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Human hnRNP A2/B1 is an RNA-binding protein that plays important roles in many biological processes, including maturation, transport, and metabolism of mRNA, and gene regulation of long noncoding RNAs. hnRNP A2/B1 was reported to control the microRNAs sorting to exosomes and promote primary microRNA processing as a potential m 6 A “reader.” hnRNP A2/B1 contains two RNA recognition motifs that provide sequence-specific recognition of RNA substrates. Here, we determine crystal structures of tandem RRM domains of hnRNP A2/B1 in complex with various RNA substrates, elucidating specific recognitions of AGG and UAG motifs by RRM1 and RRM2 domains, respectively. Further structural and biochemical results demonstrate multivariant binding modes for sequence-diversified RNA substrates, supporting a RNA matchmaker mechanism in hnRNP A2/B1 function. Moreover, our studies in combination with bioinformatic analysis suggest that hnRNP A2/B1 may mediate effects of m 6 A through a “m 6 A switch” mechanism, instead of acting as a direct “reader” of m 6 A modification. RNA-binding protein hnRNP A2/B1 is suggested to promote miRNA processing as a m 6 A 'reader'. Here, the authors determine crystal structures of RRM domains of hnRNP A2/B1 in complex with various RNA substrates and determine that hnRNP A2/B1 may function as an auxiliary factor in 'm 6 A switch' instead.

N 6 -methyladenosine (m 6 A) is the most abundant ribonucleotide modification among eukaryotic messenger RNAs. The m 6 A “writer” consists of the catalytic subunit m 6 A-METTL complex (MAC) and the regulatory subunit m 6 A-METTL-associated complex (MACOM), the latter being essential for enzymatic activity. Here, we report the cryo-electron microscopy (cryo-EM) structures of MACOM at a 3.0-Å resolution, uncovering that WTAP and VIRMA form the core structure of MACOM and that ZC3H13 stretches the conformation by binding VIRMA. Furthermore, the 4.4-Å resolution cryo-EM map of the MACOM–MAC complex, combined with crosslinking mass spectrometry and GST pull-down analysis, elucidates a plausible model of the m 6 A writer complex, in which MACOM binds to MAC mainly through WTAP and METTL3 interactions. In combination with in vitro RNA substrate binding and m 6 A methyltransferase activity assays, our results illustrate the molecular basis of how MACOM assembles and interacts with MAC to form an active m 6 A writer complex.

Besides the canonical RNA-based RNase P, pre-tRNA 5’-end processing can also be catalyzed by protein-only RNase P (PRORP). To date, various PRORPs have been discovered, but the basis underlying substrate binding and cleavage by HARPs (homolog of Aquifex RNase P) remains elusive. Here, we report structural and biochemical studies of HARPs. Comparison of the apo- and pre-tRNA-complexed structures showed that HARP is able to undergo large conformational changes that facilitate pre-tRNA binding and catalytic site formation. Planctomycetes bacterium HARP exists as dimer in vitro, but gel filtration and electron microscopy analysis confirmed that HARPs from Thermococcus celer , Thermocrinis minervae and Thermocrinis ruber can assemble into larger oligomers. Structural analysis, mutagenesis and in vitro biochemical studies all supported one cooperative pre-tRNA processing mode, in which one HARP dimer binds pre-tRNA at the elbow region whereas 5’-end removal is catalyzed by the partner dimer. Our studies significantly advance our understanding on pre-tRNA processing by PRORPs. HARP are member of protein-only RNase P, which catalyzes pre-tRNA 5’-end processing and maturation. Here, the authors present crystal structure and provide mechanistic insights into pre-tRNA binding and cleavage by HARP proteins.

Small interference RNAs are the key components of RNA interference, a conserved RNA silencing or viral defense mechanism in many eukaryotes. In Drosophila melanogaster , Dicer-2 ( Dm Dcr-2)-mediated RNAi pathway plays important roles in defending against viral infections and protecting genome integrity. During the maturation of siRNAs, two cofactors can regulate Dm Dcr-2’s functions: Loqs-PD that is required for dsRNA processing, and R2D2 that is essential for the subsequent loading of siRNAs into effector Ago2 to form RISC complexes. However, due to the lack of structural information, it is still unclear whether R2D2 and Loqs-PD affect the functions of Dm Dcr-2 simultaneously. Here we present several cryo-EM structures of Dm Dcr-2/R2D2/Loqs-PD complex bound to dsRNAs with various lengths by the Helicase domain. These structures revealed that R2D2 and Loqs-PD can bind to different regions of Dm Dcr-2 without interfering with each other. Furthermore, the cryo-EM results demonstrate that these complexes can form large oligomers and assemble into fibers. The formation and depolymerization of these oligomers are associated with ATP hydrolysis. These findings provide insights into the structural mechanism of Dm Dcr-2 and its cofactors during siRNA processing. R2D2 and Loqs-PD are cofactors of Drosophila Dicer-2 (DmDcr-2), which generates siRNAs. Here the authors report the cryo-EM structures of DmDcr-2/R2D2/Loqs-PD with dsRNAs showing that these complexes can form oligomers and assemble into fibers.

Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant V Zn -related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O 2 ) is a vacancy cluster (most likely a V Zn -nV O complex with n = 2, 3) rather than the isolated V Zn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the V Zn  at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more V Zn . Increasing the P(O 2 ) during growth also lead to the formation of the vacancy cluster with relatively more V Zn . For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the V Zn -related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed.

With the advent of wearable device technology, fabrication of inorganic semiconductor devices on flexible organic substrates is of great interest. In this paper, a fascinating method and a low-cost flexible substrate material polyvinyl alcohol (PVAL) have been utilized to embed ZnO microwire (MW) array to produce ultraviolet (UV) photodetector (PD) with decent photoresponsivity. The flexible PVAL substrate is relatively cheap and has better bendability as compared to polyethylene terephthalate (PET) and other traditional flexible substrate materials, which makes it unique in comparison to traditional devices. The device shows a current photoresponsivity of 29.6 A/W in the UV spectral range (350 to 380 nm) and maintains an excellent detection performance with even a bending angle of 180°. In dark, a low current of 1.4 μA at 5 V bias and response time of 4.27 ms was observed. In addition to the excellent device performance at wide bending angles, the fabricated device also performs well with the bending radii close to 0. Therefore, ZnO MW array PD has a great potential for the real-time monitoring of harmful UV exposure to warn the users for the appropriate arrangement avoidance.

MgtE is a Mg 2+ channel conserved in organisms ranging from prokaryotes to eukaryotes, including humans, and plays an important role in Mg 2+ homeostasis. The previously determined MgtE structures in the Mg 2+ -bound, closed-state, and structure-based functional analyses of MgtE revealed that the binding of Mg 2+ ions to the MgtE cytoplasmic domain induces channel inactivation to maintain Mg 2+ homeostasis. There are no structures of the transmembrane (TM) domain for MgtE in Mg 2+ -free conditions, and the pore-opening mechanism has thus remained unclear. Here, we determined the cryo-electron microscopy (cryo-EM) structure of the MgtE-Fab complex in the absence of Mg 2+ ions. The Mg 2+ -free MgtE TM domain structure and its comparison with the Mg 2+ -bound, closed-state structure, together with functional analyses, showed the Mg 2+ -dependent pore opening of MgtE on the cytoplasmic side and revealed the kink motions of the TM2 and TM5 helices at the glycine residues, which are important for channel activity. Overall, our work provides structure-based mechanistic insights into the channel gating of MgtE.

In the progress of nonlinear optics, multiphoton absorption (MPA) upconversion lasing enables many vital applications in bioimaging, three-dimensional optical data storage, and photodynamic therapy. Here, efficient four-photon absorption upconversion lasing from the ZnO/ZnMgO multiple quantum wells (MQWs) at room temperature is realized. Moreover, the MPA upconversion lasing and third-harmonic generation peak generated in the MQWs under the excitation of a femtosecond (fs) laser pulse were observed concurrently, and the essential differences between each other were studied comprehensively. Compared with the ZnO film, the upconversion lasing peak of the ZnO/ZnMgO MQWs exhibits a clear blue shift. In addition, the four-photon absorption upconversion photoluminescence (PL) intensity was enhanced in the MQWs/Au nanoparticles (NPs) by the metal-localized surface plasmons (LSPs). The work paves the way for short-wavelength lasers by taking advantage of the high stability and large exciton binding energy of the MQWs’ structures.

Small interfering RNAs (siRNAs) are the key components for RNA interference (RNAi), a conserved RNA-silencing mechanism in many eukaryotes 1 , 2 . In Drosophila , an RNase III enzyme Dicer-2 (Dcr-2), aided by its cofactor Loquacious-PD (Loqs-PD), has an important role in generating 21 bp siRNA duplexes from long double-stranded RNAs (dsRNAs) 3 , 4 . ATP hydrolysis by the helicase domain of Dcr-2 is critical to the successful processing of a long dsRNA into consecutive siRNA duplexes 5 , 6 . Here we report the cryo-electron microscopy structures of Dcr-2–Loqs-PD in the apo state and in multiple states in which it is processing a 50 bp dsRNA substrate. The structures elucidated interactions between Dcr-2 and Loqs-PD, and substantial conformational changes of Dcr-2 during a dsRNA-processing cycle. The N-terminal helicase and domain of unknown function 283 (DUF283) domains undergo conformational changes after initial dsRNA binding, forming an ATP-binding pocket and a 5′-phosphate-binding pocket. The overall conformation of Dcr-2–Loqs-PD is relatively rigid during translocating along the dsRNA in the presence of ATP, whereas the interactions between the DUF283 and RIIIDb domains prevent non-specific cleavage during translocation by blocking the access of dsRNA to the RNase active centre. Additional ATP-dependent conformational changes are required to form an active dicing state and precisely cleave the dsRNA into a 21 bp siRNA duplex as confirmed by the structure in the post-dicing state. Collectively, this study revealed the molecular mechanism for the full cycle of ATP-dependent dsRNA processing by Dcr-2–Loqs-PD. Structures of the Dcr-2–Loqs-PD complex while it is processing a double-stranded RNA (dsRNA) substrate elucidate the interactions between Dcr-2 and Loqs-PD, and show that Dcr-2 undergoes substantial conformational changes during a dsRNA-processing cycle.

Zinc Oxide (ZnO) nanostructures have been grown by electrochemical deposition on the indium-tin oxide (ITO) substrates. The influences and mechanism of pre-treatment of ITO on the morphology, density and size of ZnO nanostructures have been studied. Hexagonal dumbbell-like shape of ZnO bipods have been obtained because of the absence of nucleation sites on the unetched ITO substrate. An ITO etching process in dilute HCl results in the instantaneous and high density nucleation process, which was introduced to reduce the system energy and increase the growth density of ZnO nanostructures. Therefore, well-defined hexagonal and dense ZnO nanorods array are deposited on the etched ITO surface.