Ca²⁺-mediated inhibition of a nuclear protein that recognizes UV-damaged DNA and is constitutively overexpressed in resistant human cells: DNA-binding assay

A nuclear protein that recognizes UV-damaged DNA was detected from HeLa cells using DNA-binding assay. Treatment of cells with Ca²⁺ ionophore (A23187) caused a dramatic inhibition of the damage-recognition activity. In contrast, in vitro treatment of nuclear extracts with agents that affect protein conformation (such as urea, NP40 and Ca²⁺) did not significantly affect on the damage-recognition activity. The Ca²⁺-mediated inhibition of UV damage recognition was reconstituted by the addition of the cytosolic extracts, suggesting that the Ca²⁺ effect does not directly act on the UV damage-recognition protein. The expression of the detected nuclear protein was increased in UV-resistant HeLa cells. In contrast, the level of this protein was dramatically reduced in UV-sensitive xeroderma pigmentosum group A cells. In addition, UV damage-recognition protein is resistant to RNase, and is independent of the previously identified proteins that bind cisplatin-DNA adduct. These findings implied that the recognition of UV-DNA adduct is modulated by the intracellular level of Ca²⁺.