Identification of a type I Ca(2+)/Mg(2+)-dependent endonuclease induced in maize cells exposed to camptothecin

BACKGROUND: Camptothecin is a plant alkaloid that specifically binds topoisomerase I, inhibiting its activity and inducing double stranded breaks in DNA and activating the cell responses to DNA damage. RESULTS: Maize cultured cells were incubated in the presence of different concentrations of camptothecin. Camptothecin inhibits cultured cell growth, induces genomic DNA degradation, and induces a 32 kDa Ca(2+)/Mg(2+)-dependent nuclease activity. This nuclease, we called CaMNUC32, is inhibited by Zn(2+) and by acid pH, it is mainly localized in the nucleus and it cleaves single- and double-stranded DNA, with a higher activity against single-stranded DNA. Two-dimensional electrophoresis combined with mass spectrometry suggests that CaMNUC32 is a member of the type I S1/P1 nuclease family. This type of nucleases are usually Zn(2+)-dependent but our results support previous indications that S1-type nucleases have a wide variety of enzyme activities, including Ca(2+)/Mg(2+)-dependent. CONCLUSIONS: We have identified and characterized CaMNUC32, a 32 kDa Ca(2+)/Mg(2+)-dependent nuclease of the S1/P1 family induced by the topoisomerase I inhibitor camptothecin in maize cultured cells.