Highlights of Coronavirus Structural Studies

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Reader's Corner Archive

19 Apr

Structures and mechanisms of tRNA methylation by METTL1–WDR4 (Nature)

Specific, regulated modification of RNAs is important for proper gene expression(1,2). tRNAs are rich with various chemical modifications that affect their stability and function(3,4). 7-Methylguanosine (m(7)G) at tRNA position 46 is a conserved modification that modulates steady-state tRNA levels to affect cell growth(5,6). The METTL1-WDR4 complex generates m(7)G46 in humans, and dysregulation of METTL1-WDR4 has been linked to brain malformation and multiple cancers(7-22). Here we show how METTL1 and WDR4 cooperate to recognize RNA substrates and catalyse methylation. A crystal structure of METTL1-WDR4 and cryo-electron microscopy structures of METTL1-WDR4-tRNA show that the composite protein surface recognizes the tRNA elbow through shape complementarity. The cryo-electron microscopy structures of METTL1-WDR4-tRNA with S-adenosylmethionine or S-adenosylhomocysteine along with METTL1 crystal structures provide additional insights into the catalytic mechanism by revealing the active site in multiple states. The METTL1 N terminus couples cofactor binding with conformational changes in the tRNA, the catalytic loop and the WDR4 C terminus, acting as the switch to activate m(7)G methylation. Thus, our structural models explain how post-translational modifications of the METTL1 N terminus can regulate methylation. Together, our work elucidates the core and regulatory mechanisms underlying m(7)G modification by METTL1, providing the framework to understand its contribution to biology and disease.

22 Mar

Structure of the NuA4 acetyltransferase complex bound to the nucleosom (Nature)

Deoxyribonucleic acid in eukaryotes wraps around the histone octamer to form nucleosomes, the fundamental unit of chromatin. The N termini of histone H4 interact with nearby nucleosomes and play an important role in the formation of high-order chromatin structure and heterochromatin silencing. NuA4 in yeast and its homologue Tip60 complex in mammalian cells are the key enzymes that catalyse H4 acetylation, which in turn regulates chromatin packaging and function in transcription activation and DNA repair. Here we report the cryo-electron microscopy structure of NuA4 from Sacchromyces cerevisiae bound to the nucleosome.NuA4 comprises two major modules: the catalytic histone acetyltransferase (HAT) module and the transcription activator-binding (TRA) module. The nucleosome is mainly bound by the HAT module and is positioned close to a polybasic surface of the TRA module, which is important for the optimal activity of NuA4. The nucleosomal linker DNA carrying the upstream activation sequence is oriented towards the conserved, transcription activator-binding surface of the Tra1 subunit, which suggests a potential mechanism of NuA4 to act as a transcription co-activator. The HAT module recognizes the disk face of the nucleosome through the H2A–H2B acidic patch and nucleosomal DNA, projecting the catalytic pocket of Esa1 to the N-terminal tail of H4 and supporting its function in selective acetylation of H4. Together, our findings illustrate how NuA4 is assembled and provide mechanistic insights into nucleosome recognition and transcription co-activation by a HAT.

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