Cleavage of the Bloom's syndrome gene product during apoptosis by caspase-3 results in an impaired interaction with topoisomerase III alpha

In higher eukaryotes, the integration of signals triggered in response to c ertain types of stress can result in programmed cell death. Central to thes e events is the sequential activation of a cascade of proteinases known as caspases. The final activated effector caspases of this cascade digest a nu mber of cellular proteins, in some cases increasing their enzymatic activit y, in others destroying their function. Of the proteins shown to be targets for caspase-mediated proteolysis, a surprisingly large proportion are prot eins involved in the signalling or repair of DNA damage. Here we investigat e whether BLM, the product of the gene mutated in Bloom's syndrome, a human autosomal disease characterised by cancer predisposition and sunlight sens itivity, is cleaved during apoptosis. BLM interacts with topoisomerase Ilia and has been proposed to play an important role in maintaining genomic int egrity through its roles in DNA repair and replication. We show that BLM is cleaved during apoptosis by caspase-3 and reveal that the main cleavage si te is located at the junction between the N-terminal and central helicase d omains of BLM. Proteolytic cleavage by caspase-3 produces a 120 kDa fragmen t, which contains the intact helicase domain and three smaller fragments, t he relative amounts of which depend on time of incubation with caspase-3. T he 120 kDa fragment retains the helicase activity of the intact BLM protein . However, its interaction with topoisomerase Ilia is severely impaired. Si nce the BLM-topoisomerase interaction is believed to be necessary for many of the replication and recombination functions of BLM, we suggest that casp ase-3 cleavage of BLM could alter the localisation and/or function of BLM a nd that these changes may be important in the process of apoptosis.