2017年6月29日 木曜日 午後5時から6時10分 4号館 3階セミナー室です。
 ホスト 児玉研

Chromatin proteomics - decoding the epigenome

Constitutive heterochromatin is typically defined by high levels of DNA methylation and H3 lysine 9 tri- methylation (H3K9Me3), whereas facultative het- erochromatin displays DNA hypomethylation and high H3 lysine 27 trimethylation (H3K27Me3). The two chromatin types generally do not coexist at the same loci, suggesting mutual exclusivity. During development or in cancer, pericentromeric regions can adopt either epigenetic state, but the switching mechanism is unknown. We used a quantitative locus specific purification method to characterize changes in pericentromeric chromatin-associated proteins in mouse embryonic stem cells deficient for either the methyltransferases required for DNA methylation or H3K9Me3. DNA methylation controls heterochromatin architecture and inhibits Polycomb recruitment. To analyze the inheritance of epigenetic information we also combined nascent chromatin capture and SILAC labeling to track histone modifications and histone variants during replication and across the cell cycle. We show that old histones maintain methylation marks upon recycling onto newly replicated DNA offering a blueprint for chromatin restoration. Methylation of new histones is slow, step-wise and continues into the next cell cycle. Indeed, H3K9 and H3K27 tri-methylation requires passage through mitosis and extents beyond one cell cycle. Therefore, methylation state increases with histone age and exit from the cell cycle is accompanied with global increase in histone methylation. Our work reveals that DNA replication has a global long-lasting impact on chromatin state and provides a foundation to understand how epigenetic states are propagated.

Prof. Dr. Axel Imhof
 Biomedical Center Ludwig Maximilians University of Munich, Germany

Splicing Snapshots: Cryo-EM Structures of the Spliceosome

Splicing is a fundamental step in eukaryotic gene expression, where non-coding introns are removed from messenger RNAs, and alternative splicing is a major contributor to proteome diversity in higher organisms. A large, dynamic protein-RNA complex called the spliceosome catalyses splicing by assembling on pre-mRNA substrates and performing two transesterification reactions. Although the spliceosome was discovered more than 30 years ago, it has defied structural characterisation by X-ray crystallography due to its low abundance and extreme flexibility. Recent developments in cryo-electron microscopy (cryo-EM), including new detectors and statistical image processing algorithms, have caused a revolution in structural biology. In this talk I will present near-atomic resolution cryo-EM structures of the spliceosome carrying out the two chemical steps of splicing. These structures reveal the architecture of this immense machine, the roles of step-specific splicing factors, and show how the entirely RNA-based active site performs catalysis.

Mr. Max Wilkinson
 MRC Laboratory of Molecular Biology, University of Cambridge, UK

Supervisor: Dr Kiyoshi Nagai