Structural and mechanistic insights into the DNA glycosylase AAG-mediated base excision in nucleosome

Abstract The engagement of a DNA glycosylase with a damaged DNA base marks the initiation of base excision repair. Nucleosome-based packaging of eukaryotic genome obstructs DNA accessibility, and how DNA glycosylases locate the substrate site on nucleosomes is currently unclear. Here, we report cryo-electron microscopy structures of nucleosomes bearing a deoxyinosine (DI) in various geometric positions and structures of them in complex with the DNA glycosylase AAG. The apo nucleosome structures show that the presence of a DI alone perturbs nucleosomal DNA globally, leading to a general weakening of the interface between DNA and the histone core and greater flexibility for the exit/entry of the nucleosomal DNA. AAG makes use of this nucleosomal plasticity and imposes further local deformation of the DNA through formation of the stable enzyme–substrate complex. Mechanistically, local distortion augmentation, translation/rotational register shift and partial opening of the nucleosome are employed by AAG to cope with substrate sites in fully exposed, occluded and completely buried positions, respectively. Our findings reveal the molecular basis for the DI-induced modification on the structural dynamics of the nucleosome and elucidate how the DNA glycosylase AAG accesses damaged sites on the nucleosome with different solution accessibility

Structural snapshot of cytoplasmic pre-60S ribosomal particles bound by Nmd3, Lsg1, Tif6 and Reh1.

A key step in ribosome biogenesis is the nuclear export of pre-ribosomal particles. Nmd3, a highly conserved protein in eukaryotes, is a specific adaptor required for the export of pre-60S particles. Here we used cryo-electron microscopy (cryo-EM) to characterize Saccharomyces cerevisiae pre-60S particles purified with epitope-tagged Nmd3. Our structural analysis indicates that these particles belong to a specific late stage of cytoplasmic pre-60S maturation in which ribosomal proteins uL16, uL10, uL11, eL40 and eL41 are deficient, but ribosome assembly factors Nmd3, Lsg1, Tif6 and Reh1 are present. Nmd3 and Lsg1 are located near the peptidyl-transferase center (PTC). In particular, Nmd3 recognizes the PTC in its near-mature conformation. In contrast, Reh1 is anchored to the exit of the polypeptide tunnel, with its C terminus inserted into the tunnel. These findings pinpoint a structural checkpoint role for Nmd3 in PTC assembly, and provide information about functional and mechanistic roles of these assembly factors in the maturation of the 60S ribosomal subunit

Structural and functional insights into the tetrameric photosystem I from heterocyst-forming cyanobacteria

Two large protein-cofactor complexes, photosystem I and photosystem II, are the central components of photosynthesis in the thylakoid membranes. Here, we report the 2.37-angstrom structure of a tetrameric photosystem I complex from a heterocyst-forming cyanobacterium Anabaena sp. PCC 7120. Four photosystem I monomers, organized in a dimer of dimer, form two distinct interfaces that are largely mediated by specifically orientated polar lipids, such as sulfoquinovosyl diacylglycerol. The structure depicts a more closely connected network of chlorophylls across monomer interfaces than those seen in trimeric PSI from thermophilic cyanobacteria, possibly allowing a more efficient energy transfer between monomers. Our physiological data also revealed a functional link of photosystem I oligomerization to cyclic electron flow and thylakoid membrane organization.</p