Calcium-induced cleavage of DNA topoisomerase I involves the cytoplasmic-nuclear shuttling of calpain 2

Important to the function of calpains is temporal and spatial regulation of their proteolytic activity. Here, we demonstrate that cytoplasm-resident calpain 2 cleaves human nuclear topoisomerase I (hTOP1) via Ca²⁺-activated proteolysis and nucleoplasmic shuttling of proteases. This proteolysis of hTOP1 was induced by either ionomycin-caused Ca²⁺ influx or addition of Ca²⁺ in cellular extracts. Ca²⁺ failed to induce hTOP1 proteolysis in calpain 2-knockdown cells. Moreover, calpain 2 cleaved hTOP1 in vitro. Furthermore, calpain 2 entered the nucleus upon Ca²⁺ influx, and calpastatin interfered with this process. Calpain 2 cleavage sites were mapped at K¹⁵⁸ and K¹⁸³ of hTOP1. Calpain 2-truncated hTOP1 exhibited greater relaxation activity but remained able to interact with nucleolin and to form cleavable complexes. Interestingly, calpain 2 appears to be involved in ionomycin-induced protection from camptothecin-induced cytotoxicity. Thus, our data suggest that nucleocytoplasmic shuttling may serve as a novel type of regulation for calpain 2-mediated nuclear proteolysis.