Isolation of active recombinant XPG protein, a human DNA repair endonuclease

Complementation group G of xeroderma pigmentosum (XP-G) is one of the most rare and phenotypically heterogeneous forms of this inherited disorder. XP-G patients vary from having a very mild defect in DNA repair to being severely affected, and a few cases are also associated with the neurological complications of Cockayne's syndrome. The XPG gene encodes an acidic protein with a predicted molecular mass of 133 kDa that confers normal UV resistance when expressed in XP-G cells. Here we report the isolation of full-length XPG as a soluble protein expressed from a recombinant baculovirus. The purified polypeptide corrects the DNA nucleotide excision repair defect of XP-G cell extracts in vitro, and it acts as a magnesium-dependent single-stranded DNA endonuclease. This is the first direct evidence for a human protein with properties that implicate it in the incision step of nucleotide excision repair.