A human homolog of Drosophila lethal(3)malignant brain tumor (1(3)mbt) protein associates with condensed mitotic chromosomes
ABSTRACT The lethal(3)malignant brain tumor (D-l(3)mbt) gene is considered to be one of the tumor suppressor genes of Drosophila, and its recessive mutations are associated with malignant transformation of the neuroblasts in the larval brain. The structure of D-l(3)mbt protein is similar to Drosophila sex comb on midleg (Scm) protein which is a member of Polycomb group (PcG) proteins. We have isolated here the first human homolog of the D-l(3)mbt gene, designated h-l(3)mbt. Radiation hybrid mapping and fluorescence in situ hybridization (FISH) analysis localized the h-l(3)mbt gene to chromosome 20q12. The h-l(3)mbt transcript is expressed in most of the human adult normal tissues and cultured cell lines. However, some cancer cells markedly reduce the h-l(3)mbt protein expression. Immunocytochemical study revealed that the h-l(3)mbt protein shows a speckled and scattered distribution in interphase nuclei and completely associates with condensed chromosomes in mitotic cells. This subcellular localization has been shown to be different from that of Bmi1 protein which is a component of PcG complex. Furthermore, overexpression of h-l(3)mbt protein by using a Cre-mediated gene activation system leads to failures of proper chromosome segregation and cytokinesis, which result in formation of multinuclei in U251MG cells. These observations suggest that h-l(3)mbt protein has functions distinct from those of PcG proteins and may play a role in proper progression of cell division.
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ABSTRACT: Mitosis is accompanied by dramatic changes in chromatin organization and nuclear architecture. Transcription halts globally and most sequence-specific transcription factors and co-factors are ejected from mitotic chromatin. How then does the cell maintain its transcriptional identity throughout the cell division cycle? It has become clear that not all traces of active transcription and gene repression are erased within mitotic chromatin. Many histone modifications are stable or only partially diminished throughout mitosis. In addition, some sequence-specific DNA binding factors have emerged that remain bound to select sites within mitotic chromatin, raising the possibility that they function to transmit regulatory information through the transcriptionally silent mitotic phase, a concept that has been termed "mitotic bookmarking." Here we review recent approaches to studying potential bookmarking factors with regards to their mitotic partitioning, and summarize emerging ideas concerning the in vivo functions of mitotically bound nuclear factors.Epigenetics & Chromatin 04/2013; 6(1):6. DOI:10.1186/1756-8935-6-6 · 4.46 Impact Factor
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ABSTRACT: Polycomb group (PcG) proteins are epigenetic transcriptional factors that repress key developmental regulators, and maintain cellular identity through mitosis via a poorly understood mechanism. Using quantitative live cell imaging in mouse ES cells and tumor cells, we demonstrate that, while Polycomb Repressive Complex (PRC) 1 proteins (Cbx family proteins, Ring1b, Mel18 and Phc1) exhibit variable capacities of association with mitotic chromosomes, Cbx2 overwhelmingly binds to mitotic chromosomes. The recruitment of Cbx2 to mitotic chromosomes is independent of PRC1 or PRC2, and Cbx2 is needed to recruit PRC1 complex to mitotic chromosomes. Quantitative fluorescence recovery after photobleaching (FRAP) analysis indicates that PRC1 proteins rapidly exchange at interphasic chromatin. On entry into mitosis, Cbx2, Ring1b, Mel18, and Phc1 proteins become immobilized at mitotic chromosomes, while other Cbx family proteins dynamically bind to mitotic chromosomes. Depletion of PRC1 or PRC2 proteins has no effect on the immobilization of Cbx2 on mitotic chromosomes. We identify that the N-terminus of Cbx2 is needed for its recruitment to mitotic chromosomes, while the C-terminus is required for its immobilization. Thus, these results provide fundamental insights to the molecular mechanisms of epigenetic inheritance.Molecular Biology of the Cell 09/2014; 25(23). DOI:10.1091/mbc.E14-06-1109 · 4.55 Impact Factor
Article: Malignant Brain Tumor Repeats[Show abstract] [Hide abstract]
ABSTRACT: We report on the X-ray structure of three 100-amino acid mbt repeats in h-l(3)mbt, a polycomb group protein involved in transcriptional repression, whose gene is located in a region of chromosome 20 associated with hematopoietic malignancies. Interdigitation between the extended arms and cores of the mbt repeats results in a three-leaved propeller-like architecture, containing a central cavity. We have identified one ligand binding pocket per mbt repeat, which accommodates either the morphilino ring of MES or the proline ring of the C-terminal peptide segment, within a cavity lined by aromatic amino acids. Strikingly, phenotypic alterations resulting from point mutations or deletions in the mbt repeats of the related Drosophila SCM protein are clustered in and around the ligand binding pocket.Structure 07/2003; 11(7):775-789. DOI:10.1016/S0969-2126(03)00127-8 · 6.79 Impact Factor