Villalobos, P. | bioRxiv Science

bioRxiv2026

Structural, dynamic, and evolutionary determinants of substrate binding in the tetrameric 6-phosphogluconate dehydrogenase from Gluconobacter oxydans

6-Phosphogluconate dehydrogenases (6PGDHs) catalyze a key oxidative step in the oxidative pentose phosphate pathway (oxPPP), a route essential for NAD(P)H generation and carbon metabolism in bacteria and eukaryotes. While the structural basis of substrate recognition is well established for long-chain dimeric 6PGDHs, the mechanisms used by short-chain tetrameric enzymes remain poorly defined. Here, we present a 2.0 [A] crystal structure of tetrameric 6PGDH from Gluconobacter oxydans (Go6PGDH) in complex with 6-phosphogluconate (6PG) and integrate it with evolutionary, computational, and functional analyses. The structure shows that, unlike dimeric homologs, tetrameric Go6PGDH does not undergo a domain-closure transition upon ligand binding. Instead, 6PG induces a compaction of the tetramer mediated by two conserved C-terminal elements: an inter-protomer ionic "lock" and an intra-subunit C-terminal "latch" that together stabilize a closed catalytic pocket. Molecular-dynamics simulations identify His328 as a central residue that couples C-terminal tail closure to direct ligand coordination, and mutagenesis analysis confirms its essential role in catalytic efficiency. Thermodynamic measurements reveal that 6PG binding is strongly enthalpy-driven, consistent with the formation of an ordered hydrogen-bonding and electrostatic network in the closed conformation. These findings define a substrate-induced quaternary-tightening mechanism unique to tetrameric 6PGDHs and illustrate how a conserved C-terminal module has been adapted across the family to regulate substrate binding and catalysis.

biochemistry↗

bioRxiv2025

Clade-wide proteome analysis shows widespread non-canonical Dcr proteins in fungi.

Dicers (Dcrs) are central proteins involved in the biogenesis of small RNAs (sRNAs) in eukaryotes. Most of the knowledge on Dcr structure, function and evolution comes from studies conducted in animal and plant species. Comparatively, much less is known in fungi, which are a genetically and ecologically diverse group with important roles in ecosystems, agriculture, medicine, and biotechnology. While canonical Dcrs in plants and animals contain a well-defined domain architecture, most fungal Dcrs with experimentally validated functions lack one or more identifiable canonical domains, raising questions about how RNA-binding and precise sRNA processing is retained. Here, we conducted the most extensive survey of fungal Dcr proteins, analyzing 1,593 proteomes across eight phyla. We found a diversity of Dcr domain architectures, with some of them lacking an identifiable PAZ, Helicase, and/or double-stranded RNA binding domains. Phylogenetic analyses showed that different Dcr classes are distributed across distinct clades that often align with fungal taxonomic groups. Despite the lack of canonical domain architectures, we found that fungal Dcrs fold into a characteristic L-shaped structure and show PAZ-like folds, even in proteins without detectable PAZ sequences. Molecular docking and electrostatic analyses further indicate that these divergent Dcrs maintain key RNA-binding surfaces for proper sRNA processing. Our results indicate a remarkable evolutionary plasticity of Dcr in fungi, showing that essential sRNA processing functions can be retained through structural conservation, and highlighting fungi as models to study the modular evolution of the RNAi machinery in eukaryotes. Significance statementDicer (Dcr) proteins are central to RNA interference (RNAi), a gene regulatory mechanism conserved across eukaryotes. However, current models of Dcr structure, function, and evolution are largely based on studies in animals and plants. Here, we present the most comprehensive analysis to date of Dcr proteins in fungi, a diverse eukaryotic group including many societally important pathogens and symbiotes which are reliant on RNAi. Our findings reveal that despite widespread divergence from canonical Dcr architecture, fungal Dcrs conserve critical folds and RNA-binding features, further suggesting that core RNAi functions are maintained. This work establishes fungi as key models for studying the evolution and functional robustness of the RNAi machinery, offering broader insight into the diversity and plasticity of sRNA biogenesis pathways across eukaryotes.

genomics↗