Structural Basis for Control of Methylation Extent in Polyketide Synthase Metal-Dependent C-Methyltransferases

Redefining the Role of the EryM Acetyltransferase in Natural Product Biosynthetic Pathways

The GNAT (GCN5-related N -acetyltransferase) superfamily comprises enzymes with a conserved fold and diverse catalytic activities, including primarily acyl transfer, with a few examples of decarboxylation. EryM, a GNAT enzyme from Saccharopolyspora erythraea , has been implicated in both erythromycin and erythrochelin biosynthesis, with dual functionality as an acetyltransferase and a decarboxylase. Despite an historical association with malonyl-coenzyme A decarboxylation activity, the structural basis for this dual activity has remained unknown as its close homologs were identified with only acyl transfer activity. Here, crystal structures of EryM in its free form (2.5 Å) and in complex with acetyl-CoA (2.9 Å) reveal insights into the active site architecture and substrate interactions. Functional assays demonstrate that EryM catalyzes acyl transfer but lacks decarboxylation activity, challenging long-standing assumptions about its biosynthetic role. Comparative analysis of EryM and homologs in siderophore biosynthetic pathways highlights a conserved catalytic pocket with an essential His and identically positioned side chains common to GNAT enzymes for N -acyl transfer from CoA to primary hydroxylamine substrates. Bioinformatic analysis defines a large GNAT subfamily that is broadly distributed in the microbial world. These findings redefine EryM as an acetyltransferase and provide a foundation for understanding GNAT functional diversity in natural product biosynthesis.

Structures and Protein Engineering of the α-Keto Acid C-Methyltransferases SgvM and MrsA for Rational Substrate Transfer

S-adenosyl-l-methionine-dependent methyltransferases (MTs) are involved in the C-methylation of a variety of natural products. The MTs SgvM from Streptomyces griseoviridis and MrsA from Pseudomonas syringae pv. syringae catalyze the methylation of the β-carbon atom of α-keto acids in the biosynthesis of the antibiotic natural products viridogrisein and 3-methylarginine, respectively. MrsA shows high substrate selectivity for 5-guanidino-2-oxovalerate, while other α-keto acids, such as the SgvM substrates 4-methyl-2-oxovalerate, 2-oxovalerate, and phenylpyruvate, are not accepted. Here we report the crystal structures of SgvM and MrsA in the apo form and bound with substrate or S-adenosyl-l-methionine. By investigating key residues for substrate recognition in the active sites of both enzymes and engineering MrsA by site-directed mutagenesis, the substrate range of MrsA was extended to accept α-keto acid substrates of SgvM with uncharged and lipophilic β-residues. Our results showcase the transfer of the substrate scope of α-keto acid MTs from different biosynthetic pathways by rational design.

Structure of methyltransferase RedM that forms the dimethylpyrrolinium of the bisindole reductasporine

Bisindoles are biologically active natural products that arise from the oxidative dimerization of two molecules of l-tryptophan. In bacterial bisindole pathways, a core set of transformations is followed by the action of diverse tailoring enzymes that catalyze reactions that lead to diverse bisindole products. Among bisindoles, reductasporine is distinct due to its dimethylpyrrolinium structure. Its previously reported biosynthetic gene cluster encodes two unique tailoring enzymes, the imine reductase RedE and the dimethyltransferase RedM, which were shown to produce reductasporine from a common bisindole intermediate in recombinant E. coli. To gain more insight into the unique tailoring enzymes in reductasporine assembly, we reconstituted the biosynthetic pathway to reductasporine in vitro and then solved the 1.7 Å resolution structure of RedM. Our work reveals RedM adopts a variety of conformational changes with distinct open and closed conformations, and site-directed mutagenesis alongside sequence analysis identifies important active site residues. Finally, our work sets the stage for understanding how RedM evolved to react with a pyrrolinium scaffold and may enable the development of new dimethyltransferase catalysts.

HIF in Gastric Cancer: Regulation and Therapeutic Target

HIF means hypoxia-inducible factor gene family, and it could regulate various biological processes, including tumor development. In 2021, the FDA approved the new drug Welireg for targeting HIF-2a, and it is mainly used to treat von Hippel-Lindau syndrome, which demonstrated its good prospects in tumor therapy. As the fourth deadliest cancer worldwide, gastric cancer endangers the health of people all across the world. Currently, there are various treatment methods for patients with gastric cancer, but the five-year survival rate of patients with advanced gastric cancer is still not high. Therefore, here we reviewed the regulatory role and target role of HIF in gastric cancer, and provided some references for the treatment of gastric cancer.