A tri-serine cluster within the topoisomerase IIα-interaction domain of the BLM helicase is required for regulating chromosome breakage in human cells

Abstract

The recQ-like helicase BLM interacts directly with topoisomerase IIα to regulate chromosome breakage in human cells.We demonstrate that a phosphosite tri-serine cluster (S577/S579/S580) within the BLM topoisomerase IIα-interaction region is required for this function. Enzymatic activities of BLM and topoisomerase IIα are reciprocally stimulated in vitro by ten-fold for topoisomerase IIα decatenation/relaxation activity and three-fold for BLM unwinding of forked DNA duplex substrates. A BLM transgene encoding alanine substitutions of the tri-serine cluster in BLM-/- transfected cells increasesmicronuclei, DNA double strand breaks and anaphase ultrα-fine bridges (UFBs), and decreases cellular co-localization of BLM with topoisomerase IIα. In vitro, these substitutions significantly reduce the topoisomerase IIα-mediated stimulation of BLM unwinding of forked DNA duplexes. Substitution of the tri-serine cluster with aspartic acids tomimic serine phosphorylation reverses these effects in vitro and in vivo. Our findings implicate themodification of this BLMtri-serine cluster in regulating chromosomal stability.