The major human AP-endonuclease 1 (APE1) is a multifunctional protein that plays a central role in the repair of damaged DNA by acting as a dual-function nuclease in the base excision repair pathway. This enzyme was also independently identified as a redox activator of AP-1 DNA-binding activity and has subsequently been shown to activate a variety of transcription factors via a redox mechanism. In a third distinct role, APE1 was identified as a component of a trans-acting complex that acts as a repressor by binding to the negative calcium responsive elements (nCaRE)-A and nCaRE-B, which were first discovered in the promoter of the human parathyroid gene and later in the APE1 promoter itself. Here we show that the nuclear protein complex which binds to the nCaRE-B2 of the hAPE1 gene contains APE1 itself and the heterogeneous nuclear ribonucleoprotein L (hnRNP-L). The interaction between the APE1 and hnRNP-L proteins does not require the represence of nCaRE-B2. Our results support the possibility that the APE1 gene is down-regulated by its own product, which would be the first such example of the regulation of a DNA repair enzyme, and identify a novel function of hnRNP-L in transcriptional regulation.
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