BLAP75/RMI1 promotes the BLM-dependent dissolution of homologous recombination intermediates

doi:10.1073/pnas.0508295103

BLAP75/RMI1 promotes the BLM-dependent dissolution of homologous recombination intermediates

*Cancer Research UK, Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DS, United Kingdom;
^†Department of Biochemistry, University of Toronto, Toronto, ON, Canada M5S 1A8;
^‡Experimental Radiation Oncology, University of Texas M. D. Anderson Cancer Center, Houston, TX 77030; and
^§Laboratory of Genetics, National Institute on Aging, National Institutes of Health, Baltimore, MD 21224

Edited by Philip C. Hanawalt, Stanford University, Stanford, CA, and approved January 10, 2006 (received for review September 22, 2005)

Abstract

BLM encodes a member of the highly conserved RecQ DNA helicase family, which is essential for the maintenance of genome stability. Homozygous inactivation of BLM gives rise to the cancer predisposition disorder Bloom’s syndrome. A common feature of many RecQ helicase mutants is a hyperrecombination phenotype. In Bloom’s syndrome, this phenotype manifests as an elevated frequency of sister chromatid exchanges and interhomologue recombination. We have shown previously that BLM, together with its evolutionarily conserved binding partner topoisomerase IIIα (hTOPO IIIα), can process recombination intermediates that contain double Holliday junctions into noncrossover products by a mechanism termed dissolution. Here we show that a recently identified third component of the human BLM/hTOPO IIIα complex, BLAP75/RMI1, promotes dissolution catalyzed by hTOPO IIIα. This activity of BLAP75/RMI1 is specific for dissolution catalyzed by hTOPO IIIα because it has no effect in reactions containing either Escherichia coli Top1 or Top3, both of which can also catalyze dissolution in a BLM-dependent manner. We present evidence that BLAP75/RMI1 acts by recruiting hTOPO IIIα to double Holliday junctions. Implications of the conserved ability of type IA topoisomerases to catalyze dissolution and how the evolution of factors such as BLAP75/RMI1 might confer specificity on the execution of this process are discussed.

Footnotes

^¶To whom correspondence should be addressed. E-mail: ian.hickson{at}cancer.org.uk
Author contributions: L.W. designed research; L.W., C.Z.B., and C.X. performed research; G.W.B., W.W., J.O., C.X., J.Y., and M.C. contributed new reagents/analytic tools; L.W., C.Z.B., J.Y., W.W., L.L., G.W.B., and I.D.H. analyzed data; and L.W. and I.D.H. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations:

BS,

Bloom’s syndrome;

SCE,

sister chromatid exchange;

DHJ,

double Holliday junction;

MBP,

maltose-binding protein;

hTOPO IIIα,

human topoisomerase IIIα;

Top1 and Top3,

Escherichia coli topoisomerases 1 and 3.