Centrin 2 stimulates nucleotide excision repair by interacting with xeroderma pigmentosum group C protein

Xeroderma pigmentosum group C (XPC) protein plays a key role in DNA damage recognition in global genome nucleotide excision repair (NER). The protein forms in vivo a heterotrimeric complex involving one of the two human homologs of Saccharomyces cerevisiae Rad23p and centrin 2, a centrosomal protein. Because centrin 2 is dispensable for the cell-free NER reaction, its role in NER has been unclear. Binding experiments with a series of truncated XPC proteins allowed the centrin 2 binding domain to be mapped to a presumed alpha-helical region near the C terminus, and three amino acid substitutions in this domain abrogated interaction with centrin 2. Human cell lines stably expressing the mutant XPC protein exhibited a significant reduction in global genome NER activity. Furthermore, centrin 2 enhanced the cell-free NER dual incision and damaged DNA binding activities of XPC, which likely require physical interaction between XPC and centrin 2. These results reveal a novel vital function for centrin 2 in NER, the potentiation of damage recognition by XPC.     

Flexibility and plasticity of human centrin 2 binding to the xeroderma pigmentosum group C protein (XPC) from nuclear excision repair

Human centrin 2 is a component of the nucleotide excision repair system, as a subunit of the heterotrimer including xeroderma pigmentosum group C protein (XPC) and hHR23B. The C-terminal domain of centrin (C-HsCen2) binds strongly a peptide from the XPC protein (P1-XPC: N(847)-R(863)). Here, we characterize the solution Ca(2+)-dependent structural and molecular features of the C-HsCen2 in complex with P1-XPC, mainly using NMR spectroscopy and molecular modeling. The N-terminal half of the peptide, organized as an alpha helix is anchored into a deep hydrophobic cavity of the protein, because of three bulky hydrophobic residues in position 1-4-8 and electrostatic contacts with the centrin helix E. Investigation of the whole centrin interactions shows that the N-terminal domain of the protein is not involved in the complex formation and is structurally independent from the peptide-bound C-terminal domain. The complex may exist in three different binding conformations corresponding to zero, one, and two Ca(2+)-bound states, which may exchange with various rates and have distinct structural stability. The various features of the intermolecular interaction presented here constitute a centrin-specific mode for the target binding.     

Structural, thermodynamic, and cellular characterization of human centrin 2 interaction with xeroderma pigmentosum group C protein

Human centrin 2 (HsCen2), an EF-hand calcium binding protein, plays a regulatory role in the DNA damage recognition during the first steps of the nucleotide excision repair. This biological action is mediated by the binding to a short fragment (N847-R863) from the C-terminal region of xeroderma pigmentosum group C (XPC) protein. This work presents a detailed structural and energetic characterization of the HsCen2/XPC interaction. Using a truncated form of HsCen2 we obtained a high resolution (1.8 A) X-ray structure of the complex with the peptide N847-R863 from XPC. Structural and thermodynamic analysis of the interface revealed the existence of both electrostatic and apolar inter-molecular interactions, but the binding energy is mainly determined by the burial of apolar bulky side-chains into the hydrophobic pocket of the HsCen2 C-terminal domain. Binding studies with various peptide variants showed that XPC residues W848 and L851 constitute the critical anchoring side-chains. This enabled us to define a minimal centrin binding peptide variant of five residues, which accounts for about 75% of the total free energy of interaction between the two proteins. Immunofluorescence imaging in HeLa cells demonstrated that HsCen2 binding to the integral XPC protein may be observed in living cells, and is determined by the same interface residues identified in the X-ray structure of the complex. Overexpression of XPC perturbs the cellular distribution of HsCen2, by inducing a translocation of centrin molecules from the cytoplasm to the nucleus. The present data confirm that the in vitro structural features of the centrin/XPC peptide complex are highly relevant to the cellular context.     

Structure of the mouse peptide N-glycanase-HR23 complex suggests co-evolution of the endoplasmic reticulum-associated degradation and DNA repair pathways

Peptide N-glycanase removes N-linked oligosaccharides from misfolded glycoproteins as part of the endoplasmic reticulum-associated degradation pathway. This process involves the formation of a tight complex of peptide N-glycanase with Rad23 in yeast and the orthologous HR23 proteins in mammals. In addition to its function in endoplasmic reticulum-associated degradation, HR23 is also involved in DNA repair, where it plays an important role in damage recognition in complex with the xeroderma pigmentosum group C protein. To characterize the dual role of HR23, we have determined the high resolution crystal structure of the mouse peptide N-glycanase catalytic core in complex with the xeroderma pigmentosum group C binding domain from HR23B. Peptide N-glycanase features a large cleft between its catalytic cysteine protease core and zinc binding domain. Opposite the zinc binding domain is the HR23B-interacting region, and surprisingly, the complex interface is fundamentally different from the orthologous yeast peptide N-glycanase-Rad23 complex. Different regions on both proteins are involved in complex formation, revealing an amazing degree of divergence in the interaction between two highly homologous proteins. Furthermore, the mouse peptide N-glycanase-HR23B complex mimics the interaction between xeroderma pigmentosum group C and HR23B, thereby providing a first structural model of how the two proteins interact within the nucleotide excision repair cascade in higher eukaryotes. The different interaction interfaces of the xeroderma pigmentosum group C binding domains in yeast and mammals suggest a co-evolution of the endoplasmic reticulum-associated degradation and DNA repair pathways.     

Structure, Dynamics and Thermodynamics of the Human Centrin 2/hSfi1 Complex

Centrin, an EF-hand calcium-binding protein, has been shown to be involved in the duplication of centrosomes, and Sfi1 (Suppressor of fermentation-induced loss of stress resistance protein 1) is one of its centrosomal targets. There are three isoforms of human centrin, but here we only considered centrin 2 (HsCen2). This protein has the ability to bind to any of the ∼ 25 repeats of human Sfi1 (hSfi1) with more or less affinity. In this study, we mainly focused on the 17th repeat (R17-hSfi1-20), which presents the highest level of similarity with a well-studied 17-residue peptide (P17-XPC) from human xeroderma pigmentosum complementation group C protein, another centrin target for DNA repair. The only known structure of HsCen2 was resolved in complex with P17-XPC. The 20-residue peptide R17-hSfi1-20 exhibits the motif L8L4W1, which is the reverse of the XPC motif, W1L4L8. Consequently, the dipole of the helix formed by this motif has a reverse orientation. We wished to ascertain the impact of this reversal on the structure, dynamics and affinity of centrin. To address this question, we determined the structure of C-HsCen2 [the C-terminal domain of HsCen2 (T94-Y172)] in complex with R17-hSfi1-20 and monitored its dynamics by NMR, after having verified that the N-terminal domain of HsCen2 does not interact with the peptide. The structure shows that the binding mode is similar to that of P17-XPC. However, we observed a 2 -Å translation of the R17-hSfi1-20 helix along its axis, inducing less anchorage in the protein and the disruption of a hydrogen bond between a tryptophan residue in the peptide and a well-conserved nearby glutamate in C-HsCen2. NMR dynamic studies of the complex strongly suggested the existence of an unusual calcium secondary binding mode in calcium-binding loop III, made possible by the uncommon residue composition of this loop. The secondary metal site is only populated at high calcium concentration and depends on the type of bound ligand.     

Centrin/Cdc31 is a novel regulator of protein degradation

Rad23 is required for efficient protein degradation and performs an important role in nucleotide excision repair. Saccharomyces cerevisiae Rad23, and its human counterpart (hHR23), are present in a complex containing the DNA repair factor Rad4 (termed XPC, for xeroderma pigmentosum group C, in humans). XPC/hHR23 was also reported to bind centrin-2, a member of the superfamily of calcium-binding EF-hand proteins. We report here that yeast centrin, which is encoded by CDC31, is similarly present in a complex with Rad4/Rad23 (called NEF2). The interaction between Cdc31 and Rad23/Rad4 varied by growth phase and reflected oscillations in Cdc31 levels. Strikingly, a cdc31 mutant that formed a weaker interaction with Rad4 showed sensitivity to UV light. Based on the dual function of Rad23, in both DNA repair and protein degradation, we questioned if Cdc31 also participated in protein degradation. We report here that Cdc31 binds the proteasome and multiubiquitinated proteins through its carboxy-terminal EF-hand motifs. Moreover, cdc31 mutants were highly sensitive to drugs that cause protein damage, failed to efficiently degrade proteolytic substrates, and formed altered interactions with the proteasome. These findings reveal for the first time a new role for centrin/Cdc31 in protein degradation.     

Binding surface mapping of intra- and interdomain interactions among hHR23B,ubiquitin, and polyubiquitin binding site 2 of S5a

hHR23B is the human homologue of the yeast protein RAD23 and is known to participate in DNA repair by stabilizing xeroderma pigmentosum group C protein. However, hHR23B and RAD23 also have many important functions related to general proteolysis. hHR23B consists of N-terminal ubiquitin-like (UbL), ubiquitin association 1 (UBA1), xeroderma pigmentosum group C binding, and UBA2 domains. The UBA domains interact with ubiquitin (Ub) and inhibit the assembly of polyubiquitin. On the other hand, the UbL domain interacts with the poly-Ub binding site 2 (PUbS2) domain of the S5a protein, which can carry polyubiquitinated substrates into the proteasome. We calculated the NMR structure of the UbL domain of hHR23B and determined binding surfaces of UbL and Ub to UBA1, UBA2, of hHR23B and PUbS2 of S5a by using chemical shift perturbation. Interestingly, the surfaces of UbL and Ub that bind to UBA1, UBA2, and PUbS2 are similar, consisting of five beta-strands and their connecting loops. This is the first report that an intramolecular interaction between UbL and UBA domains is possible, and this interaction could be important for the control of proteolysis by hHR23B. The binding specificities of UbL and Ub for PUbS1, PUbS2, and general ubiquitin-interacting motifs, which share the LALA motif, were evaluated. The UBA domains bind to the surface of Ub including Lys-48, which is required for multiubiquitin assembly, possibly explaining the observed inhibition of multiubiquitination by hHR23B. The UBA domains bind to UbL through electrostatic interactions supported by hydrophobic interactions and to Ub mainly through hydrophobic interactions supported by electrostatic interactions.     

Calcium-Binding Capacity of Centrin2 Is Required for Linear POC5 Assembly but Not for Nucleotide Excision Repair

Centrosomes, the principal microtubule-organising centres in animal cells, contain centrins, small, conserved calcium-binding proteins unique to eukaryotes. Centrin2 binds to xeroderma pigmentosum group C protein (XPC), stabilising it, and its presence slightly increases nucleotide excision repair (NER) activity in vitro. In previous work, we deleted all three centrin isoforms present in chicken DT40 cells and observed delayed repair of UV-induced DNA lesions, but no centrosome abnormalities. Here, we explore how centrin2 controls NER. In the centrin null cells, we expressed centrin2 mutants that cannot bind calcium or that lack sites for phosphorylation by regulatory kinases. Expression of any of these mutants restored the UV sensitivity of centrin null cells to normal as effectively as expression of wild-type centrin. However, calcium-binding-deficient and T118A mutants showed greatly compromised localisation to centrosomes. XPC recruitment to laser-induced UV-like lesions was only slightly slower in centrin-deficient cells than in controls, and levels of XPC and its partner HRAD23B were unaffected by centrin deficiency. Interestingly, we found that overexpression of the centrin interactor POC5 leads to the assembly of linear, centrin-dependent structures that recruit other centrosomal proteins such as PCM-1 and NEDD1. Together, these observations suggest that assembly of centrins into complex structures requires calcium binding capacity, but that such assembly is not required for centrin activity in NER.     

Xeroderma pigmentosum group C protein possesses a high affinity binding site to human centrin 2 and calmodulin

Human centrin 2 (HsCen2), a member of the EF-hand superfamily of Ca2+-binding proteins, is commonly associated with centrosome-related structures. The protein is organized in two domains, each containing two EF-hand motifs, but only the C-terminal half exhibits Ca2+ sensor properties. A significant fraction of HsCen2 is localized in the nucleus, where it was recently found associated with the xeroderma pigmentosum group C protein (XPC), a component of the nuclear excision repair pathway. Analysis of the XPC sequence (940 residues), using a calmodulin target recognition software, enabled us to predict two putative binding sites. The binding properties of the two corresponding peptides were investigated by isothermal titration calorimetry. Only one of the peptides (P1-XPC) interacts strongly (Ka = 2.2 x 10(8) m-1, stoichiometry 1:1) with HsCen2 in a Ca2+-dependent manner. This peptide also binds, with a similar affinity (Ka = 1.1 x 10(8) m-1) to a C-terminal construct of HsCen2, indicating that the interaction with the integral protein is mainly the result of the contribution of the C-terminal half. The second peptide (P2-XPC) failed to show any detectable binding either to HsCen2 or to its C-terminal lobe. The two peptides interact with different affinities and mechanisms with calmodulin. Circular dichroism and nuclear magnetic resonance were used to structurally characterize the complex formed by the C-terminal domain of HsCen2 with P1-XPC.     

The cloning and expression characterization of the centrosome protein genes family (centrin genes) in rat testis

Centrins are members of the centrosome protein family, which is highly conserved during revolution. The homologous genes of centrin in many organisms had been cloned, but the sequences of the rat centrin genes were not reported yet in GenBank. We cloned the cDNA fragments of centrin-1, -2 and -3 from the rat testis by RT-PCR, and analyzed the homology of the deduced amino acid sequences. The expression characterization of centrin genes in rat spermatogenesis was carried out by semi-quantitative RT-PCR. The results show that the homology of the corresponding centrin proteins in human, mouse and rat is high. The expression of centrin-1 is testis-specific, spermatogenic cell-specific and developmental stage-related. Centrin-1 begins to be transcribed when the meiosis occurs, and its mRNA level reaches the peak in round spermatids. Centrin-2 and centrin-3 are highly expressed in spermatogonia and their mRNA level decreases markedly when meiosis occurs. These results suggest that centrin-1 may play roles in meiosis and spermiogenesis, and centrin-2 and centrin-3 may be related to mitosis.     

Identification and characterization of XPC-binding domain of hHR23B.

hHR23B was originally isolated as a component of a protein complex that specifically complements nucleotide excision repair (NER) defects of xeroderma pigmentosum group C cell extracts in vitro and was identified as one of two human homologs of the Saccharomyces cerevisiae NER gene product Rad23. Recombinant hHR23B has previously been shown to significantly stimulate the NER activity of recombinant human XPC protein (rhXPC). In this study we identify and functionally characterize the XPC-binding domain of hHR23B protein. We prepared various internal as well as terminal deletion products of hHR23B protein in a His-tagged form and examined their binding with rhXPC by using nickel-chelating Sepharose. We demonstrate that a domain covering 56 amino acids of hHR23B is required for binding to rhXPC as well as for stimulation of in vitro NER reactions. Interestingly, a small polypeptide corresponding to the XPC-binding domain is sufficient to exert stimulation of XPC NER activity. Comparison with known crystal structures and analysis with secondary structure programs provided strong indications that the binding domain has a predominantly amphipathic alpha-helical character, consistent with evidence that the affinity with XPC is based on hydrophobic interactions. Our work shows that binding to XPC alone is required and sufficient for the role of hHR23B in in vitro NER but does not rule out the possibility that the protein has additional functions in vivo.     

The cloning and expression characterization of the centrosome protein genes family (centrin genes) in rat testis

Centrins are members of the centrosome protein family, which is highly conserved during revolution. The homologous genes of centrin in many organisms had been cloned, but the sequences of the rat centrin genes were not reported yet in GenBank. We cloned the cDNA fragments of centrin-1, -2 and -3 from the rat testis by RT-PCR, and analyzed the homology of the deduced amino acid sequences. The expression characterization of centrin genes in rat spermato-genesis was carried out by semi-quantitative RT-PCR. The results show that the homology of the corresponding centrin proteins in human, mouse and rat is high. The expression of centrin-1 is testis-specific, spermatogenic cell-specific and developmental stage-related. Centrin-1 begins to be transcribed when the meiosis occurs, and its mRNA level reaches the peak in round spermatids. Centrin-2 and centrin-3 are highly expressed in spermatogonia and their mRNA level decreases markedly when meiosis occurs. These results suggest that centrin-1 may play roles in     

Structure of the N-terminal calcium sensor domain of centrin reveals the biochemical basis for domain-specific function

Centrin is an essential component of microtubule-organizing centers in organisms ranging from algae and yeast to humans. It is an EF-hand calcium-binding protein with homology to calmodulin but distinct calcium binding properties. In a previously proposed model, the C-terminal domain of centrin serves as a constitutive anchor to target proteins, and the N-terminal domain serves as the sensor of calcium signals. The three-dimensional structure of the N-terminal domain of Chlamydomonas rheinhardtii centrin has been determined in the presence of calcium by solution NMR spectroscopy. The domain is found to occupy an open conformation typical of EF-hand calcium sensors. Comparison of the N- and C-terminal domains of centrin reveals a structural and biochemical basis for the domain specificity of interactions with its cellular targets and the distinct nature of centrin relative to other EF-hand proteins. An NMR titration of the centrin N-terminal domain with a fragment of the known centrin target Sfi1 reveals binding of the peptide to a discrete site on the protein, which supports the proposal that the N-terminal domain serves as a calcium sensor in centrin.     

The mode of action of centrin. Binding of Ca2+ and a peptide fragment of Kar1p to the C-terminal domain

Centrin is an EF-hand calcium-binding protein closely related to the prototypical calcium sensor protein calmodulin. It is found in microtubule-organizing centers of organisms ranging from algae and yeast to man. In vitro, the C-terminal domain of centrin binds to the yeast centrosomal protein Kar1p in a calcium-dependent manner, whereas the N-terminal domain does not show any appreciable affinity for Kar1p. To obtain deeper insights into the structural basis for centrin's function, we have characterized the affinities of the C-terminal domain of Chlamydomonas reinhardtii centrin for calcium and for a peptide fragment of Kar1p using CD, fluorescence, and NMR spectroscopy. Calcium binding site IV in C. reinhardtii centrin was found to bind Ca2+ approximately 100-fold more strongly than site III. In the absence of Ca2+, the protein occupies a mixture of closed conformations. Binding of a single ion in site IV is sufficient to radically alter the conformational equilibrium, promoting occupancy of an open conformation. However, an exchange between closed and open conformations remains even at saturating levels of Ca2+. The population of the open conformation is substantially stabilized by the presence of the target peptide Kar1p-(239-257) to a point where a single ion bound in site IV is sufficient to completely shift the conformational equilibrium to the open conformation. This is reflected in the enhancement of the Ca2+ affinity in this site by more than an order of magnitude. These data confirm the direct coupling of the Ca2+ binding-induced shift in the equilibrium between the closed and open conformations to the binding of the peptide. Combined with the common localization of the two proteins in the microtubule organizing center, our results suggest that centrin is constitutively bound to Kar1p through its C-terminal domain and that centrin's calcium sensor activities are mediated by the N-terminal domain.     

Structure-function analysis of the EF-hand protein centrin-2 for its intracellular localization and nucleotide excision repair

Centrin-2 is an evolutionarily conserved, calmodulin-related protein, which is involved in multiple cellular functions including centrosome regulation and nucleotide excision repair (NER) of DNA. Particularly to exert the latter function, complex formation with the XPC protein, the pivotal NER damage recognition factor, is crucial. Here, we show that the C-terminal half of centrin-2, containing two calcium-binding EF-hand motifs, is necessary and sufficient for both its localization to the centrosome and interaction with XPC. In XPC-deficient cells, nuclear localization of overexpressed centrin-2 largely depends on co-overexpression of XPC, and mutational analyses of the C-terminal domain suggest that XPC and the major binding partner in the centrosome share a common binding surface on the centrin-2 molecule. On the other hand, the N-terminal domain of centrin-2 also contains two EF-hand motifs but shows only low-binding affinity for calcium ions. Although the N-terminal domain is dispensable for enhancement of the DNA damage recognition activity of XPC, it contributes to augmenting rather weak physical interaction between XPC and XPA, another key factor involved in NER. These results suggest that centrin-2 may have evolved to bridge two protein factors, one with high affinity and the other with low affinity, thereby allowing delicate regulation of various biological processes.     

Binding of human centrin 2 to the centrosomal protein hSfi1

hSfi1, a human centrosomal protein with homologs in other eukaryotic organisms, includes 23 repeats, each of 23 amino acids, separated by 10 residue linkers. The main molecular partner in the centrosome is a small, calcium-binding EF-hand protein, the human centrin 2. Using isothermal titration calorimetry experiments, we characterized the centrin-binding capacity of three isolated hSfi1 repeats, two exhibiting the general consensus motif and the third being the unique Pro-containing human repeat. The two standard peptides bind human centrin 2 and its isolated C-terminal domain with high affinity (approximately 10(7) M(-1)) by an enthalpy-driven mechanism, with a moderate Ca2+ dependence. The Pro-containing repeat shows a binding affinity that is two orders of magnitude lower. The target binding site is localized within the C-terminal domain of human centrin 2. Fluorescence titration and NMR spectroscopy show that the well-conserved Trp residue situated in the C-terminus of each repeat is deeply embedded in a protein hydrophobic cavity, indicating that the peptide direction is reversed relative to previously studied centrin targets. The present results suggest that almost all of the repeats of the Sfi1 protein may independently bind centrin molecules. On the basis of this hypothesis and previous studies on centrin self-assembly, we propose a working model for the role of centrin-Sfi1 interactions in the dynamic structure of centrosome-associated contractile fibers.     

Synthesis of complex phosphopeptides as mimics of p53 functional domains.

A complete set of mono-, di- and triphosphorylated peptides comprising amino acids 10-27, the Mdm2 and p300 binding site(s) of p53, with and without a fluorescein label at the N-terminus, was synthesized by step-by-step solid phase synthesis. Fluorescence polarization analysis revealed that phosphorylation at Thr18 decreased binding to recombinant Mdm2 protein compared with the unphosphorylated and the two other single phosphorylated analogues. Unlabelled multiply phosphorylated peptides corresponding to this amino-terminal transactivation domain proved to be powerful tools in analysing the phosphate specificity of existing anti-p53 monoclonal and polyclonal antibodies using direct ELISA. The tetramerization domain of human p53 protein was modelled with a 53 residue-long unlabelled unphosphorylated and Ser315-phosphorylated peptide pair. CD analysis showed similar alpha-helical structures for both peptides and no major difference in the secondary structure could be observed upon phosphorylation. Size-exclusion HPLC indicated that these synthetic oligomerization domain mimics underwent a pH-dependent tetramerization process, but the presence of a phosphate group at Ser315 did not modify the oligomeric state of the 308-360 p53 fragments. Nevertheless, the fluorescein-labelled Ser315 phosphorylated peptide bound to the downstream signalling ligand DNA topoisomerase I protein with slightly higher affinity than did the unphosphorylated analogue.     

An anti-endotoxin peptide that generates from the amino-terminal domain of complement regulatory protein C1 inhibitor

C1 inhibitor (C1INH), a complement regulatory protein, prevents endotoxin shock via a direct interaction of the amino-terminal domain with gram-negative bacterial lipopolysaccharide (LPS). Importantly, the cleaved, inactive C1INH still is an anti-endotoxin effector indicating the anti-endotoxin peptide that generates from the amino-terminal domain of C1INH. In this study, we first identified that a cleaved fragment within the major part of the amino-terminal domain in in vitro proteolytic analysis of C1INH had an ability to bind to LPS. We synthesized several peptides overlapping the C1INH cleaved fragment. Among these synthetic peptides, a 13-mer derivative peptide at position from 18 to 30, named N2((18-30)), exhibited the most powerful anti-endotoxin activity in vitro, enlightening that it was most strong at binding to LPS, inhibiting the interaction of LPS with LPS-binding protein (LBP), blocking LPS binding to CD14(+) cells, and suppressing production of tumor necrosis factor (TNF)-alpha by murine macrophages, RAW 264.7. In the murine endotoxin shock model, the peptide N2((18-30)) protected mice from LPS-induced lethal septic shock by inhibiting macrophage activation. These data indicate that the peptide N2((18-30)) derived from the amino-terminal region of C1INH is anti-endotoxin.     

Structure of the XPC binding domain of hHR23A reveals hydrophobic patches for protein interaction

Rad23 proteins are involved both in the ubiquitin-proteasome pathway and in nucleotide excision repair (NER), but the relationship between these two pathways is not yet understood. The two human homologs of Rad23, hHR23A and B, are functionally redundant in NER and interact with xeroderma pigmentosum complementation group C (XPC) protein. The XPC-hHR23 complex is responsible for the specific recognition of damaged DNA, which is an early step in NER. The interaction of the XPC binding domain (XPCB) of hHR23A/B with XPC protein has been shown to be important for its optimal function in NER. We have determined the solution structure of XPCB of hHR23A. The domain consists of five amphipathic helices and reveals hydrophobic patches on the otherwise highly hydrophilic domain surface. The patches are predicted to be involved in interaction with XPC. The XPCB domain has limited sequence homology with any proteins outside of the Rad23 family except for sacsin, a protein involved in spastic ataxia of Charlevoix-Saguenay, which contains a domain with 35% sequence identity.