Trypanosoma brucei protein arginine methyltransferase 7 (TbPRMT7) exclusively generates monomethylarginine (MMA), which directs biological consequences distinct from that of symmetric dimethylarginine (SDMA) and asymmetric dimethylarginine (ADMA). However, determinants controlling the strict monomethylation activity are unknown. We present the crystal structure of the TbPRMT7 active core in complex with S-adenosyl-L-homocysteine (AdoHcy) and a histone H4 peptide substrate. In the active site, residues E172, E181, and Q329 hydrogen bond the guanidino group of the target arginine and align the terminal guanidino nitrogen in a position suitable for nucleophilic attack on the methyl group of S-adenosyl-L-methionine (AdoMet). Structural comparisons and isothermal titration calorimetry data suggest that the TbPRMT7 active site is narrower than those of protein arginine dimethyltransferases, making it unsuitable to bind MMA in a manner that would support a second turnover, thus abolishing the production of SDMA and ADMA. Our results present the structural interpretations for the monomethylation activity of TbPRMT7.

Transfer RNA (tRNA) methylation is necessary for the proper biological function of tRNA. The N1 methylation of guanine at Position 9 (m1G9) of tRNA, which is widely identified in eukaryotes and archaea, was found to be catalyzed by the Trm10 family of methyltransferases (MTases). Here, we report the first crystal structures of the tRNA MTase spTrm10 from Schizosaccharomyces pombe in the presence and absence of its methyl donor product S-adenosyl-homocysteine (SAH) and its ortholog scTrm10 from Saccharomyces cerevisiae in complex with SAH. Our crystal structures indicated that the MTase domain (the catalytic domain) of the Trm10 family displays a typical SpoU-TrmD (SPOUT) fold. Furthermore, small angle X-ray scattering analysis reveals that Trm10 behaves as a monomer in solution, whereas other members of the SPOUT superfamily all function as homodimers. We also performed tRNA MTase assays and isothermal titration calorimetry experiments to investigate the catalytic mechanism of Trm10 in vitro. In combination with mutational analysis and electrophoretic mobility shift assays, our results provide insights into the substrate tRNA recognition mechanism of Trm10 family MTases.

Constructing artificial helical structures through hierarchical self-assembly and exploring the underlying mechanism are important, and they help gain insight from the structures, processes, and functions from the biological helices and facilitate the development of material science and nanotechnology. Herein, the two enantiomers of chiral Au­(I) complexes ( S )-1 and ( R )-1 were synthesized, and they exhibited impressive spontaneous hierarchical self-assembly transitions from vesicles to helical fibers. An impressive chirality inversion and amplification was accompanied by the assembly transition, as elucidated by the results of in situ and time-dependent circular dichroism spectroscopy and scanning electron microscope imaging. The two enantiomers could serve as ideal chiral templates to co-assemble with other achiral luminogens to efficiently induce the resulting co-assembly systems to show circularly polarized luminescence (CPL). Our work has provided a simple but efficient way to explore the sophisticated self-assembly process and presented a facile and effective strategy to fabricate architectures with CPL properties.