From the Scientific Computing Laboratory and
§ Laboratory of Molecular Genetics, NIEHS, National
Institutes of Health, Research Triangle Park, North Carolina 27709 and ¶ Department of Molecular Genetics, Cancer Research Institute,
Slovak Academy of Sciences,
Vlarska 7, 833 91 Bratislava, Slovakia
Received for publication, October 3, 2002, and in revised form, November 25, 2002
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ABSTRACT |
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A molecular model for the human nucleotide
excision repair protein, XPD, was developed based on the structural and
functional relationship of the protein with a bacterial nucleotide
excision repair (NER) protein, UvrB. Whereas XPD does not share
significant sequence identity with UvrB, the proteins share seven
highly conserved helicase motifs that define a common protein
structural template. They also have similar functional roles in their
ATPase activity and the ability to unwind DNA and verify damaged
strands in the process of NER. The validity of using the crystal
structure of UvrB as a template for the development of an XPD model was
tested by mimicking human disease-causing mutations (XPD: R112H, D234N, R601L) in UvrB (E110R, D338N, R506A) and by mutating two highly conserved residues (XPD, His-237 and Asp-609; UvrB, H341A and D510A).
The XPD structural model can be employed in understanding the molecular
mechanism of XPD human disease causing mutations. The value of this XPD
model demonstrates the generalized approach for the prediction of the
structure of a mammalian protein based on the crystal structure of a
structurally and functionally related bacterial protein sharing
extremely low sequence identity (<15%).
Nucleotide excision repair
(NER)1 is a process by which
damaged nucleotides, from UV light and chemical carcinogens, are
removed from DNA. NER is one of the most highly conserved biochemical pathways, and enzymes that mediate this process appear in prokaryotes, archaea, and eukaryotes (1, 2). NER can be viewed in the following five
continuous steps: 1) damage recognition and verification, 2) incision,
3) excision, 4) repair synthesis, and 5) ligation. During the damage
recognition step, a protein complex first identifies a structural
perturbation in the helical DNA, which is then verified by additional
damage processing and strand opening proteins. Once the lesion has been
verified, endonucleases are recruited to the damaged strand performing
the initial incision 3' followed by a 5' incision facilitating the
excision of an oligonucleotide containing the damage. Repair synthesis
fills in the resulting gap. Finally DNA ligase seals the newly
completed repair patch. The evolutionary conservation of proteins
involved in NER is exemplified by human XPD, which maintains 56 and
52% (protein) sequence identity, respectively, with the equivalent
Schizosaccharomyces pombe, Rad15, and Saccharomyces
cerevisiae, RAD3, proteins over the length of the entire protein
(BLAST alignments). Human and mouse XPD retain 98% (protein) sequence
identity (3).
XPD is one of the constituent proteins forming the TFIIH complex that
is responsible for damage processing and strand verification (4, 5).
XPD and XPB have intrinsic helicase activities that are absolutely
required for NER (4). TFIIH is also required for initiation of RNA
polymerase II transcription. Mutations in the XPD gene can lead to one
of the following three disease syndromes in humans: xeroderma
pigmentosum (XP), trichothiodystrophy (TTD), and Cockayne syndrome (CS)
(6). Analysis of specific point mutations associated with each disease
has revealed that a majority of patients with XPD have mutations in one
of seven helicase motifs (6).
Helicases from bacteria to humans are believed to have evolved from a
common ancestor (7). Helicase motifs may be so highly conserved because
they represent a fundamental mechanism for DNA processing necessary for
transcription, replication, repair, and recombination (8). Mutations in
several helicase proteins have been linked to cancer-related and
accelerated aging disorders (Werner's syndrome, Bloom's syndrome,
trichothiodystrophy, Cockayne's syndrome, and xeroderma pigmentosum)
(9-11).
Helicases have been classified into three large superfamilies
(designated SF1, SF2, and SF3) and two smaller families (F4 and F5)
(12). Proteins belonging to SF1 and SF2 are characterized as such based
on possessing a set of seven conserved motifs ranging in length from
about 20 to 40 amino acids. These helicase motifs demonstrate both a
sequence and three-dimensional structure conservation. SF1 and SF2
helicase proteins were predicted to have a conserved In general, structural homology between related proteins is much more
highly conserved than sequence homology (25). Thus the helicase motifs
possess similar folds and relative three-dimensional relational
positions within the proteins despite their low overall sequence
identity. Over the entire length of the protein, XPD shares 12.8%
sequence identity and 61% sequence similarity with PcrA, 15% identity
and 59% similarity with Rep, 15% identity and 62% similarity with
UvrB (Fig. 1), and 12.5% identity and
56% similarity with NS3. Both UvrB and XPD belong to SF2 and exhibit greater similarity in sequence as compared with the three other helicases whose structures have been solved. In addition, UvrB serves a
homologous function as TFIIH in bacterial NER (24, 26, 27) in that it
assists in verifying a damaged site and processing the lesion into an
open conformation apparently by inserting a
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
-pleated sheet
structure based on other solved NTPase structures sharing these motifs
(12). Structural studies have confirmed this prediction. Helicases
contain the Walker box A and B motifs found in many NTP-binding
proteins (in helicase motif I and II). Individual specificity of the
protein function is conferred by the insertion of other domains not
shared between the proteins. Crystal structures of three known
monomeric/dimeric helicases and one repair protein that shares a common
helicase fold have been determined as follows: 1) Escherichia
coli Rep with ssDNA alone, and a ternary complex with ADP (13,
14); 2) Bacillus stearothermophilus PcrA with ADP-PNP,
ssDNA, and Mg2+ (15-17); 3) hepatitis C virus NS3 with
single-stranded oligonucleotide (14, 18-20); and 4) UvrB, from
Thermus thermophilus (21, 22), and Bacillus
caldotenax with ATP and Mg2+ (23, 24).
-hairpin between the two
DNA strands (23, 24, 26). Finally, both XPD (as part of TFIIH) and UvrB
serve as a scaffold on which to recruit the nucleases that perform the dual incisions. Structurally UvrB is folded into four domains, 1a, 1b, 2, and 3 (Fig. 2A). Domains 1a and 3 contain
helicase motifs I-III and IV-VI, respectively, sharing a common fold
to other monomeric helicases such as NS3 (23).
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Fig. 1.
The sequence alignment of UvrB
(B. caldotenax and T. thermophilus)
and XPD (human) that served as the basis for the development of the XPD
model. The helicase motifs are colored corresponding to their
color in the UvrB and XPD structures shown.
The goal of this study was to create a molecular model of the human NER
protein XPD, based on the solved crystal structure of the related
bacterial NER protein, UvrB (23), as a template. These proteins are
closely related based on their common helicase structural motifs and
biological function, to open double-stranded DNA during NER (28).
Accuracy of the XPD model was confirmed experimentally by expressing
B. caldotenax UvrB proteins containing mutations
corresponding to known disease-causing mutations in XPD. In addition,
two residues highly evolutionarily conserved throughout all bacterial
UvrB species and also RAD3 and XPD (mouse and human) were also mutated.
The behavior of these mutant UvrB proteins was examined through
incision, DNA binding, and oligonucleotide-destabilizing assays in a
reconstituted system containing purified UvrA and UvrC from B. caldotenax. The XPD model serves as a guide for
structural-functional studies of the protein and assists in gaining a
molecular understanding of the NER process in humans, XPD
disease-causing mutations, and XPD protein-protein and protein-DNA interactions.
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MATERIALS AND METHODS |
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Molecular Modeling of XPD
Multiple Sequence Alignment-- A structure-based multiple sequence alignment was developed for the DNA helicase proteins, using the location of domains and helicase motifs, as these sequences do not possess high sequence identity. The alignment included sequences of all the solved monomeric helicase structures (Rep, PcrA, NS3, and UvrB), five bacterial UvrB sequences, human and mouse XPD, and RAD3 the yeast XPD homologue sequences. Part of this multiple alignment, the alignment of UvrB and XPD sequences, is included in Fig. 1. The alignment method used was based on the work of Gorbalenya et al. (29) and Gorbalenya and Koonin (30) using the programs DIAGON (31) and OPTAL (32) and the Sankoff algorithm (33) for multiple sequence alignment. The XPD sequence was aligned by hand to the multiple sequence alignment described for SF2 helicases by Gorbalenya et al. (29) and Gorbalenya and Koonin (30) using the RAD3 sequence as a guide.
XPD Model Construction-- The XPD protein model was constructed using the bacterial B. caldotenax UvrB crystal structure coordinates (Fig. 2A (Protein Data Bank code 1D9Z) (23)) as the template. The sequence alignment indicated that XPD shared closer sequence similarity within the helicase motifs with UvrB, and the alignment resulted in the fewest deletions and insertions throughout the protein sequence as compared with Rep, PcrA, and NS3. UvrB and XPD also share an analogous function as nucleotide excision repair proteins. The XPD homology model was constructed using the Accelrys Homology software package (MSI Homology Users Guide, San Diego). There were 15 loop insertions constructed using the Molecular Simulations homology modeling loop generation software. Because the structure for these loops is less well determined than the rest of the model, as they are not built from experimentally determined structure, they are indicated in black in the model structure in Fig. 2B. UvrB residues 180-301 were deleted from the XPD model structure, as there are no corresponding residues in the XPD helicase. XPD contains an additional domain, XPD residues 283-448, not present in UvrB. This domain is not similar to any other helicase protein domains and does not possess sequence homology to any other solved protein or protein domain structures. A suggestion of a possible structure for this domain is made from threading and inverse folding algorithms in combination with secondary structure prediction methods (as described in the Supplemental Material). The structural model of this domain is shown in Fig. 3.
Modeling the Inserted (Non-homologous) XPD Domain (Residues
283-448)--
Three protein-threading algorithms were applied
independently to identify a protein fold to model the XPD domain
represented by residues 283-447 (not present in UvrB) as follows: the
GenThreader algorithm (34, 35), the 3D-PSSM algorithm (36, 37), and the
Bioinbgu algorithm (42). The top protein folds for this XPD domain
given by all three algorithms independently were all -helical proteins, and all shared a fold with nucleic acid-binding proteins. The consensus from these three methods is that this domain
has a protein fold similar to that of the AraC family transcriptional activator (Protein Data Bank entry 1BL0) or DNA-binding domain of the
centromere binding protein B (CENP-B) (Protein Data Bank entry 1BW6).
Both of these proteins are members of the same SCOP data base protein
fold class characteristic of all
-helical DNA/RNA binding, three
-helical bundle proteins. The structural model of this domain based
on this fold is shown in Fig. 3.
Modeling the XPD C Terminus (Residues 713-761)-- The NMR and crystal structure of the UvrB C terminus were solved independently of the rest of the protein (38, 39). This part of the protein was found to have a helix-turn-helix fold and to interact with a homologous domain in UvrC. There was no homology or sequence similarity between the XPD and UvrB C termini.
As the C-terminal XPD sequence does not exhibit high sequence identity
to any previously solved structure, threading was used to search for a
"structural" similarly folded homologue. Consensus results from
seven protein threading algorithms (LOOPP (40), The Sausage Machine
(41), 3D-PSSM (36, 37), Bioinbgu (42), GenThreader (34, 35), COBLATH
(43), and Fugue (44)) were used in concert with secondary structure
prediction algorithms (PSIPRED (45), PHD (46), SSPRO (47), PROF (48),
and PREDATOR (49)) to develop a threaded model for the XPD C terminus
(49 residues, 713-761). The top structural hits from all
threading methods were all -helical proteins (all members of the
CATH 1 10 10 60 class), and secondary structure prediction
methods also predicted the 49 C-terminal residues would form an
-helical bundle. Similar protein structures that were selected by
more than one of the seven different threading methods are as follows:
1OCTC, the oct-1 POU domain (a DNA binding protein); 1BH9A (TBP
associated factor); and 4HB1, a designed four-helical bundle.
A structural model was developed for the XPD C terminus based on the Protein Data Bank structure entry 4HB1, which is the structure of a designed 4-helical bundle. This structure consists of two 2-helical bundles related by a 2-fold axis of symmetry through the center. Only the coordinates from two of the helices were used to model the XPD C terminus. The fold of the XPD C terminus is shown to be similar to that of UvrB. The threaded model structure developed for the XPD C terminus is shown and compared with the UvrB experimentally determined C-terminal structure (38, 39) in Fig. 3. A significant number of frequently occurring XPD disease-causing mutations of XP or TTD patients have mutations occurring in the C terminus of the protein (50). C-terminal XPD residues 658, 675, 683, 713, and 722 have been shown experimentally to bind to the N-terminal domain of another component TFIIH protein, p44 (50). p44 residues Asp-66, Glu-166, Asp-178, and Gly-200 have been shown experimentally to interact with XPD (51).
Experimental Assays
Construction and Expression of UvrB Mutants-- UvrA, UvrB, and UvrC proteins from B. caldotenax were purified by standard procedures (New England Biolabs IMPACTTM T7 system manual) with some modifications for each protein, which will be published elsewhere. T4 polynucleotide kinase was purchased from Invitrogen. Pfu DNA polymerase was purchased from Stratagene. All UvrB mutants were constructed by PCR using QuikChange site-directed mutagenesis method (Stratagene) with Pfu Turbo DNA polymerase, pUC18uvrBBca as a template DNA and two oligonucleotide primers (Genosys), each complementary to opposite strands of the template DNA, containing the desired mutation. Resulting mutant plasmids were verified by sequencing for the absence of additional PCR-induced mutations in the uvrB gene sequence. Mutated uvrB gene was finally subcloned into pTYB1 expression vector.
DNA Substrates--
Fluorescein-containing DNA substrates were
synthesized by Sigma Genosys. The DNA sequence of a 50-bp dsDNA
substrate containing a single internal fluorescein adduct
(F26-50 dsDNA) is shown in Fig.2A. For 5'
labeling 10 pmol of 50-mer fluorescein-containing top strand was
incubated with 25 units of T4 polynucleotide kinase in 70 mM Tris/Cl (pH 7.6), 10 mM MgCl2,
100 mM KCl, 1 mM 2-mercaptoethanol, and 15 pmol
of [-32P]ATP (3000 Ci/mmol). After incubation at
37 °C for 1 h, the reaction was terminated by incubation at
80 °C for 10 min in the presence of 20 mM EDTA.
Annealing of the top and the bottom strand was performed in the
presence of 50 mM NaCl, followed by purification through
Bio-Spin P-30 polyacrylamide gel column (Bio-Rad) for removal of
unincorporated nucleotides. The double-stranded character and
homogeneity of the 50 bp substrate were examined by a restriction assay
(38) and analyzed on a 10% polyacrylamide sequencing gel under
denaturating conditions. The cholesterol containing substrate was a
generous gift from N. Goosen (Leiden, The Netherlands) (52).
The DNA sequence of the helicase substrate (HS1F-M13mp19) was described previously (26). Five pmol of a 26-mer containing an internal fluorescein adduct (HS1F) were labeled at its 5' terminus under the same conditions as the F26-50 top strand. The helicase substrate was constructed by hybridizing 0.4 pmol of 5'-labeled HS1F oligonucleotide with equimolar amounts of M13mp19 (+) strand (Invitrogen) and purified as described above.
Gel Mobility Shift Assay-- Binding reactions were performed with 2 nM DNA substrate (5'-32P-labeled fluorescein-50 dsDNA) with 20 nM B. caldotenax UvrA and 60 nM B. caldotenax UvrB in 20 µl of UvrABC buffer (50 mM Tris/Cl (pH 7.5), 10 mM MgCl2, 50 mM KCl, 1 mM ATP, 5 mM DTT) for 20 min at 55 °C. Glycerol was then added to the reaction (8% v/v), and the reaction mixture was loaded onto a 4% native polyacrylamide gel (80:1). The gel and the running buffer (89 mM Tris, 89 mM boric acid, 2 mM EDTA) contained 1 mM ATP and 10 mM MgCl2. The electrophoresis was performed for 3 h at 100 V at room temperature. The gel was dried and exposed against Storage Phosphor Screen (Amersham Biosciences) overnight at room temperature.
UvrABC Incision Assay-- The 5'-terminally labeled cholesterol-50 or fluorescein-50 dsDNA substrate (2 nM) was incised by UvrABC (20 nM UvrA, 60 nM UvrB, and 50 nM UvrC) in 20 µl of UvrABC buffer at 55 °C for 1 h. The reaction was terminated by ethanol precipitation. The samples were denatured with formamide and heated to 90 °C for 5 min and then quick-chilled on ice. The incision products were analyzed by electrophoresis on a 10% polyacrylamide sequencing gel under denaturating conditions at 400-500 V with TBE buffer. In the case of the helicase substrate incision, the reaction mixture (15 µl) contained ~8 fmol (in ssDNA circles) of DNA substrate, 50 nM UvrA, 100 nM UvrB, and 100 nM UvrC in buffer A2 (50 mM Tris/Cl (pH 7.5), 100 mM KCl, 15 mM MgCl2, 1 mM EDTA, 5 mM ATP, 2 mM DTT) and was incubated at 42 °C for 1 h. The reaction was quenched with 5 µl of stop solution (25% (v/v) Ficoll, 1% SDS, 0.1 mM EDTA, 0.25% orange G) and heated for 2 min at 85 °C, and the entire sample was then loaded onto a 15% denaturating polyacrylamide gel equilibrated with TBE running buffer. Electrophoresis was carried out at 400-500 V for 1-2 h. The gels were processed as described above.
Oligonucleotide Destabilizing Assay-- The reaction mixture contained 50 nM UvrA, 100 nM UvrB, and ~8 fmol (in ssDNA circles) of helicase substrate (HS1F-M13mp19) in buffer A1 (50 mM Tris/Cl (pH 7.5), 150 mM NaCl, 10 mM MgCl2, 2 mM ATP, 5 mM DTT) and was incubated at 37 °C for various time intervals. The reaction was stopped with 5 µl of stop solution (50% (v/v) glycerol, 1% SDS, 0.1 M EDTA, 0.25% orange G), and the entire sample was loaded on 10% non-denaturating acrylamide gel in TBE running buffer. Electrophoresis was run at 120-150 V for 1-2 h, and the gels were processed as described previously.
ATP Hydrolysis Assay--
The conversion of ATP to ADP by the
UvrABC system was determined by a coupled enzyme assay system
consisting of pyruvate kinase and lactate dehydrogenase to link the
hydrolysis of ATP to the oxidation of NADH. The assay mixture consisted
of 50 mM Tris/Cl (pH 7.5), 50 mM NaCl, 4 mM MgCl2, 1 mM DTT, 20 units/ml
lactate dehydrogenase, 20 units/ml pyruvate kinase, 2 mM
phosphoenolpyruvate, 0.15 mM NADH, and 200 nM
Uvr proteins in the presence or absence of 50 ng of UV-irradiated DNA
substrate. The continuous assay was performed at 55 °C for 60 min;
the ATPase activity was determined from the slope of the turnover of
NADH versus time.
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RESULTS AND DISCUSSION |
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The XPD Model
The structure of the molecular model developed for the human XPD
protein, based on the x-ray crystal structure of B. caldotenax UvrB (23, 24), is shown in Fig.
2B. This model includes all the defined SF2 helicase motifs, domains 1a, 1b, 2, and 3, and structure for XPD residues 10-282 and 448-712 based on the UvrB structure coordinates as the template. The model of the additional XPD
domain, residues 283-448, absent in UvrB, and location relative to the
helicase motifs are indicated in Fig. 3.
The XPD C-terminal model structure (residues 713-761) and location
relative to the helicase motifs are shown in Fig. 3. Fig.
4 indicates the location of all the
currently known disease (XP, TTD, and CS)-associated mutations found in
XPD on the structural model. It is interesting to note that the
majority of the disease-associated mutations are found in domains 1a
and 3 whose structure is highly conserved across all helicases. Other
frequently occurring disease-causing mutations are located in helicase
motifs III, V, and VI (3, 6, 53, 54). No disease-associated mutations
have been observed in the XPD-inserted threaded domain (residues
283-447).
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Overall comparison of the modeled XPD structures with other solved
monomeric helicase structures illustrates the striking conservation of
the central -pleated sheet structure of domain 1a with surrounding
-helices common to NTPases as first evidenced in the structure of
the nucleotide binding domain of e1F4A (55). This domain contains the
Walker A and B motifs common to NTPases and is involved in ATP binding
and hydrolysis. Domain 3 of the XPD model also conserves the central
-pleated sheet structure with surrounding
-helices of the other
solved SF1 and SF2 family helicases. Significant differences between
the XPD modeled structure and UvrB structure are observed in domain 2, with the insertion of a large loop in place of the antiparallel
-sheets in UvrB, and domain 1B where the
-helices of UvrB are
absent in XPD. In addition, the
-hairpin believed to play an
essential role in DNA binding in UvrB (26) is not as distinct and
extended in the XPD model structure as the two proteins exhibited low
sequence similarity in this region. The overall root mean square
deviation between the backbone UvrB structure and XPD model
(superposition of 3392 atoms) is 1.89 Å (excluding the XPD-inserted
domain and C terminus).
Validation of XPD Model through Site-directed Mutagenesis of UvrB
Experimental Strategy, Selection of Mutants, and Biochemical Analyses-- The validity of our modeling approach was tested through biochemical experiments with mutant UvrB proteins containing homologous changes as some of the disease-causing mutations in the XPD gene. Two categories of experimental mutations were made (Table I). One group of UvrB mutants was selected as they represent the residues analogous to the XPD disease-causing mutations (E110A/E110R, D338N, and R506A). The second group of mutants was selected based on their high conservation in UvrB, RAD3, and XPD. This second group of UvrB mutants (H341A, D510N/D510A) can be thought of as "predictive" in that they are based on their conservation to have functional significance. The structural locations of the UvrB mutants constructed and purified are shown in Fig. 3. The biochemical properties of these UvrB mutants were studied by incision (Fig. 5), gel mobility shift, oligonucleotide destabilization (Fig. 6), and ATP hydrolysis assays (Table II).
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Mutations in Helicase Motif II-- The D338N and H341A mutants were selected due to their location in the "DEAD" box Walker B highly conserved helicase II motif found in many NTPases. In addition, His-341 (motif II) is believed to interact with residues in motif VI and III (56). Asp-338 corresponds in sequence alignment with XPD to a high prevalence D234N XP disease-causing mutation. Overall, the UvrB mutations in the helicase motif regions identical to XPD disease-causing mutations had reduced DNA repair capacity. Previously, XPD disease-causing mutations (XPDG675R, R722W) have been isolated and purified and have been shown experimentally to perform poorly in helicase activity assays (50). The RAD3 K48R ATP-binding site mutant, which is part of the GXGKT Walker A Box, helicase I motif, has been shown to lose ATPase and DNA helicase activities but still binds ATP (57). This same mutation has been made previously in E. coli UvrB and shown to reduce greatly its ATPase activity (58). Helicase SF2 members contain the conserved consensus "DEAD/H" box in helicase motif II. The first (Asp) residue of this motif binds the Mg+2 ion; the second residue (Glu) is proposed to bind water during ATP hydrolysis. Studies in other helicases have shown mutations in motif II to be ATPase-deficient and incapable of DNA unwinding (7). The UvrB D338N mutant 1) is strongly impaired in its ability to support incision of damaged fluorescein or cholesterol-containing DNA (Fig. 5); 2) did not form a complex with DNA (Fig. 6A); 3) is defective in the destabilization of a damaged oligonucleotide (Fig. 6B); and 4) displayed significantly reduced ATP hydrolysis activity (Table II). The D338A mutation has been reported previously (59) in E. coli UvrB, although it was made under the assumption that the protein was functioning as a nuclease and not to test unwinding activity. In contrast mutating the nearby His-341 to alanine (conserved in 29/29 bacterial species, Table I) in helicase motif II resulted in a milder defect in the four functional assays.
Mutations in Helicase Motif V-- The UvrB Arg-506 mutation corresponds to the high prevalence R601L/R601W XP disease-causing mutation. Mutating this residue to alanine had a mild phenotype in the four biochemical assays. However, mutating the nearby Asp-510 residue to alanine (corresponding to XPD Asp-609), in the highly conserved helicase V motif, resulted in drastic reduction in activity across all the biochemical assays. The Asp-510 residue was mutated because it is part of a highly conserved motif found in helicase motif V, EG(I/L)D that is conserved from UvrB to RAD3 (S. cerevisiae) and XPD (human). The Glu residue of this motif is believed to be involved in long distance interactions across the protein with motif Ia (56). This mutation was reported previously by Lin et al. (59) in E. coli UvrB and shown to reduce its activity. Other residues in helicase motifs V and VI have been shown to be important for ATP hydrolysis; UvrB cannot hydrolyze ATP when Arg-540 and Arg-543 are mutated (24). It is interesting to note that the D510N mutation has a mild phenotype in most biochemical assays but has a significantly higher UvrA/UV DNA-stimulated ATPase activity than the wild type UvrB.
TTD-like Mutation in the -Hairpin--
The E110A/E110R UvrB
mutant was selected for analysis due to its correspondence (in location
according to the sequence alignment) to the R112H high prevalence TTD
disease-causing mutant and location in the UvrB
-hairpin proposed to
participate in DNA-binding interactions (23, 24, 26, 52). The Glu-110
to arginine or alanine UvrB mutations behaved the same as wild type
protein in all assays and did not exhibit any deficiency in the ability
to bind, cleave, or destabilize oligomeric DNA. Surprisingly, this
mutation which lies outside the ATP binding domain leads to a
significant increase in the UvrA/UV DNA-stimulated ATPase activity.
Although this mutation was selected because it corresponded to the XPD
R112H TTD-causing mutation, this region of the protein, however, shows
little sequence conservation between the UvrB and XPD proteins. The
-hairpin turn has already had been proposed to be one of the
structural loop regions of the XPD model predicted with lower
reliability than the rest of the structure and where differences in
behavior between UvrB and XPD were anticipated. Additionally, the
positively charged arginine residue would be expected to exhibit
different biochemical behavior than the negatively charged glutamic
acid. The Arg-112 XPD mutation is a TTD disease-causing mutation unlike the others selected which are XP disease-causing mutations. It is
possible because TTD is viewed as a disease of transcription and not
NER that some other mechanism is involved in the TTD disease syndrome
for which the bacterial NER enzyme UvrB is an inappropriate model
(6).
CONCLUSIONS
The goal of these studies was to construct a molecular model of human DNA repair protein XPD based on the experimentally determined crystal structure of the bacterial DNA repair protein, UvrB. Model building was validated by experimental verification of XPD mutations in corresponding residues in the UvrB protein. The XPD model helps to give structural insights into some of the mutations in the XPD gene which when mutated yield the XP phenotype. For example, one prediction of the XPD model is that Arg-683, which is located on the "backside" of XPD on domain 3, is part of a binding surface for interaction with p44. This residue is in close proximity to another mutation, D681N, which is the most common XPD mutation. These two mutations help define a p44 interaction site on XPD that is essential for XPD to be tightly bound to TFIIH (50). This XPD model in conjunction with the future biochemical analysis of these mutations will be useful formulating structure-function explanations for the various XPD mutations leading to TTD, XP, or CS phenotypes.
In summary, we found that XP disease-like mutations could be
successfully mimicked experimentally in UvrB (D338N, H341A, and R506A)
and had a greater impact on the activity of UvrB than a TTD
disease-like mutation (E110A/E110R). This model, and the concept that
XPD and UvrB both play a vital step in damage-strand verification, is
supported by recent DNA cross-linking experiments with XPD (60).
Reardon and Sancar (60) show that XPD is making intimate contact with a
psoralen monoadduct. Similar studies have shown UvrB can also be
cross-linked to psoralen (61)or bis-platinum adducts (62). This general
approach, the development and validation of a structural model for
other mammalian proteins, should be generally applicable where
structural information exists for closely functional related bacterial,
archaeal, or yeast proteins, which share significant protein structural
motifs or a structural protein fold.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed: NIEHS, Laboratory
of Molecular Genetics, P. O. Box 12233 Mail Drop D3-01, 111 T.W.
Alexander Dr., Research Triangle Park, NC 27709. Tel.: 919-541-2799; Fax: 919-541-5064; E-mail: vanhout1@niehs.nih.gov.
Published, JBC Papers in Press, November 27, 2002, DOI 10.1074/jbc.M210159200
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ABBREVIATIONS |
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The abbreviations used are:
NER, nucleotide
excision repair;
XP, xeroderma pigmentosum;
TTD, trichothiodystrophy;
CS, Cockayne syndrome;
ADP-PNP, 5'-adenylyl-,
-imidodiphosphate;
ssDNA, single-stranded DNA;
dsDNA, double-stranded DNA;
DTT, dithiothreitol.
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REFERENCES |
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