The Drosophila mus101 Gene, Which Links DNA Repair, Replication and Condensation of Heterochromatin in Mitosis, Encodes a Protein With Seven BRCA1 C-Terminus Domains

Abstract

The mutagen-sensitive-101 (mus101) gene of Drosophila melanogaster was first identified 25 years ago through mutations conferring larval hypersensitivity to DNA-damaging agents. Other alleles of mus101 causing different phenotypes were later isolated: a female sterile allele results in a defect in a tissue-specific form of DNA synthesis (chorion gene amplification) and lethal alleles cause mitotic chromosome instability that can be observed genetically and cytologically. The latter phenotype presents as a striking failure of mitotic chromosomes of larval neuroblasts to undergo condensation of pericentric heterochromatic regions, as we show for a newly described mutant carrying lethal allele mus101lcd. To gain further insight into the function of the Mus101 protein we have molecularly cloned the gene using a positional cloning strategy. We report here that mus101 encodes a member of the BRCT (BRCA1 C terminus) domain superfamily of proteins implicated in DNA repair and cell cycle checkpoint control. Mus101, which contains seven BRCT domains distributed throughout its length, is most similar to human TopBP1, a protein identified through its in vitro association with DNA topoisomerase IIβ. Mus101 also shares sequence similarity with the fission yeast Rad4/Cut5 protein required for repair, replication, and checkpoint control, suggesting that the two proteins may be functional homologs.

Sld3, which interacts with Cdc45 (Sld4), functions for chromosomal DNA replication in Saccharomyces cerevisiae

“…Thus, either higher eukaryotic cells do not need Sld3 for Cdc45 loading to Mcm or Sld3 is diversified from organism to organism. Although we can not find a good candidate for Dpb11 in higher eukaryotes by its amino acid sequence, Drosophila Mus101 (Yamamoto et al ., 2000) and human TopBP1 (J.Syväoya, personal communication), which have seven and eight BRCT domains, respectively, and play roles in DNA replication and repair, are suggested as functional homologs of Dpb11 and Cut5/Rad4, a counterpart of Dpb11 in S.pombe . Moreover, Sld3 and Psl3 of S.pombe are similar in not only amino acid sequences, but also biological functions (R.Nakajima and H.Masukata, personal communication).…”

CK2 Phosphorylation of Human Papillomavirus 16 E2 on Serine 23 Promotes Interaction with TopBP1 and Is Critical for E2 Interaction with Mitotic Chromatin and the Viral Life Cycle

“…It is part of the replication complex in mammalian cells, interacting with Treslin to promote the initiation of replication ( 39 – 43 ). TopBP1 contains nine BRCT ( BR CA1 carboxyl-terminal ) domains that act as hydrophobic pockets mediating interaction with cellular proteins, including proteins that are phosphorylated following cell signaling events and are involved in replication initiation and the DNA damage response (DDR) ( 44 – 65 ). TopBP1 is required for the activation of the ATR (ataxia-telangiectasia and Rad3-related) kinase via interaction with ATRIP (ATR-interacting protein), and TopBP1 is also a substrate for ATM (ataxia-telangiectasia mutated).…”

“…Both ATM and ATR are activated during the viral life cycle in order to promote viral genome replication; therefore, TopBP1 is an essential mediator of the HPV16 life cycle ( 66 – 70 ). TopBP1 also has several roles during mitosis as it prevents transmission of DNA damage (including DNA double-strand breaks and catenated DNA) to G 1 daughter cells ( 45 , 47 , 65 , 71 – 74 ).…”

Overexpression of TopBP1 leads to transformation with a TP53 mutation of non-tumorigenic breast epithelial cells

“…Further human TopBP1 was identified as an interactor of Topoisomerase IIβ in a yeast two-hybrid screen (Wang and Elledge, 2002). The other known homologues of TopBP1 are Dpb11 ( S. cerevisiae ) (Araki et al, 1995), Mus101 ( D. melanogaster ) (Yamamoto et al, 2000), Mei1 ( A. thaliana ) and Xmus101 ( X. laevis ).…”