The XPC protein carries out the initial damage-recognition step of NER. The XP-C complementation group is one of the most frequent in the Western population, but it is rare in Japan. Although XP-C patients have severe skin problems, they rarely suffer from neurological abnormalities (see Table 1). Cells derived from this complementation group are totally deficient in global genome repair of UV-induced DNA damage but maintain the ability to remove photoproducts from the transcribed strand of transcriptionally active genes (17,18). Consequently, RNA synthesis recovers with kinetics similar to those of normal cells following UV irradiation. The XPC protein, in contrast to XPA, XPB, XPD, XPF, and XPG proteins, is therefore involved in only one of the NER subpathways. This likely accounts for the milder phenotype of XP-C patients compared with those from completely NER-deficient complementation groups. Since XP-C patients can repair damage in transcribing DNA, these crucial regions of DNA, which are in active use, are repaired normally.

The human XPC gene, located on chromosome 3p25.1, encodes a protein of 125 kD, which forms a tight complex with the 58-kD HR23B protein and is required for the initial steps of NER. HR23B and HR23A are two human homologs of the yeast NER protein Rad23 (19). HR23B harbors an XPC-binding domain of 54 amino acids, which is sufficient to stimulate the catalytic activity of XPC in vitro (20). Both HR23 proteins are more abundant than XPC in mammalian cells, and they contain a ubiquitin-like region at their amino-terminus (19) that, at least in their yeast homolog, is essential for its repair function (21,22). Recent studies have shown that the XPC-HR23B complex contains a third subunit, the centroso-mal component centrin2/caltractin1 (CEN2), which stabilizes the complex and stimulates XPC activity (23). The significance of this association is not yet clear.

Although the involvement of XPC in an early step of GGR has been known for a long time (24), its role as the first protein to recognize the damage has been recognized only recently (25-27). This function was for a long time attributed to XPA—the first NER protein shown to have preferential affinity for DNA lesions. It has now been demonstrated both in vitro and in vivo (25,27) that XPA is involved in a later step of the NER pathway (see Sec. IV.C and Fig. 1).

XPC-HR23B possesses binding affinity for both single- and double-stranded DNA, and its affinity for damaged DNA is at least 10 times greater than it is for undamaged DNA (19,24,25). By using plasmids of different sizes that contained known NER-substrate lesions in an in vitro damage-recognition competition assay, Sugasawa and coworkers (25) showed that repair is detected only when the damaged plasmids are incubated with purified XPC-HR23B complex or with XPC+/+ extracts, before XPA, but not vice-versa. Agreeing with the above findings are the results from an in vivo study (27), in which irradiation through a filter was used to introduce damage to specific sites within the nucleus. Immunoflu-orescence was then used to show the migration of different NER components to the site of the damage. This work showed that assembly of the NER components at the site of the damage is sequential. Localization of XPC to the damage was independent of all other components, whereas migration of any of the other components did not occur in XP-C cells. These data strongly suggest that the XPC-HR23B protein complex is the prerequisite factor for the initial step of dam age recognition of GGR by NER and for the recruitment of all subsequent NER factors in the preincision complex.

XPC-HR23B binds to small bubble structures, irrespective of whether they contain damage, but incisions by the NER nucleases (see Sec. IV.F and G) will only be effected if damage is present in the bubble. This is consistent with a two-step mechanism for damage recognition. First, the helix distortion is recognized by XPC-HR23B, and the presence of damaged bases is verified subsequently by other factors (28). XPC interacts with TFIIH, and this interaction is independent of XPA (26). As described in Sec. IV.D and E, TFIIH contains the XPB and XPD proteins and is used to open up the DNA structure at the site of the damage. It is likely that recruitment of TFIIH to damaged sites by XPC is the next step in the NER process.

Two independently generated knockout mouse models for Xpc are viable and develop normally, indicating that the XPC protein is not essential for life (29-31). Although age-dependent spontaneous mutagenesis is increased in Xpc-defective mice, there is no increased incidence of spontaneous tumorigenesis or premature aging at least until 1 year of age (29,30,32). Xpc mice are, however, sensitive to UVB irradiation with a high predisposition to skin and eye tumors compared with wild-type mice (29,30,33,34).

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