The xeroderma pigmentosum complementation group A (XP-A) protein, XPA, has recently been expressed in Escherichia coli in a soluble and fully functional form. An affinity column was prepared by linking the XPA protein to a solid support. When HeLa cell-free extract capable of excision repair was applied to the column, > 99.9% of the proteins were in the flow-through. However, the flow-through fraction lacked excision activity. The activity was restored by adding the high salt (1 M KCl) eluate of the column to the flow-through fraction. The XPA protein-bound fraction was tested for specific proteins by an in vitro complementation assay with a panel of cell-free extracts from DNA repair-deficient human and rodent cell lines. The XPA-bound fraction complemented cell-free extracts of excision repair cross-complementing 1 (ERCC-1), ERCC-4 (XP-F), and XP-A mutants. We conclude that the XPA damage recognition protein makes a ternary complex with the ERCC1/ERCC4(XPF) heterodimer with a potential nuclease function.
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