Upon damage of DNA in eukaryotic cells, several repair and checkpoint proteins undergo a dramatic intranuclear relocalization, translocating to nuclear foci thought to represent sites of DNA damage and repair. Examples of such proteins include the checkpoint kinase ATR (ATM and Rad3-related) as well as replication protein A (RPA), a single-stranded DNA binding protein required in DNA replication and repair. Here, we used a microscopy-based approach to investigate whether the damage-induced translocation of RPA is an active process regulated by ATR. Our data show that in undamaged cells, ATR and RPA are uniformly distributed in the nucleus or localized to promyelocytic leukemia protein (PML) nuclear bodies. In cells treated with ionizing radiation, both ATR and RPA translocate to punctate, abundant nuclear foci where they continue to colocalize. Surprisingly, an ATR mutant that lacks kinase activity fails to relocalize in response to DNA damage. Furthermore, this kinase-inactive mutant blocks the translocation of RPA in a cell cycle-dependent manner. These observations demonstrate that the kinase activity of ATR is essential for the irradiation-induced release of ATR and RPA from PML bodies and translocation of ATR and RPA to potential sites of DNA damage.

Survivin, which is the smallest member of the inhibitor of apoptosis protein (IAP) family, is a chromosomal passenger protein that mediates the spindle assembly checkpoint and cytokinesis, and also functions as an inhibitor of apoptosis. Frequently overexpressed in human cancers and not expressed in most adult tissues, survivin has been proposed as an attractive target for anticancer therapies and, in some cases, has even been touted as a cancer-specific gene. Survivin is, however, expressed in proliferating adult cells, including human hematopoietic stem cells, T-lymphocytes, and erythroid cells throughout their maturation. Therefore, it is unclear how survivin-targeted anticancer therapies would impact steady-state blood development. To address this question, we used a conditional gene-targeting strategy and abolished survivin expression from the hematopoietic compartment of mice. We show that inducible deletion of survivin leads to ablation of the bone marrow, with widespread loss of hematopoietic progenitors and rapid mortality. Surprisingly, heterozygous deletion of survivin causes defects in erythropoiesis in a subset of the animals, with a dramatic reduction in enucleated erythrocytes and the presence of immature megaloblastic erythroblasts. Our studies demonstrate that survivin is essential for steady-state hematopoiesis and survival of the adult, and further, that a high level of survivin expression is critical for proper erythroid differentiation.

Chromosome condensation is essential for proper segregation of duplicated sister chromatids in mitosis. Mammalian erythroid maturation is also associated with gradual nuclear condensation. However, few proteins that are directly involved in chromosome condensation during erythropoiesis have been identified. In this report, we show that MTB (more than blood), which was initially isolated in a yeast two-hybrid screen for proteins that interact with the basic helix-loop-helix (bHLH) protein stem cell leukemia (SCL), and later identified as the murine homolog of the condensin II subunit CAP-G2, participates in erythroid cell development. MTB interacts with SCL and another hematopoietic bHLH protein, E12, and is recruited to the nucleus by SCL and E12. In addition, MTB can repress SCL/E12-mediated transcriptional activation. Consistent with the model that MTB may function together with SCL/E12 heterodimer during erythroid cell development, MTB is highly expressed in the erythroid lineage and is upregulated upon erythroid differentiation. Moreover, overexpression of MTB promotes the terminal differentiation of the murine erythroleukemia erythroid cell line. Together, these findings demonstrate that the condensin II subunit MTB/mCAP-G2 plays a novel function during erythropoiesis and suggest that key hematopoietic transcription factors such as SCL and E12 may regulate the terminal differentiation of hematopoietic cells through the interaction with condensin complexes.