The Bloom’s syndrome helicase BLM functions as a homodimer within the Bloom’s complex

Homologous recombination (HR) is an essential and highly regulated cellular process. Unintended HR in somatic cells is potentially deleterious; it can result in translocations and/or somatic cell loss of heterozygosity. This is suppressed by the BLM protein in combination with topoisomerase III, RMI1 and RMI2 in a hetero-tetrameric complex (the ‘Bloom’s complex’). To understand how the activities of BLM helicase and topoisomerase III are coupled, we purified the four-subunit complex. Chemical crosslinking and mass spectrometry revealed a unique architecture that couples helicase and topoisomerase domains.