Assembly of the spliceosomal small nuclear ribonucleoparticle (snRNP) core requires the participation of themultisubunit SMN (survival of motor neuron) complex, which contains SMN and several Gemin proteins. The SMNand Gemin2 subunits directly bind Sm proteins, and Gemin5 is required for snRNP biogenesis and has been im-plicated in snRNA recognition. The RNA sequence required for snRNP assembly includes the Sm site and an ad-jacent 3′stem–loop, but a precise understanding of Gemin5’s RNA-binding specificity is lacking. Here we show thatthe N-terminal half of Gemin5, which is composed of two juxtaposed seven-bladed WD40 repeat domains, recog-nizes the Sm site. The tandem WD40 repeat domains are rigidly held together to form a contiguous RNA-bindingsurface. RNA-contacting residues are located mostly on loops betweenβstrands on the apical surface of the WD40domains. Structural and biochemical analyses show that base-stacking interactions involving four aromatic residuesand hydrogen bonding by a pair of arginines are crucial for specific recognition of the Sm sequence. We also show thatan adenine immediately 5′to the Sm site is required for efficient binding and that Gemin5 can bind short RNA oligosin an alternative mode. Our results provide mechanistic understandings of Gemin5’s snRNA-binding specificity aswell as valuable insights into the molecular mechanism of RNA binding by WD40 repeat proteins in general.

Aim: To report our methods for expression and purification of α7 nicotinic acetylcholine receptor (α7-nAChR), a ligand-gated pentameric ion channel and an important drug target. Methods: α7-nAChRs of 10 different species were cloned into an inducible BacMam vector with an N-terminal tag of a tandem maltose-binding protein (MBP) and a TEV cleavage site. This α7-nAChR fusion receptor was expressed in mammalian HEK293F cells and detected by Western blot. The expression was scaled up to liters. The receptor was purified using amylose resin and size-exclusion chromatography. The quality of the purified receptor was assessed using SDS-PAGE gels, thermal stability analysis, and negative stain electron microscopy (EM). The expression construct was optimized through terminal truncations and site-directed mutagenesis. Results: Expression screening revealed that α7-nAChR from Taeniopygia guttata had the highest expression levels. The fusion receptor was expressed mostly on the cell surface, and it could be efficiently purified using one-step amylose affinity chromatography. One to two milligrams of the optimized α7-nAChR expression construct were purified from one liter of cell culture. The purified α7-nAChR samples displayed high thermal stability with a Tm of 60 °C, which was further enhanced by antagonist binding but decreased in the presence of agonist. EM analysis revealed ring-like structures with a central hydrophilic hole, which was consistent with the pentameric assembly of the α7-nAChR channel. Conclusion: We have established methods for crystallization scale expression and purification of α7-nAChR, which lays a foundation for high-resolution structural studies using X-ray crystallography or single particle cryo-EM analysis.

Aim: To establish a method for efficient expression and purification of the human serotonin type 3A receptor (5-HT3A) that is suitable for structural studies. Methods: Codon-optimized cDNA of human 5-HT3A was inserted into a modified BacMam vector, which contained an IgG leader sequence, an 8×His tag linked with two-Maltose Binding Proteins (MBP), and a TEV protease cleavage site. The BacMam construct was used to generate baculoviruses for expression of 5-HT3A in HEK293F cells. The proteins were solubilized from the membrane with the detergent C12E9, and purified using MBP affinity chromatography. The affinity tag was removed by TEV protease treatment and immobilized metal ion affinity chromatography. The receptors were further purified by size-exclusion chromatography (SEC). Western blot and SDS-PAGE were used to detect 5-HT3A during purification. The purified receptor was used in crystallization and analyzed with negative stain electron microscopy (EM). Results: The BacMam system yielded 0.5 milligram of the human 5-HT3A receptor per liter of cells. MBP affinity purification resulted in good yields with high purity and homogeneity. SEC profiles indicated that the purified receptors were pentameric. No protein crystals were obtained; however, a reconstructed 3D density map generated from the negative stain EM data fitted well with the mouse 5-HT3A structure. Conclusion: With the BacMam system, robust expression of the human 5-HT3A receptor is obtained, which is monodisperse, therefore enabling 3D reconstruction of an EM map. This method is suitable for high-throughput screening of different constructs, thus facilitating structural and biochemical studies of the 5-HT3A receptor.

DNA methylation is an important epigenetic modification that is essential for various developmental processes through regulating gene expression, genomic imprinting, and epigenetic inheritance1,2,3,4,5. Mammalian genomic DNA methylation is established during embryogenesis by de novo DNA methyltransferases, DNMT3A and DNMT3B6,7,8, and the methylation patterns vary with developmental stages and cell types9,10,11,12. DNA methyltransferase 3-like protein (DNMT3L) is a catalytically inactive paralogue of DNMT3 enzymes, which stimulates the enzymatic activity of Dnmt3a13. Recent studies have established a connection between DNA methylation and histone modifications, and revealed a histone-guided mechanism for the establishment of DNA methylation14. The ATRX–DNMT3–DNMT3L (ADD) domain of Dnmt3a recognizes unmethylated histone H3 (H3K4me0)15,16,17. The histone H3 tail stimulates the enzymatic activity of Dnmt3a in vitro17,18, whereas the molecular mechanism remains elusive. Here we show that DNMT3A exists in an autoinhibitory form and that the histone H3 tail stimulates its activity in a DNMT3L-independent manner. We determine the crystal structures of DNMT3A–DNMT3L (autoinhibitory form) and DNMT3A–DNMT3L-H3 (active form) complexes at 3.82 and 2.90 Å resolution, respectively. Structural and biochemical analyses indicate that the ADD domain of DNMT3A interacts with and inhibits enzymatic activity of the catalytic domain (CD) through blocking its DNA-binding affinity. Histone H3 (but not H3K4me3) disrupts ADD–CD interaction, induces a large movement of the ADD domain, and thus releases the autoinhibition of DNMT3A. The finding adds another layer of regulation of DNA methylation to ensure that the enzyme is mainly activated at proper targeting loci when unmethylated H3K4 is present, and strongly supports a negative correlation between H3K4me3 and DNA methylation across the mammalian genome9,10,19,20. Our study provides a new insight into an unexpected autoinhibition and histone H3-induced activation of the de novo DNA methyltransferase after its initial genomic positioning.

Enterovirus 71 (EV71) is a major causative agent of hand, food, and mouth disease, which frequently occurs in young children. Since there are 11 subgenotypes (A, B1 to B5, and C1 to C5) within EV71, an EV71 vaccine capable of protecting against all of these subgenotypes is desirable. We report here the vaccine potential and protective mechanism of two chimeric virus-like particles (VLPs) presenting conserved neutralizing epitopes of EV71. We show that fusions of hepatitis B core antigen (HBc) with the SP55 or SP70 epitope of EV71, designated HBcSP55 and HBcSP70, respectively, can be rapidly generated and self-assembled into VLPs with the epitopes displayed on the surface. Immunization with the chimeric VLPs induced carrier- and epitope-specific antibody responses in mice. Anti-HBcSP55 and anti-HBcSP70 sera, but not anti-HBc sera, were able to neutralize in vitro multiple genotypes and strains of EV71. Importantly, passive immunization with anti-HBcSP55 or anti-HBcSP70 sera protected neonatal mice against lethal EV71 infections. Interestingly, anti-HBcSP70 sera could inhibit EV71 attachment to susceptible cells, whereas anti-HBcSP55 sera could not. However, both antisera were able to neutralize EV71 infection in vitro at the postattachment stage. The divergent mechanism of neutralization and protection conferred by anti-SP70 and anti-SP55 sera is in part attributed to their respective ability to bind authentic viral particles. Collectively, our study not only demonstrates that chimeric VLPs displaying the SP55 and SP70 epitopes are promising candidates for a broad-spectrum EV71 vaccine but also reveals distinct mechanisms of neutralization by the SP55- and SP70-targeted antibodies.