Sequestration of mammalian Rad51-recombination protein into micronuclei

The mammalian Rad51 protein is involved in homologous recombination and in DNA damage repair. Its nuclear distribution after DNA damage is highly dyna mic, and distinct foci of Rad51 protein, distributed throughout the nuclear volume, are induced within a few hours after gamma irradiation; these-foci then coalesce into larger clusters. Rad51-positive cells do not undergo DN A replication. Rad51 foci colocalize with both replication protein A and si tes of unscheduled DNA repair synthesis and may represent a nuclear domain for recombinational DNA repair. By 24 h postirradiation, most foci are sequ estered into micronuclei or assembled into Rad51-coated DNA fibers. These m icronuclei and DNA fibers display genome fragmentation typical of apoptotic cell death. Other repair proteins, such as Rad52 and Gadd45, are not elimi nated from the nucleus. DNA double strand breaks in repair-deficient cells or induced by the clastogen etoposide are also accompanied by the sequester ing of Rad51 protein before cell death. The spindle poison colcemid causes cell cycle arrest and Rad51-foci formation without directly damaging DNA. C ollectively, these observations suggest that mammalian Rad51 protein associ ates with damaged DNA and/or with DNA that is temporarily or irreversibly u nable to replicate and these foci may subsequently be eliminated from the n ucleus.