Structural basis for dimerization of a paramyxovirus polymerase complex.

The transcription and replication processes of non-segmented, negative-strand RNA viruses (nsNSVs) are catalyzed by a multi-functional polymerase complex composed of the large protein (L) and a cofactor protein, such as phosphoprotein (P). Previous studies have shown that the nsNSV polymerase can adopt a dimeric form, however, the structure of the dimer and its function are poorly understood. Here we determine a 2.7 Å cryo-EM structure of human parainfluenza virus type 3 (hPIV3) L-P complex with the connector domain (CD') of a second L built, while reconstruction of the rest of the second L-P obtains a low-resolution map of the ring-like L core region. This study reveals detailed atomic features of nsNSV polymerase active site and distinct conformation of hPIV3 L with a unique ß-strand latch. Furthermore, we report the structural basis of L-L dimerization, with CD' located at the putative template entry of the adjoining L. Disruption of the L-L interface causes a defect in RNA replication that can be overcome by complementation, demonstrating that L dimerization is necessary for hPIV3 genome replication. These findings provide further insight into how nsNSV polymerases perform their functions, and suggest a new avenue for rational drug design.

Tuning the magnetic ordering driven by cationic antisite defects in the Li(ZnMn)As system.

The electronic structure and magnetic properties of Li(ZnMn)As with antisite defects have been investigated by using first-principles calculations within the Perdew-Burke-Ernzerhof generalized gradient approximation. The cation antisite defect induced by Zn substitution for As was considered. Mn-3d, As-4p, Zn-4s, and Zn-4p were involved in the formation of d-sp hybrid orbitals, which enhanced the non-localized properties of Mn-3d electrons and provided a channel of Mn(↑)-As(↓)-ZnAs(↓)-Mn(↑) for indirect exchange of electrons between the magnetic ions. The antisite defect of Zn-substituted As belonged to the acceptor doping, rendering the compound p-type characteristics. The existence of the extra free hole carriers regulated the magnetic ordering transition. The ferromagnetic coupling between the Mn magnetic dopants was more favorable in the system with an antisite defect. In this paper, a novel type of dilute magnetic semiconductor with controllable carriers was designed and the mechanism of ferromagnetic coupling was revealed, which provided a theoretical reference for the subsequent studies.

Structural basis of human SNAPc recognizing proximal sequence element of snRNA promoter.

In eukaryotes, small nuclear RNAs (snRNAs) function in many fundamental cellular events such as precursor messenger RNA splicing, gene expression regulation, and ribosomal RNA processing. The snRNA activating protein complex (SNAPc) exclusively recognizes the proximal sequence element (PSE) at snRNA promoters and recruits RNA polymerase II or III to initiate transcription. In view that homozygous gene-knockout of SNAPc core subunits causes mouse embryonic lethality, functions of SNAPc are almost housekeeping. But so far, the structural insight into how SNAPc assembles and regulates snRNA transcription initiation remains unclear. Here we present the cryo-electron microscopy structure of the essential part of human SNAPc in complex with human U6-1 PSE at an overall resolution of 3.49 Å. This structure reveals the three-dimensional features of three conserved subunits (N-terminal domain of SNAP190, SNAP50, and SNAP43) and explains how they are assembled into a stable mini-SNAPc in PSE-binding state with a "wrap-around" mode. We identify three important motifs of SNAP50 that are involved in both major groove and minor groove recognition of PSE, in coordination with the Myb domain of SNAP190. Our findings further elaborate human PSE sequence conservation and compatibility for SNAPc recognition, providing a clear framework of snRNA transcription initiation, especially the U6 system.

Electron energy loss spectroscopy and first-principles study of GaN via Zn doping.

The electronic structure of GaN and GaN:Zn was investigated by electron energy loss spectroscopy and first-principles calculations. In the low-loss spectrum, the interband transitions are assigned to the observed energy loss peaks. After Zn doping, impurity levels are introduced to the density of states and hybrid orbitals of N 2p and Zn 3d are formed around the Fermi level. In the nitrogen K-edge, an additional peak was observed due to the formation of donor defect states. A core-hole effect is believed to be significant for simulation of the N K-edge for both GaN and GaN:Zn.

Structures of cell wall arabinosyltransferases with the anti-tuberculosis drug ethambutol.

The arabinosyltransferases EmbA, EmbB, and EmbC are involved in Mycobacterium tuberculosis cell wall synthesis and are recognized as targets for the anti-tuberculosis drug ethambutol. In this study, we determined cryo-electron microscopy and x-ray crystal structures of mycobacterial EmbA-EmbB and EmbC-EmbC complexes in the presence of their glycosyl donor and acceptor substrates and with ethambutol. These structures show how the donor and acceptor substrates bind in the active site and how ethambutol inhibits arabinosyltransferases by binding to the same site as both substrates in EmbB and EmbC. Most drug-resistant mutations are located near the ethambutol binding site. Collectively, our work provides a structural basis for understanding the biochemical function and inhibition of arabinosyltransferases and the development of new anti-tuberculosis agents.

Mycobacterial dynamin-like protein IniA mediates membrane fission.

Mycobacterium tuberculosis infection remains a major threat to human health worldwide. Drug treatments against tuberculosis (TB) induce expression of several mycobacterial proteins, including IniA, but its structure and function remain poorly understood. Here, we report the structures of Mycobacterium smegmatis IniA in both the nucleotide-free and GTP-bound states. The structures reveal that IniA folds as a bacterial dynamin-like protein (BDLP) with a canonical GTPase domain followed by two helix-bundles (HBs), named Neck and Trunk. The distal end of its Trunk domain exists as a lipid-interacting (LI) loop, which binds to negatively charged lipids for membrane attachment. IniA does not form detectable nucleotide-dependent dimers in solution. However, lipid tethering indicates nucleotide-independent association of IniA on the membrane. IniA also deforms membranes and exhibits GTP-hydrolyzing dependent membrane fission. These results confirm the membrane remodeling activity of BDLP and suggest that IniA mediates TB drug-resistance through fission activity to maintain plasma membrane integrity.