Regulation of Rad51 function by c-Abl in response to DNA damage

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 dynamic, 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 DNA replication. Rad51 foci colocalize with both replication protein A and sites of unscheduled DNA repair synthesis and may represent a nuclear domain for recombinational DNA repair. By 24 h postirradiation, most foci are sequestered into micronuclei or assembled into Rad51-coated DNA fibers. These micronuclei and DNA fibers display genome fragmentation typical of apoptotic cell death. Other repair proteins, such as Rad52 and Gadd45, are not eliminated from the nucleus. DNA double strand breaks in repair-deficient cells or induced by the clastogen etoposide are also accompanied by the sequestering of Rad51 protein before cell death. The spindle poison colcemid causes cell cycle arrest and Rad51-foci formation without directly damaging DNA. Collectively, these observations suggest that mammalian Rad51 protein associates with damaged DNA and/or with DNA that is temporarily or irreversibly unable to replicate and these foci may subsequently be eliminated from the nucleus.

The contribution of homologous recombination in preserving genome integrity in mammalian cells

Although it is clear that mammalian somatic cells possess the enzymatic machinery to perform homologous recombination of DNA molecules, the importance of this process in mitigating DNA damage has been uncertain. An initial genetic framework for studying homologous recombinational repair (HRR) has come from identifying relevant genes by homology or by their ability to correct mutants whose phenotypes are suggestive of recombinational defects. While yeast has been an invaluable guide, higher eukaryotes diverge in the details and complexity of HRR. For eliminating DSBs, HRR and end-joining pathways share the burden, with HRR contributing critically during S and G2 phases. It is likely that the removal of interstrand cross-links is absolutely dependent on efficient HRR, as suggested by the extraordinary sensitivity of the ercc1, xpf/ercc4, xrcc2, and xrcc3 mutants to cross-linking chemicals. Similarly, chromosome stability in untreated cells requires intact HRR, which may eliminate DSBs arising during DNA replication and thereby prevent chromosome aberrations. Complex regulation of HRR by cell cycle checkpoint and surveillance functions is suggested not only by direct interactions between human Rad51 and p53, c-Abl, and BRCA2, but also by very high recombination rates in p53-deficient cells.

Tyrosine phosphorylation of the TATA element modulatory factor by the FER nuclear tyrosine kinases

The FER locus in the mouse encodes two tyrosine kinases, p94fer and p51ferT. While p94fer accumulates in the cytoplasm and nucleus of most mammalian cells the expression of p51ferT is restricted to the nucleus of meiotic primary spermatocytes. The cellular function of the FER kinases is not understood, nor has a substrate for these enzymes been characterized. To identify putative substrates of p94fer and p51ferT, the two enzymes were used as 'baits' in the yeast two-hybrid screening system. cDNAs encoding the mouse TATA element modulatory factor (TMF) were repeatedly isolated in this assay. TMF was previously shown to bind the TATA element in RNA polymerase II promoters and impaired their functioning in a cell free transcription system. Both p94fer and p51ferT phosphorylated the TMF protein in in vitro and in vivo kinase assays. Sequential deletions showed that the carboxy-terminal region of TMF was essential for phosphorylation. In situ hybridization analysis revealed the preferential accumulation of TMF transcripts in meiotic spermatogenic and oogenic cells. p94fer and p51ferT may thus modulate the suppressive activity of TMF during cellular growth and in defined differentiation processes.