XPC AND HUMAN HOMOLOGS OF RAD23 - INTRACELLULAR-LOCALIZATION AND RELATIONSHIP TO OTHER NUCLEOTIDE EXCISION-REPAIR COMPLEXES

The xeroderma pigmentosum syndrome complementation group C (XP-C) is d ue to a defect in the global genome repair subpathway of nucleotide ex cision repair (NER), The XPC protein is complexed with HHR23B, one of the two human homologs of the yeast NER protein, RAD23 [Masutani at al . (1994) EMBO J. 8, 1831-1843], Using heparin chromatography, gel filt ration and native gel electrophoresis we demonstrate that the majority of HHR23B is in a free, non-complexed form, and that a minor fraction is tightly associated with XPC, In contrast, we cannot detect any bou nd HHR23A, Thus the HHR23 proteins may have an additional function ind ependent of XPC. The fractionation behaviour suggests that the non-bou nd forms of the HHR23 proteins are not necessary for the core of the N ER reaction, Although both HHR23 proteins share a high level of overal l homology, they migrate very differently on native gels, pointing to a difference in conformation, Gel filtration suggests the XPC-HHR23B h eterodimer resides in a high MW complex, However, immunodepletion stud ies starting from repair-competent Manley extracts fail to reveal a st able association of a significant fraction of the HHR23 proteins or th e XPC-HHR23B complex with the basal transcription/repair factor TFIIH, or with the ERCC1 repair complex, Consistent with a function in repai r or DNA/chromatin metabolism, immunofluorescence studies show all XPC , HHR23B and (the free) HHR23A to reside in the nucleus.