at sharp DNA bends. We propose that this indirect recognition function contributes to damage verification by probing the susceptibility of the DNA substrate to be kinked during the assembly of NER complexes.

DNA-XPA interactions: a (31)P NMR and molecular modeling study of dCCAATAACC association with the minimal DNA-binding domain (M98-F219) of the nucleotide excision repair protein XPA

Recent NMR-based, chemical shift mapping experiments with the minimal DNA-binding domain of XPA (XPA-MBD: M98-F219) suggest that a basic cleft located in the loop-rich subdomain plays a role in DNA-binding. Here, XPA-DNA interactions are further characterized by NMR spectroscopy from the vantage point of the DNA using a single-stranded DNA nonamer, dCCAATAACC (d9). Up to 2.5 molar equivalents of XPA-MBD was titrated into a solution of d9. A subset of (31)P resonances of d9 were observed to broaden and/or shift providing direct evidence that XPA-MBD binds d9 by a mechanism that perturbs the phosphodiester backbone of d9. The interior five residues of d9 broadened and/or shifted before (31)P resonances of phosphate groups at the termini, suggesting that when d9 is bound to XPA-MBD the internal residues assume a correlation time that is characteristic of the molecular weight of the complex while the residues at the termini undergo a fraying motion away from the surface of the protein on a timescale such that the line widths are more characteristic of the molecular weight of ssDNA. A molecular model of the XPA-MBD complex with d9 was calculated based on the (15)N (XPA-MBD) and (31)P (d9) chemical shift mapping studies and on the assumption that electrostatic interactions drive the complex formation. The model shows that a nine residue DNA oligomer fully covers the DNA-binding surface of XPA and that there may be an energetic advantage to binding DNA in the 3'-->5' direction rather than in the 5'-->3' direction (relative to XPA-MBD alpha-helix-3).

Human nucleotide excision repair protein XPA: NMR spectroscopic studies of an XPA fragment containing the ERCC1-binding region and the minimal DNA-binding domain (M59-F219)

XPA is a central protein component of nucleotide excision repair (NER), a ubiquitous, multi-component cellular pathway responsible for the removal and repair of many structurally distinct DNA lesions from the eukaryotic genome. The solution structure of the minimal DNA-binding domain of XPA (XPA-MBD: M98-F219) has recently been determined and chemical shift mapping experiments with 15N-labeled XPA-MBD show that XPA binds DNA along a basic surface located in the C-terminal loop-rich subdomain. Here, XPA-DNA interactions are further characterized using an XPA fragment containing the minimal DNA-binding domain plus the ERCC1-binding region (XPA-EM: M59-F219). The 15N/1H HSQC spectrum of XPA-EM closely maps onto the 15N/1H HSQC spectrum of XPA-MBD, suggesting the DNA-binding domain is intact in the larger XPA fragment. Such a conclusion is corroborated by chemical shift mapping experiments of XPA-EM with a single strand DNA oligomer, dCCAATAACC (d9), that show the same set of 15N/1H HSQC cross peaks are effected by the addition of DNA. However, relative to DNA-free XPA-MBD, the 15N/1H HSQC cross peaks of many of the basic residues in the loop-rich subdomain of DNA-free XPA-EM are less intense, or gone altogether, suggesting the acidic ERRC1-binding region of XPA-EM may associate transiently with the basic DNA-binding surface. While the DNA-binding domain in XPA-EM is structured and functional, 15N-edited NOESY spectra of XPA-EM indicate that the acidic ERRC1-binding region is unstructured. If the structural features observed for XPA-EM persist in XPA, transient intramolecular association of the ERCC1-binding domain with the DNA-binding region may play a role in the sequential assembly of the NER components.

Zinc and the gene

A significant portion of cellular zinc is found in the nucleus where it appears to be critically involved in maintaining genetic stability and in the process of gene expression. With regard to gene expression zinc functions mechanistically at several levels but recent interest has focussed especially on the involvement of zinc in DNA transcription through the activity of transcription factors which contain specific zinc-finger regions which bind to DNA and, in conjunction with other families of transcription factors, control cell proliferation, differentiation and cell death. Because of the central importance of zinc in cell division and growth, considerable attention is paid to zinc as an essential trace element and much has been written concerning dietary sources of zinc and recommended dietary intakes of the metal.

Xeroderma pigmentosum and related disorders: defects in DNA repair and transcription

The genetic disorders xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD) are all associated with defects in nucleotide excision repair (NER) of DNA damage. Their clinical features are very different, however, XP being a highly cancer-prone skin disorder, whereas CS and TTD are cancer-free multisystem disorders. All three are genetically complex, with at least eight complementation groups for XP (XP-A to -G and variant), five for CS (CS-A, CS-B, XP-B, XP-D, and XP-G), and three for TTD (XP-B, XP-D, and TTD-A). With the exception of the variant, the products of the XP genes are proteins involved in the different steps of NER, and comprise three damage-recognition proteins, two helicases, and two nucleases. The two helicases, XPB and XPD, are components of the basal transcription factor TFIIH, which has a dual role in NER and initiation of transcription. Different mutations in these genes can affect NER and transcription differentially, and this accounts for the different clinical phenotypes. Mutations resulting in defective repair without affecting transcription result in XP, whereas if transcription is also affected, TTD is the outcome. CS proteins are only involved in transcription-coupled repair, a subpathway of NER in which damage in the transcribed strands of active genes is rapidly and preferentially repaired. Current evidence suggests that they also have an important but not essential role in transcription. The variant form of XP is defective in a novel DNA polymerase, which is able to synthesise DNA past UV-damaged sites.

Three-dimensional structural views of damaged-DNA recognition: T4 endonuclease V, E. coli Vsr protein, and human nucleotide excision repair factor XPA

Genetic information is frequently disturbed by introduction of modified or mismatch bases into duplex DNA, and hence all organisms contain DNA repair systems to restore normal genetic information by removing such damaged bases or nucleotides and replacing them by correct ones. The understanding of this repair mechanism is a central subject in cell biology. This review focuses on the three-dimensional structural views of damaged DNA recognition by three proteins. The first protein is T4 endonuclease V (T4 endo V), which catalyzes the first reaction step of the excision repair pathway to remove pyrimidine-dimers (PD) produced within duplex DNA by UV irradiation. The crystal structure of this enzyme complexed with DNA containing a thymidine-dimer provided the first direct view of DNA lesion recognition by a repair enzyme, indicating that the DNA kink coupled with base flipping-out is important for damaged DNA recognition. The second is very short patch repair (Vsr) endonuclease, which recognizes a TG mismatch within the five base pair consensus sequence. The crystal structure of this enzyme in complex with duplex DNA containing a TG mismatch revealed a novel mismatch base pair recognition scheme, where three aromatic residues intercalate from the major groove into the DNA to strikingly deform the base pair stacking but the base flipping-out does not occur. The third is human nucleotide excision repair (NER) factor XPA, which is a major component of a large protein complex. This protein has been shown to bind preferentially to UV- or chemical carcinogen-damaged DNA. The solution structure of the XPA central domain, essential for the interaction of damaged DNA, was determined by NMR. This domain was found to be divided mainly into a (Cys)4-type zinc-finger motif subdomain for replication protein A (RPA) recognition and the carboxyl terminal subdomain responsible for DNA binding.

SRB, Isern NG, Lingbeck JM, Taylor JS, Wold MS, Gochin M, Spicer LD, Lowry DF, Kennedy MA. Interactions of human nucleotide excision repair protein XPA with DNA and RPA70 Delta C327: chemical shift mapping and 15N NMR relaxation studies

Human XPA is an essential component in the multienzyme nucleotide excision repair (NER) pathway. The solution structure of the minimal DNA binding domain of XPA (XPA-MBD: M98-F219) was recently determined [Buchko et al. (1998) Nucleic Acids Res. 26, 2779-2788, Ikegami et al. (1998) Nat. Struct. Biol. 5, 701-706] and shown to consist of a compact zinc-binding core and a loop-rich C-terminal subdomain connected by a linker sequence. Here, the solution structure of XPA-MBD was further refined using an entirely new class of restraints based on pseudocontact shifts measured in cobalt-substituted XPA-MBD. Using this structure, the surface of XPA-MBD which interacts with DNA and a fragment of the largest subunit of replication protein A (RPA70 Delta C327: M1-Y326) was determined using chemical shift mapping. DNA binding in XPA-MBD was highly localized in the loop-rich subdomain for DNA with or without a lesion [dihydrothymidine (dhT) or 6-4-thymidine-cytidine (64TC)], or with DNA in single- or double-stranded form, indicating that the character of the lesion itself is not the driving force for XPA binding DNA. RPA70 Delta C327 was found to contact regions in both the zinc-binding and loop-rich subdomains. Some overlap of the DNA and RPA70 Delta C327 binding regions was observed in the loop-rich subdomain, indicating a possible cooperative DNA-binding mode between XPA and RPA70 Delta C327. To complement the chemical shift mapping data, the backbone dynamics of free XPA-MBD and XPA-MBD bound to DNA oligomers containing dhT or 64TC lesions were investigated using 15N NMR relaxation data. The dynamic analyses for the XPA-MBD complexes with DNA revealed localized increases and decreases in S2 and an increase in the global correlation time. Regions of XPA-MBD with the largest increases in S2 overlapped regions having the largest chemical shifts changes upon binding DNA, indicating that the loop-rich subdomain becomes more rigid upon binding DNA. Interestingly, S2 decreased for some residues in the zinc-binding core upon DNA association, indicating a possible concerted structural rearrangement on binding DNA.

Extended X-ray absorption fine structure evidence for a single metal binding domain in Xenopus laevis nucleotide excision repair protein XPA

Nucleotide excision repair (NER) is an important cellular mechanism, conserved from bacteria to humans, responsible for eliminating multiple types of structurally distinct DNA lesions from the genome. The protein XPA appears to play a central role in NER, recognizing and/or verifying damaged DNA and recruiting other proteins, including RPA, ERCC1, and TFIIH, to repair the damage. Sequence analysis and genetic evidence suggest that zinc, which is essential for DNA binding, is associated with a C4-type motif, C-X2-C-X17-C-X2-C. Sequence analysis suggests that a second, H2C2-type zinc-binding motif may be present near the C-terminal. Seventy percent of the amino acid sequence of Xenopus laevis XPA (xXPA) is identical to human XPA and both putative zinc-binding motifs are conserved in all known XPA proteins. Electrospray ionization-mass spectroscopy data show that xXPA contains only one zinc atom per molecule. EXAFS spectra collected on full-length xXPA in frozen (77 K) 15% glycerol aqueous solution unequivocally show that the zinc atom is coordinated to four sulfur atoms with an average Zn--S bond length of 2.33 +/- 0.02 A. Together, the EXAFS and mass spectroscopy data indicate that xXPA contains just one C4-type zinc-binding motif.