Search

Type I restriction–modification (R–M) systems are widespread in prokaryotic genomes and provide robust protection against foreign DNA. They are multisubunit enzymes with methyltransferase, endonuclease and translocase activities. Despite extensive studies over the past five decades, little is known about the molecular mechanisms of these sophisticated machines. Here, we report the cryo-electron microscopy structures of the representative EcoR124I R–M system in different assemblies (R2M2S1, R1M2S1 and M2S1) bound to target DNA and the phage and mobile genetic element-encoded anti-restriction proteins Ocr and ArdA. EcoR124I can precisely regulate different enzymatic activities by adopting distinct conformations. The marked conformational transitions of EcoR124I are dependent on the intrinsic flexibility at both the individual-subunit and assembled-complex levels. Moreover, Ocr and ArdA use a DNA-mimicry strategy to inhibit multiple activities, but do not block the conformational transitions of the complexes. These structural findings, complemented by mutational studies of key intermolecular contacts, provide insights into assembly, operation and inhibition mechanisms of type I R–M systems. This study provides new insights into the structure, assembly and dynamics of type I restriction–modification systems, and their inhibition by phage proteins.

Summary DNA-induced liquid-liquid phase separation of cyclic GMP-AMP synthase (cGAS) triggers a potent response to detect pathogen infection and promote innate immune signaling. Whether and how pathogens manipulate cGAS-DNA condensation to mediate immune evasion is unknown. We report the identification of a structurally related viral tegument protein family, represented by ORF52 and VP22 from gamma- and alpha-herpesvirinae, respectively, that employs a conserved mechanism to restrict cGAS-DNA phase separation. ORF52/VP22 proteins accumulate into, and effectively disrupt, the pre-formed cGAS-DNA condensation both in vitro and in cells. The inhibition process is dependent on DNA-induced liquid-liquid phase separation of the viral protein rather than a direct interaction with cGAS. Moreover, highly abundant ORF52 proteins carried within viral particles are able to target cGAS-DNA phase separation in early infection stage. Our results define ORF52/VP22-type tegument proteins as a family of inhibitors targeting cGAS-DNA phase separation and demonstrate a mechanism for how viruses overcome innate immunity.

The co-translational targeting or insertion of secretory and membrane proteins into the endoplasmic reticulum (ER) is a key biological process mediated by the signal recognition particle (SRP). In eukaryotes, the SRP68–SRP72 (SRP68/72) heterodimer plays an essential role in protein translocation. However, structural information on the two largest SRP proteins, SRP68 and SRP72, is limited, especially regarding their interaction. Herein, we report the first crystal structures of human apo-SRP72 and the SRP68/72 complex at 2.91Å and 1.7Å resolution, respectively. The SRP68-binding domain of SRP72 contains four atypical tetratricopeptide repeats (TPR) and a flexible C-terminal cap. Apo-SRP72 exists mainly as dimers in solution. To bind to SRP68, the SRP72 homodimer disassociates, and the indispensable C-terminal cap undergoes a pronounced conformational change to assist formation of the SRP68/72 heterodimer. A 23-residue polypeptide of SRP68 is sufficient for tight binding to SRP72 through its unusually hydrophobic and extended surface. Structural, biophysical, and mutagenesis analyses revealed that cancer-associated mutations disrupt the SRP68–SRP72 interaction and their co-localization with ER in mammalian cells. The results highlight the essential role of the SRP68–SRP72 interaction in SRP-mediated protein translocation and provide a structural basis for disease diagnosis, pathophysiology, and drug design.

Summary: Zinc-finger antiviral protein (ZAP) is a host antiviral factor that specifically restricts a wide range of viruses. ZAP selectively binds to CG-dinucleotide-enriched RNA sequences and recruits multiple RNA degradation machines to degrade target viral RNA. However, the molecular mechanism and structural basis for ZAP recognition of specific RNA are not clear. Here, we report the crystal structure of the ZAP N-terminal domain bound to a CG-rich single-stranded RNA, providing the molecular basis for its specific recognition of a CG dinucleotide and additional guanine and cytosine. The four zinc fingers of ZAP adopt a unique architecture and form extensive interactions with RNA. Mutations of both protein and RNA at the RNA-ZAP interacting surface reduce the in vitro binding affinity and cellular antiviral activity. This work reveals the molecular mechanism of ZAP recognition of specific target RNA and also provides insights into the mechanism by which ZAP coordinates downstream RNA degradation processes. : ZAP is a host antiviral factor that specifically restricts a wide range of viruses. Luo et al. determine the structural and molecular basis for the specific recognition of a CG dinucleotide and additional guanine and cytosine by ZAP. Keywords: ▪▪▪

The superfine high crystallinity TiO 2 nanocrystals with tunable anatase/rutile ratios have been synthesized in an ethanol aqueous solution of HCl and sodium dodecylbenzene sulfonate (DBS) by a “low temperature dissolution–reprecipitation process” (LTDRP) and solvethermal treatment composite method. The effect of the synthesis parameters on the crystal structure, crystal size and the photocatalytic properties of the TiO 2 nanocrystallines were investigated. The growth and aggregation of TiO 2 nanocrystallines can be inhibited, and the high crystallinity spherical nanoparticles with small size are generated in the presence of DBS. The ratios of anatase and rutile can be easily controlled by sample adjusting aging time. The pure phase anatase is obtained by solvethermal treatment without aging process, the contents and the crystallite size of rutile are increased with increase of the aging time and only rutile exists while aging time is 16 h or more. The well-crystallized TiO 2 nanocrystals exhibit higher photocatalytic activities. Their high photocatalytic activities could be attributed to high crystallinity, small crystal size (approximately 5 nm for anatase and 8 to 15 nm for rutile), the synergistic effect between anatase and rutile and strong electron-withdrawing characteristic of the sulfoacid radical of DBS.

The Sn–TiO 2− X nanoparticles have been prepared via a rapid and simple stannous chemical reducing method. The as-prepared Sn–TiO 2− X nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), XPS, and DRS technology as well as photocatalytic degradation of RhB were studied under illumination. The experiment results revealed that the reduction of the TiO 2 particles raised their Fermi level, which can enhance the driven force of photoinduced electrons transferring from TiO 2 to adsorbed O 2 and SnO 2 on the surface of TiO 2 . On the other hand, the amount of oxygen vacancies of the Sn–TiO 2− X increased after the stannous chemical reduction. The oxygen vacancies can also effectively inhibit the recombination of photoinduced electrons and holes pairs. These factors are favorable to the photocatalytic reaction.

Flexibility is an intrinsic property of proteins and essential for their biological functions. However, because of structural flexibility, obtaining high-quality crystals of proteins with heterogeneous conformations remain challenging. Here, we show a novel approach to immobilize traditional precipitants onto molecularly imprinted polymers (MIPs) to facilitate protein crystallization, especially for flexible proteins. By applying this method, high-quality crystals of the flexible N-terminus of human fragile X mental retardation protein are obtained, whose absence causes the most common inherited mental retardation. A novel KH domain and an intermolecular disulfide bond are discovered, and several types of dimers are found in solution, thus providing insights into the function of this protein. Furthermore, the precipitant-immobilized MIPs (piMIPs) successfully facilitate flexible protein crystal formation for five model proteins with increased diffraction resolution. This highlights the potential of piMIPs for the crystallization of flexible proteins.

The present study aimed to ascertain whether suppression of midkine (MK) expression in pancreatic cancer cells inhibits metastasis to the liver. Human pancreatic cancer AsPC-1 cells were transfected with small interfering RNA (siRNA) targeting MK. siRNA against MK was observed to reduce the expression of MK mRNA and protein in a concentration- and time-dependent manner, and to decrease the number of migrating and tissue-penetrating cells in a concentration-dependent manner (P