Multiple nuclear localization signals in XPG nuclease

XPG: a multitasking genome caretaker

The XPG/ERCC5 endonuclease was originally identified as the causative gene for Xeroderma Pigmentosum complementation group G. Ever since its discovery, in depth biochemical, structural and cell biological studies have provided detailed mechanistic insight into its function in excising DNA damage in nucleotide excision repair, together with the ERCC1–XPF endonuclease. In recent years, it has become evident that XPG has additional important roles in genome maintenance that are independent of its function in NER, as XPG has been implicated in protecting replication forks by promoting homologous recombination as well as in resolving R-loops. Here, we provide an overview of the multitasking of XPG in genome maintenance, by describing in detail how its activity in NER is regulated and the evidence that points to important functions outside of NER. Furthermore, we present the various disease phenotypes associated with inherited XPG deficiency and discuss current ideas on how XPG deficiency leads to these different types of disease.

Revisiting the structural features of the xeroderma pigmentosum proteins: Focus on mutations and knowledge gaps

The nucleotide excision repair pathway is a broadly studied DNA repair mechanism because impairments of its key players, the xeroderma pigmentosum proteins (XPA to XPG), are associated with multiple hereditary diseases. Due to the massive number of novel mutations reported for these proteins and new structural data published every year, proper categorization and discussion of relevant observations is needed to organize this extensive inflow of knowledge. This review aims to revisit the structural data of all XP proteins while updating it with the information developed in of the past six years. Discussions and interpretations of mutation outcomes, mechanisms of action, and knowledge gaps regarding their structures are provided, as well as new perspectives based on recent research.

Structural and Calorimetric Studies Demonstrate that Xeroderma Pigmentosum Type G (XPG) Can Be Imported to the Nucleus by a Classical Nuclear Import Pathway via a Monopartite NLS Sequence

Xeroderma pigmentosum type G (XPG) proteins are involved in DNA lesion recognition and promotion of nucleotide excision repair. Specific mutations in these proteins may lead to Cockayne syndrome, in which the patients may display severe developmental retardation and neurological abnormalities. No structural information is available for their spacer region or the C-terminal domain, which are important, respectively, for specific nucleotide excision repair activity and substrate specificity, as well as nuclear translocation. Immunofluorescence studies suggested two specific regions of the XPG C-terminus as potential bipartite nuclear localization sequences, which would be responsible for its translocation to the nucleus by the classical nuclear import pathway mediated by the importin-α (Impα). Thus, in order to test these hypotheses and gain insight into the structural basis for the nuclear import process for the XPG protein, we solved the crystal structures of complexes formed by the Impα and peptides corresponding to both putative nuclear localization signal (NLS) sequences (XPG1 and XPG2) and performed isothermal titration calorimetry assays to determine their binding affinities. Structural experiments confirm the binding of both NLS peptides to Impα but, unexpectedly, they bind to the receptor as monopartite NLSs. The isothermal titration calorimetry assays demonstrated that XPG1 and XPG2 peptides bind to two separate binding sites, but with high affinity to the major NLS-binding site of the Impα, resembling classical monopartite SV40 TAg NLS. The results lead to insights about what distinguishes monopartite and bipartite NLSs, as well as the differential roles of XPG1 and XPG2 NLSs in the nuclear localization of XPG.

Regulation of base excision repair in eukaryotes by dynamic localization strategies

This chapter discusses base excision repair (BER) and the known mechanisms defined thus far regulating BER in eukaryotes. Unlike the situation with nucleotide excision repair and double-strand break repair, little is known about how BER is regulated to allow for efficient and accurate repair of many types of DNA base damage in both nuclear and mitochondrial genomes. Regulation of BER has been proposed to occur at multiple, different levels including transcription, posttranslational modification, protein-protein interactions, and protein localization; however, none of these regulatory mechanisms characterized thus far affect a large spectrum of BER proteins. This chapter discusses a recently discovered mode of BER regulation defined in budding yeast cells that involves mobilization of DNA repair proteins to DNA-containing organelles in response to genotoxic stress.

Nuclear translocation contributes to regulation of DNA excision repair activities

DNA mutations are circumvented by dedicated specialized excision repair systems, such as the base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR) pathways. Although the individual repair pathways have distinct roles in suppressing changes in the nuclear DNA, it is evident that proteins from the different DNA repair pathways interact [Y. Wang, D. Cortez, P. Yazdi, N. Neff, S.J. Elledge, J. Qin, BASC, a super complex of BRCA1-associated proteins involved in the recognition and repair of aberrant DNA structures, Genes Dev. 14 (2000) 927-939; M. Christmann, M.T. Tomicic, W.P. Roos, B. Kaina, Mechanisms of human DNA repair: an update, Toxicology 193 (2003) 3-34; N.B. Larsen, M. Rasmussen, L.J. Rasmussen, Nuclear and mitochondrial DNA repair: similar pathways? Mitochondrion 5 (2005) 89-108]. Protein interactions are not only important for function, but also for regulation of nuclear import that is necessary for proper localization of the repair proteins. This review summarizes the current knowledge on nuclear import mechanisms of DNA excision repair proteins and provides a model that categorizes the import by different mechanisms, including classical nuclear import, co-import of proteins, and alternative transport pathways. Most excision repair proteins appear to contain classical NLS sequences directing their nuclear import, however, additional import mechanisms add alternative regulatory levels to protein import, indirectly affecting protein function. Protein co-import appears to be a mechanism employed by the composite repair systems NER and MMR to enhance and regulate nuclear accumulation of repair proteins thereby ensuring faithful DNA repair.

XPG: its products and biological roles

Xeroderma pigmetosum patients of the complementation group G are rare. One group of XP-G patients displays a rather mild and typical XP phenotype. Mutations in these patients interfere with the function of XPG in the nucleotide excision repair, where it has a structural role in the assembly of the preincision complex and a catalytic role in making the incision 3' to the damaged site in DNA. Another set of XP-G patient is much more severely affected, displaying combined symptoms of xeroderma pigmentosum and Cockayne syndrome, referred to as XP/CS complex. Although the molecular basis leading to the XP/CS complex has not yet been fully established, current evidence suggests that these patients suffer from a mild defect in transcription in addition to a repair defect. Here, the history of how the XPG gene was discovered, the biochemical properties of the XPG protein and the molecular defects found in XP-G patients and mouse models are reviewed.

Cross-Talk between Nucleotide Excision and Homologous Recombination DNA Repair Pathways in the Mechanism of Action of Antitumor Trabectedin

Trabectedin (Yondelis) is a potent antitumor drug that has the unique characteristic of killing cells by poisoning the DNA nucleotide excision repair (NER) machinery. The basis for the NER-dependent toxicity has not yet been elucidated but it has been proposed as the major determinant for the drug's cytotoxicity. To study the in vivo mode of action of trabectedin and to explore the role of NER in its cytotoxicity, we used the fission yeast Schizosaccharomyces pombe as a model system. Treatment of S. pombe wild-type cells with trabectedin led to cell cycle delay and activation of the DNA damage checkpoint, indicating that the drug causes DNA damage in vivo. DNA damage induced by the drug is mostly caused by the NER protein, Rad13 (the fission yeast orthologue to human XPG), and is mainly repaired by homologous recombination. By constructing different rad13 mutants, we show that the DNA damage induced by trabectedin depends on a 46-amino acid region of Rad13 that is homologous to a DNA-binding region of human nuclease FEN-1. More specifically, an arginine residue in Rad13 (Arg961), conserved in FEN1 (Arg314), was found to be crucial for the drug's cytotoxicity. These results lead us to propose a model for the action of trabectedin in eukaryotic cells in which the formation of a Rad13/DNA-trabectedin ternary complex, stabilized by Arg961, results in cell death.

Nuclear localization of the Epstein-Barr virus EBNA3B protein

The Epstein-Barr virus nuclear antigen (EBNA) 3B is a hydrophilic, proline-rich, charged protein that is thought to be involved in transcriptional regulation and is targeted exclusively to the cell nucleus, where it localizes to discrete subnuclear granules. Co-localization studies utilizing a fusion protein between enhanced green fluorescent protein (EGFP) and EBNA3B with FLAG-tagged EBNA3A and EBNA3C proteins demonstrated that EBNA3B co-localized with both EBNA3A and EBNA3C in the nuclei of cells when overexpressed. Computer analyses identified four potential nuclear-localization signals (NLSs) in the EBNA3B amino acid sequence. By utilizing fusion proteins with EGFP, deletion constructs of EBNA3B and site-directed mutagenesis, three of the four NLSs (aa 160-166, 430-434 and 867-873) were shown to be functional in truncated forms of EBNA3B, whilst an additional NLS (aa 243-246) was identified within the N-terminal region of EBNA3B. Only two of the NLSs were found to be functional in the context of the full-length EBNA3B protein.

Identification of nuclear import and export signals within Fli-1: roles of the nuclear import signals in Fli-1-dependent activation of megakaryocyte-specific promoters

The Ets factor Friend leukemia integration 1 (Fli-1) is an important regulator of megakaryocytic (Mk) differentiation. Here, we demonstrate two novel nuclear localization signals (NLSs) within Fli-1: one (NLS1) is located at the N terminus, and another (NLS2) is within the Ets domain. Nuclear accumulation of Fli-1 reflected the combined functional effects of the two discrete NLSs. Each NLS can independently direct nuclear transport of a carrier protein, with mutations within the NLSs affecting nuclear accumulation. NLS1 has a bipartite motif, whereas the NLS2 region contains a nonclassical NLS. Both NLSs bind importin alpha (IMPalpha) and IMPbeta, with NLS1 and NLS2 being predominantly recognized by IMPalpha and IMPbeta, respectively. Fli-1 also contains one nuclear export signal. Leptomycin B abolished its cytoplasmic accumulation, showing CRM1 dependency. We demonstrate that Ets domain binding to specific target DNA effectively blocks IMP binding, indicating that the targeted DNA binding plays a role in localizing Fli-1 to its destination and releasing IMPs for recycling back to the cytoplasm. Finally, by analyzing full-length Fli-1 carrying NLS1, NLS2, and combined NLS1-NLS2 mutations, we conclude that two functional NLSs exist in Fli-1 and that each NLS is sufficient to target Fli-1 to the nucleus for activation of Mk-specific genes.

Bioinformatic analysis of the nucleolus

The nucleolus is a plurifunctional, nuclear organelle, which is responsible for ribosome biogenesis and many other functions in eukaryotes, including RNA processing, viral replication and tumour suppression. Our knowledge of the human nucleolar proteome has been expanded dramatically by the two recent MS studies on isolated nucleoli from HeLa cells [Andersen, Lyon, Fox, Leung, Lam, Steen, Mann and Lamond (2002) Curr. Biol. 12, 1-11; Scherl, Coute, Deon, Calle, Kindbeiter, Sanchez, Greco, Hochstrasser and Diaz (2002) Mol. Biol. Cell 13, 4100-4109]. Nearly 400 proteins were identified within the nucleolar proteome so far in humans. Approx. 12% of the identified proteins were previously shown to be nucleolar in human cells and, as expected, nearly all of the known housekeeping proteins required for ribosome biogenesis were identified in these analyses. Surprisingly, approx. 30% represented either novel or uncharacterized proteins. This review focuses on how to apply the derived knowledge of this newly recognized nucleolar proteome, such as their amino acid/peptide composition and their homologies across species, to explore the function and dynamics of the nucleolus, and suggests ways to identify, in silico, possible functions of the novel/uncharacterized proteins and potential interaction networks within the human nucleolus, or between the nucleolus and other nuclear organelles, by drawing resources from the public domain.

Identification of the nuclear localization signals within the Epstein–Barr virus EBNA-6 protein

Epstein-Barr virus nuclear antigen (EBNA)-6 is essential for EBV-induced immortalization of primary human B-lymphocytes in vitro. Previous studies have shown that EBNA-6 acts as a transcriptional regulator of viral and cellular genes; however at present, few functional domains of the 140 kDa EBNA-6 protein have been completely characterized. There are five computer-predicted nuclear localization signals (NLS), four monopartite and one bipartite, present in the EBNA-6 amino acid sequence. To identify which of these NLS are functional, fusion proteins between green fluorescent protein and deletion constructs of EBNA-6 were expressed in HeLa cells. Each of the constructs containing at least one of the NLS was targeted to the nucleus of cells whereas a construct lacking all of the NLS was cytoplasmic. Site-directed mutation of these NLS demonstrated that only three of the NLS were functional, one at the N-terminal end (aa 72-80), one in the middle (aa 412-418) and one at the C-terminal end (aa 939-945) of the EBNA-6 protein.

Identification of two novel nuclear import sequences on the 5-lipoxygenase protein

The nuclear import of 5-lipoxygenase modulates its capacity to produce leukotrienes from arachidonic acid. However, the molecular determinants of its nuclear import are unknown. Recently, we used structural and functional criteria to identify a novel import sequence at Arg(518) on human 5-lipoxygenase (Jones, S. M., Luo, M., Healy, A. M., Peters-Golden, M., and Brock, T. G. (2002) J. Biol. Chem. 277, 38550-38556). However, this analysis also indicated that other import sequences must exist. Here, we identify two additional sites, at Arg(112) and Lys(158), as nuclear import sequences. Both sites were found to be common to 5-lipoxygenases from different species but not found on other lipoxygenases. Both sites also appeared to be a part of structures that were predominantly random loops. Peptide sequences at these sites were sufficient to direct nuclear import of green fluorescent protein. Mutation of basic residues in these sites impaired nuclear import and combinations of mutations at different sites were additive in effect. Mutations in all three sites were required to disable nuclear accumulation of 5-lipoxygenase in all cells. Significantly, mutation in these sites did not inhibit catalytic function. Taken together, these results indicate that nuclear import of 5-lipoxygenase may reflect the combined functional effects of three discrete import sequences. Mutation of individual sites can, by itself, impair nuclear import, which in turn could impact arachidonic acid metabolism.

Relationship of neurologic degeneration to genotype in three xeroderma pigmentosum group G patients

We studied three newly diagnosed xeroderma pigmentosum complementation group G patients with markedly different clinical features. An Israeli-Palestinian girl (XP96TA) had severe abnormalities suggestive of the xeroderma pigmentosum/Cockayne syndrome complex including sun sensitivity, neurologic and developmental impairment, and death by age 6 y. A Caucasian girl (XP82DC) also had severe sun sensitivity with neurologic and developmental impairment and died at 5.8 y. In contrast, a mildly affected 14-y-old Caucasian female (XP65BE) had sun sensitivity but no neurologic abnormalities. XP96TA, XP82DC, and XP65BE fibroblasts showed marked reductions in post-ultraviolet cell survival and DNA repair but these were higher in XP65BE than in XP82DC. XP96TA fibroblasts had very low XPG mRNA expression levels whereas XP65BE fibroblasts had nearly normal levels. Host cell reactivation of an ultraviolet-treated reporter assigned all three fibroblast strains to the rare xeroderma pigmentosum complementation group G (only 10 other patients previously reported). XP96TA and XP82DC cells had mutations in both XPG alleles that are predicted to result in severely truncated proteins including stop codons and two base frameshifts. The mild XP65BE patient had an early stop codon mutation in the paternal allele. The XP65BE maternal allele had a single base missense mutation (G2817A, Ala874Thr) that showed residual ability to complement xeroderma pigmentosum complementation group G cells. These observations agree with earlier studies demonstrating that XPG mutations, which are predicted to lead to severely truncated proteins in both alleles, were associated with severe xeroderma pigmentosum/Cockayne syndrome neurologic symptoms. Retaining residual functional activity in one allele was associated with mild clinical features without neurologic abnormalities.

Cell cycle-dependent and DNA damage-inducible nuclear localization of FEN-1 nuclease is consistent with its dual functions in DNA replication and repair

Flap endonuclease-1 (FEN-1), a 43-kDa protein, is a structure-specific and multifunctional nuclease. It plays important roles in RNA primer removal of Okazaki fragments during DNA replication, DNA base excision repair, and maintenance of genome stability. Three functional motifs of the enzyme were proposed to be responsible for its nuclease activities, interaction with proliferating cell nuclear antigen, and nuclear localization. In this study, we demonstrate in HeLa cells that a signal located at the C terminus (the nuclear localization signal (NLS) motif) facilitates nuclear localization of the enzyme during S phase of the cell cycle and in response to DNA damage. Truncation of the NLS motif prevents migration of the protein from the cytoplasm to the nucleus, while having no effect on the nuclease activities and its proliferating cell nuclear antigen interaction capability. Site-directed mutagenesis further revealed that a mutation of the KRK cluster to three alanine residues completely blocked the localization of FEN-1 into the nucleus, whereas mutagenesis of the KKK cluster led to a partial defect of nuclear localization in HeLa cells without observable phenotype in yeast. Therefore, the KRKXXXXXXXXKKK motif may be a bipartite NLS driving the protein into nuclei. Yeast RAD27Delta cells transformed with human mutant M(krk) survived poorly upon methyl methanesulfonate treatment or when they were incubated at an elevated temperature.

The puzzle of PCNA's many partners

The identification of proteins that interact with proliferating cell nuclear antigen (PCNA) has recently been a rapidly expanding field of discovery. PCNA is involved in many aspects of DNA replication and processing, forming a sliding platform that can mediate the interaction of proteins with DNA. It is striking that many proteins bind to PCNA through a small region containing a conserved motif; these include proteins involved in cell cycle regulation as well as those involved in DNA processing. Sequential and regulated binding of motif-containing proteins to PCNA may contribute to the ordering of events during DNA replication and repair. Results from bacteriophages and archaea show that the structural basis for the interaction of this motif with PCNA is extremely ancient. The analysis of how such functional motifs have been recruited to proteins in present day organisms helps us to understand how these complex systems arose from ancestral organisms.

Newly discovered archaebacterial flap endonucleases show a structure-specific mechanism for DNA substrate binding and catalysis resembling human flap endonuclease-1

Mammalian flap endonuclease-1 (FEN-1) is a structure-specific metalloenzyme that acts in processing of both the Okazaki fragments during lagging strand DNA synthesis and flap intermediates during DNA damage repair. We identified and cloned three open reading frames encoding a flap endonuclease from Archaeglobus fulgidus, Methanococcus jannaschii, and Pyrococcus furiosus, respectively. The deduced FEN-1 protein sequences share approximately 75% similarity with the human FEN-1 nuclease in the conserved nuclease domains, and extensive biochemical experiments indicate that the substrate specificities and catalytic activities of these enzymes have overall similarities with those of the human enzyme. Thus, FEN-1 enzymes and likely reaction mechanisms are conserved across the eukaryotic and archaeal kingdoms. Detailed comparative analysis, however, reveals subtle differences among these four enzymes including distinctive substrate specificity, tolerance of the archaebacterial enzymes for acidic pHs and elevated temperatures, and variations in the metal-ion dependence of substrate cleavage. Although the archaebacterial enzymes were inactive at temperatures below 30 degreesC, DNA binding occurred at temperatures as low as 4 degreesC and with or without metal ions. Thus, these archaeal enzymes may provide a means to dissect the specific binding and catalytic mechanisms of the entire FEN-1 family of structure-specific nucleases.

Cytomegalovirus assembly protein precursor and proteinase precursor contain two nuclear localization signals that mediate their own nuclear translocation and that of the major capsid protein

The cytomegalovirus (CMV) assembly protein precursor (pAP) interacts with the major capsid protein (MCP), and this interaction is required for nuclear translocation of the MCP, which otherwise remains in the cytoplasm of transfected cells (L. J. Wood et al., J. Virol. 71:179-190, 1997). We have interpreted this finding to indicate that the CMV MCP lacks its own nuclear localization signal (NLS) and utilizes the pAP as an NLS-bearing escort into the nucleus. The CMV pAP amino acid sequence has two clusters of basic residues (e.g., KRRRER [NLS1] and KARKRLK [NLS2], for simian CMV) that resemble the simian virus 40 large-T-antigen NLS (D. Kalderon et al., Cell 39:499-509, 1984) and one of these (NLS1) has a counterpart in the pAP homologs of other herpesviruses. The work described here establishes that NLS1 and NLS2 are mutually independent NLS that can act (i) in cis to translocate pAP and the related proteinase precursor (pNP1) into the nucleus and (ii) in trans to transport MCP into the nucleus. By using combinations of NLS mutants and carboxy-terminal deletion constructs, we demonstrated a self-interaction of pAP and cytoplasmic interactions of pAP with pNP1 and of pNP1 with itself. The relevance of these findings to early steps in capsid assembly, the mechanism of MCP nuclear transport, and the possible cytoplasmic formation of protocapsomeric substructures is discussed.

Epstein-Barr virus DNase contains two nuclear localization signals, which are different in sensitivity to the hydrophobic regions

The DNase of Epstein-Barr virus (EBV) is a 470-amino-acid protein which possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA and single-stranded DNA as substrates. It has been reported that this protein may be found in the nucleus and/or cytoplasm of infected cells. In this study, using cell fractionation and immunoblotting to determine the distribution of EBV DNase in Akata cells stimulated with anti-human immunoglobulin G antibody (anti-IgG), the DNase was found to be located predominantly in the nucleus. To map the signals in DNase which mediate its nuclear localization, we monitored the nuclear transport of fusion proteins consisting of various fragments of EBV DNase linked to a cytoplasmic protein, beta-galactosidase (beta-Gal). The results demonstrated that two regions of the DNase with nuclear localization signal (NLS) activity, designated NLS-A (amino acids 239-266) and NLS-B (amino acids 291-306), were able independently to localize the beta-Gal to the nuclei of HEp-2 and HeLa cells. Five basic residues (R or K) were found in each NLS and distributed differently in primary structure. The basic domains and flanking residues of NLS-A and NLS-B are 250YKRPCKRSFIRFI262 and 294LKDVRKRKLGPGH306, respectively. Further examination of these sequences revealed that NLS-A contains bulky aromatic amino acids (Y and F) which may diminish its capacity to act as a strong NLS and lacks the typical proline and glycine helix-breakers. However, NLS-B contains typical proline and glycine helix-breakers and the histidine residue at amino acid 306 is required for NLS activity. In addition, two hydrophobic regions within the DNase were found to inhibit the function of NLS-A but not NLS-B, suggesting that these two domains are different types of NLSs and differ in their sensitivity to hydrophobic regions in the context of protein structure.

The DNA repair endonuclease XPG binds to proliferating cell nuclear antigen (PCNA) and shares sequence elements with the PCNA-binding regions of FEN-1 and cyclin-dependent kinase inhibitor p21

Proliferating cell nuclear antigen (PCNA) is a DNA polymerase accessory factor that is required for DNA replication during S phase of the cell cycle and for resynthesis during nucleotide excision repair of damaged DNA. PCNA binds to flap endonuclease 1 (FEN-1), a structure-specific endonuclease involved in DNA replication. Here we report the direct physical interaction of PCNA with xeroderma pigmentosum (XP) G, a structure-specific repair endonuclease that is homologous to FEN-1. We have identified a 28-amino acid region of human FEN-1 (residues 328-355) and a 29-amino acid region of human XPG (residues 981-1009) that contains the PCNA binding activity. These regions share key hydrophobic residues with the PCNA-binding domain of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1), and all three competed with one another for binding to PCNA. A conserved arginine in FEN-1 (Arg339) and XPG (Arg992) was found to be crucial for PCNA binding activity. R992A and R992E mutant forms of XPG failed to fully reconstitute nucleotide excision repair in an in vivo complementation assay. These results raise the possibility of a mechanistic linkage between excision and repair synthesis that is mediated by PCNA.

Molecular cloning and structural analysis of the functional mouse genomic XPG gene

The mouse XPG gene is a homolog of the human DNA excision repair gene known to be defective in the hereditary sun-sensitive disorder xeroderma pigmentosum (group-G). Defects in mouse XPG have been shown to directly affect the sensitivity of cultured cells to chemotherapy agents and may play a role in tumor cell drug resistance in vivo. A full-length cosmid clone of mouse XPG was isolated by complementation of the UV sensitivity and repair defect in CHO-UV135 cells. Exon mapping determined that the gene consisted of 15 exons within 32 kb of genomic DNA. Sequencing of intron-exon boundaries revealed that mouse XPG possesses a rare class of intron previously identified in only four other eukaryotic genes; it utilizes AT and AC dinucleotides instead of the expected GT and AG within the splice junctions. Promoter analysis determined that mouse XPG is expressed constitutively and probably initiates transcription from multiple start sites, yet, unlike the yeast homolog RAD2, we found no evidence that it is UVC inducible in cultured cells. Amino acid comparison with human XPG identified a highly conserved acidic region of homology not previously described.

Ultraviolet-induced movement of the human DNA repair protein, Xeroderma pigmentosum type G, in the nucleus

Xeroderma pigmentosum type G (XPG) is a human genetic disease exhibiting extreme sensitivity to sunlight. XPG patients are defective XPG endonuclease, which is an enzyme essential for DNA repair of the major kinds of solar ultraviolet (UV)-induced DNA damages. Here we describe a novel dynamics of this protein within the cell nucleus after UV irradiation of human cells. Using confocal microscopy, we have localized the immunofluorescent, antigenic signal of XPG protein to foci throughout the cell nucleus. Our biochemical studies also established that XPG protein forms a tight association with nuclear structure(s). In human skin fibroblast cells, the number of XPG foci decreased within 2 h after UV irradiation, whereas total nuclear XPG fluorescence intensity remained constant, suggesting redistribution of XPG from a limited number of nuclear foci to the nucleus overall. Within 8 h after UV, most XPG antigenic signal was found as foci. Using beta-galactosidase-XPG fusion constructs (beta-gal-XPG) transfected into HeLa cells, we have identified a single region of XPG that is evidently responsible both for foci formation and for the UV dynamic response. The fusion protein carrying the C terminus of XPG (amino acids 1146-1185) localized beta-gal specific antigenic signal to foci and to the nucleolus regions. After UV irradiation, antigenic beta-gal translocated reversibly from the subnuclear structures to the whole nucleus with kinetics very similar to the movements of XPG protein. These findings lead us to propose a model in which distribution of XPG protein may regulate the rate of DNA repair within transcriptionally active and inactive compartments of the cell nucleus.