The RAD50 gene was first identified in Saccharomyces cerevisiae based on its role in DNA repair (1). It is a member of the RAD52 epistasis group, members of which function in recombinational DNA repair. Yeast strains carrying rad50 mutations exhibit hypersensitivity to ␥-irradiation and radiomimetic chemicals such as methyl methanesulfonate and only slight sensitivity to UV irradiation (1). In addition, rad50 mutations also lead to telomere shortening and meiotic failure (1). The 153-kDa Rad50 protein contains both an N-terminal Walker A and an C-terminal Walker B NTP binding domains, linked by two extensive coiled-coil regions that are characterized by leucine heptad repeats (2). This sequence architecture is similar to that found in other members of the structural maintenance of chromosomes (SMC) 1 protein family (3, 4). The first 35 heptad repeats span amino acids 177-421 of the Rad50 sequence, whereas the second 37 heptad repeats span amino acids 743-995. These heptad repeat sequences exhibit length and significant sequence similarity to the S2 domain of rabbit myosin (2). Mutations within the Rad50 Walker A domain that alter conserved amino acids resides within the NTP binding site confer a null phenotype, and several mutations near the nucleotide binding sites cause defects in meiosis (5).The RAD50 gene product is highly conserved among species. Human and yeast Rad50 share more than 50% sequence identity within their corresponding N-and C-terminal regions as well as a similar coiled-coil structure in between. A null mutation of the mouse Rad50 gene is associated with an embryonic lethal phenotype, and cells derived from early embryos are hypersensitive to ionizing radiation, suggesting that Rad50 is essential for normal cell proliferation and DNA double-strand break repair (6). Yeast Rad50 forms a complex with Mre11 and Xrs2 (7). Similarly, human Rad50 forms a complex with hMre11 and NBS1, the potentially functional homologue of yeast Xrs2 (8,9). It is believed that the Mre11-Rad50-NBS1 protein complex plays a central role in the cellular response to DNA damage. The NBS1 gene is mutated in Nijmegen Breakage syndrome (9, 10), which is phenotypically characterized by radiosensitivity and increased chromosomal instability upon ionizing radiation. Mre11 possesses DNA end-holding as well as endonucleolytic and exonucleolytic activities (11-15) that may be utilized during the initial steps of DNA double-strand break repair. Mouse embryonic stem cells deficient in Mre11 are not viable (16), suggesting that Mre11, like Rad50, is essential for normal proliferating cells and DNA double-strand break repair. Thus, while it is clear that these three proteins function as a triplex in DNA double-strand break repair, it is also very likely that these proteins function in association with other partners to have different biological functions.To explore other potential functions of Rad50, we have identified a novel Rad50-interacting protein, named RINT-1. These two proteins specifically interact during S and M phases of the...

bIn budding yeast (Saccharomyces cerevisiae), the cell cycle-dependent telomere elongation by telomerase is controlled by the cyclin-dependent kinase 1 (Cdk1). The telomere length homeostasis is balanced between telomerase-unextendable and telomerase-extendable states that both require Cdc13. The recruitment of telomerase complex by Cdc13 promotes telomere elongation, while the formation of Cdc13-Stn1-Ten1 (CST) complex at the telomere blocks telomere elongation by telomerase. However, the cellular signaling that regulates the timing of the telomerase-extendable and telomerase-unextendable states is largely unknown. Phosphorylation of Cdc13 by Cdk1 promotes the interaction between Cdc13 and Est1 and hence telomere elongation. Here, we show that Cdk1 also phosphorylates Stn1 at threonine 223 and serine 250 both in vitro and in vivo, and these phosphorylation events are essential for the stability of the CST complexes at the telomeres. By controlling the timing of Cdc13 and Stn1 phosphorylations during cell cycle progression, Cdk1 regulates the temporal recruitment of telomerase complexes and CST complexes to the telomeres to facilitate telomere maintenance.

Faithful mitotic partitioning of the Golgi apparatus and the centrosome is critical for proper cell division. Although these two cytoplasmic organelles are probably coordinated during cell division, supporting evidence of this coordination is still largely lacking. Here, we show that the RAD50-interacting protein, RINT-1, is localized at the Golgi apparatus and the centrosome in addition to the endoplasmic reticulum. To examine the biological roles of RINT-1, we found that the homozygous deletion of Rint-1 caused early embryonic lethality at embryonic day 5 (E5) to E6 and the failure of blastocyst outgrowth ex vivo. About 81% of the Rint-1 heterozygotes succumbed to multiple tumor formation with haploinsufficiency during their average life span of 24 months. To pinpoint the cellular function of RINT-1, we found that RINT-1 depletion by RNA interference led to the loss of the pericentriolar positioning and dispersal of the Golgi apparatus and concurrent centrosome amplification during the interphase. Upon mitotic entry, RINT-1-deficient cells exhibited multiple abnormalities, including aberrant Golgi dynamics during early mitosis and defective reassembly at telophase, increased formation of multiple spindle poles, and frequent chromosome missegregation. Mitotic cells often underwent cell death in part due to the overwhelming cellular defects. Taken together, these findings suggest that RINT-1 serves as a novel tumor suppressor essential for maintaining the dynamic integrity of the Golgi apparatus and the centrosome, a prerequisite to their proper coordination during cell division.

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Genotypes associated with low levels of plasma MBL during pregnancy did not increase the risk of preterm births. A low second-trimester plasma MBL level is therefore not a predictor for the development of preterm birth.