Saccharomyces cerevisiae Mer3 Helicase Stimulates 3′–5′ Heteroduplex Extension by Rad51 Implications for Crossover Control in Meiotic Recombination

AbstractCrossover and noncrossover recombinants can form by two different pathways during meiotic recombination in Saccharomyces cerevisiae. The MER3 gene is known to affect selectively crossover, but not noncrossover, recombination. The Mer3 protein is a DNA helicase that unwinds duplex DNA in the 3′ to 5′ direction. To define the underlying molecular steps of meiotic recombination, we investigated the role of Mer3 helicase in DNA strand exchange promoted by Rad51 protein. We found that Mer3 helicase does not function as an initiator of DNA pairing events but, rather, it stimulates DNA heteroduplex extension in the 3′ → 5′ direction relative to the incoming (or displaced) single-stranded DNA. Conversely, Mer3 helicase blocks DNA heteroduplex extension in the 5′ → 3′ direction. Our results support the idea that Mer3 helicase stabilizes nascent joint molecules via DNA heteroduplex extension to permit capture of the second processed end of a double-stranded DNA break, a step which is required for crossover recombinant product formation

Redistribution of Transcription Factor AP-2α in Differentiating Cultured Human Epidermal Cells

Expression of the transcription factor AP-2α was examined in cultured human epidermal cells. Levels of AP-2α mRNA increased substantially after the cultures reached confluence, similar to the expression pattern of the differentiation markers involucrin and keratinocyte transglutaminase. The level of AP-2α protein in nuclear extracts declined markedly after confluence, however, along with its ability to form complexes with oligonucleotides containing the AP-2 response element. In contrast, the levels of AP-2α protein in cytoplasmic extracts increased dramatically after confluence, but these extracts had low DNA binding activity. Supershift experiments with specific antisera detected only AP-2α and not the β or γ isoforms. Examination of its localization by confocal microscopy revealed that AP-2α was primarily in the nucleus of basal cells and largely cytoplasmic in the most superficial cells. Localization was a dynamic phenomenon in that changing the medium resulted in accumulation of this transcription factor in the nucleus after several hours. Overall, the data indicate that AP-2α transcriptional activity is regulated in a differentiation-dependent manner in cultured keratinocytes and that this occurs by relocalization of the protein. Nuclear localization of the AP-2α protein in basal cells permits its accessibility to response elements in gene promoters, whereas sequestration in the cytoplasm as the differentiation program progresses curtails its transcriptional activity. This regulatory scheme may provide keratinocytes with the ability to restore AP-2 transcriptional activity rapidly by redistribution to the nucleus after receiving an appropriate growth signal, such as a medium change

Redistribution of Transcription Factor AP-2α in Differentiating Cultured Human Epidermal Cells

Expression of the transcription factor AP-2α was examined in cultured human epidermal cells. Levels of AP-2α mRNA increased substantially after the cultures reached confluence, similar to the expression pattern of the differentiation markers involucrin and keratinocyte transglutaminase. The level of AP-2α protein in nuclear extracts declined markedly after confluence, however, along with its ability to form complexes with oligonucleotides containing the AP-2 response element. In contrast, the levels of AP-2α protein in cytoplasmic extracts increased dramatically after confluence, but these extracts had low DNA binding activity. Supershift experiments with specific antisera detected only AP-2α and not the β or γ isoforms. Examination of its localization by confocal microscopy revealed that AP-2α was primarily in the nucleus of basal cells and largely cytoplasmic in the most superficial cells. Localization was a dynamic phenomenon in that changing the medium resulted in accumulation of this transcription factor in the nucleus after several hours. Overall, the data indicate that AP-2α transcriptional activity is regulated in a differentiation-dependent manner in cultured keratinocytes and that this occurs by relocalization of the protein. Nuclear localization of the AP-2α protein in basal cells permits its accessibility to response elements in gene promoters, whereas sequestration in the cytoplasm as the differentiation program progresses curtails its transcriptional activity. This regulatory scheme may provide keratinocytes with the ability to restore AP-2 transcriptional activity rapidly by redistribution to the nucleus after receiving an appropriate growth signal, such as a medium change

A Role for SSRP1 in Recombination-Mediated DNA Damage Response

A possible role for structure-specific recognition protein 1 (SSRP1) in replication-associated repair processes has previously been suggested based on its interaction with several DNA repair factors and the replication defects observed in SSRP1 mutants. In this study, we investigated the potential role of SSRP1 in association with DNA repair mediated by homologous recombination (HR), one of the pathways involved in repairing replication-associated DNA damage, in mammalian cells. Surprisingly, over-expression of SSRP1 reduced the number of hprt(+) recombinants generated via HR both spontaneously and upon hydroxyurea (HU) treatment, whereas knockdown of SSRP1 resulted in an increase of HR events in response to DNA double-strand break formation. In correlation, we found that the depletion of SSRP1 in HU-treated human cells elevated the number of Rad51 and H2AX foci, while over-expression of the wild-type SSRP1 markedly reduced HU-induced Rad51 foci formation. We also found that SSRP1 physically interacts with a key HR repair protein, Rad54 both in vitro and in vivo. Further, branch migration studies demonstrated that SSRP1 inhibits Rad54-promoted branch migration of Holliday junctions in vitro. Taken together, our data suggest a functional role for SSRP1 in spontaneous and replication-associated DNA damage response by suppressing avoidable HR repair events

Reappearance from Obscurity: Mammalian Rad52 in Homologous Recombination

Homologous recombination (HR) plays an important role in maintaining genomic integrity. It is responsible for repair of the most harmful DNA lesions, DNA double-strand breaks and inter-strand DNA cross-links. HR function is also essential for proper segregation of homologous chromosomes in meiosis, maintenance of telomeres, and resolving stalled replication forks. Defects in HR often lead to genetic diseases and cancer. Rad52 is one of the key HR proteins, which is evolutionarily conserved from yeast to humans. In yeast, Rad52 is important for most HR events; Rad52 mutations disrupt repair of DNA double-strand breaks and targeted DNA integration. Surprisingly, in mammals, Rad52 knockouts showed no significant DNA repair or recombination phenotype. However, recent work demonstrated that mutations in human RAD52 are synthetically lethal with mutations in several other HR proteins including BRCA1 and BRCA2. These new findings indicate an important backup role for Rad52, which complements the main HR mechanism in mammals. In this review, we focus on the Rad52 activities and functions in HR and the possibility of using human RAD52 as therapeutic target in BRCA1 and BRCA2-deficient familial breast cancer and ovarian cancer

Inhibition of Homologous Recombination in Human Cells by Targeting RAD51 Recombinase

The homologous recombination (HR) pathway plays a crucial role in the repair of DNA double-strand breaks (DSBs) and interstrand cross-links (ICLs). RAD51, a key protein of HR, possesses a unique activity: DNA strand exchange between homologous DNA sequences. Recently, using a high-throughput screening (HTS), we identified compound <b>1</b> (B02), which specifically inhibits the DNA strand exchange activity of human RAD51. Here, we analyzed the mechanism of inhibition and found that <b>1</b> disrupts RAD51 binding to DNA. We then examined the effect of <b>1</b> on HR and DNA repair in the cell. The results show that <b>1</b> inhibits HR and increases cell sensitivity to DNA damage. We propose to use <b>1</b> for analysis of cellular functions of RAD51. Because DSB- and ICL-inducing agents are commonly used in anticancer therapy, specific inhibitors of RAD51 may also help to increase killing of cancer cells