Single-molecule enzymatic reaction dynamics and mechanisms of GPX3 and TRXh9 from Arabidopsis thaliana

Glutathione peroxidases (GPXs) regulate the levels of reactive oxygen species in cells and tissues. During the redox cycling, the plant GPX is regenerated by thioredoxins (TRXs) as reductant rather than glutathione as the electron donor. However, the direct experimental observation on the interaction dynamics between GPXs and TRXs has not been reported, and the redox mechanism is unclear. In this work, the protein interactions between oxidized AtGPX3 and reduced AtTRXh9 have been studied using single-molecule fluorescence resonance energy transfer (smFRET). The obtained results indicate there are four processes in these two protein interaction, including biological recognition, binding, intermediate and unbinding state. Two enzymatic reaction intermediate states have been identified in the dissociation of AtGPX3-AtTRXh9 complex from binding to unbinding state, suggesting two types of interaction pathways and intermediate complexes. In particular, the dynamical study reveals that the redox reaction between oxidized AtGPX3 and reduced AtTRXh9 is realized through the forming and breaking of disulfide bonds via the active sites of Cys4 and Cys57 in AtTRXh9. These findings are of significant for deep understanding the redox reaction and mechanism between GPXs and TRXs enzymes, and studying other protein dynamics at single-molecule level.

High-affinity RNA binding by a hyperthermophilic single-stranded DNA-binding protein

Single-stranded DNA-binding proteins (SSBs), including replication protein A (RPA) in eukaryotes, play a central role in DNA replication, recombination, and repair. SSBs utilise an oligonucleotide/oligosaccharide-binding (OB) fold domain to bind DNA, and typically oligomerise in solution to bring multiple OB fold domains together in the functional SSB. SSBs from hyperthermophilic crenarchaea, such as Sulfolobus solfataricus, have an unusual structure with a single OB fold coupled to a flexible C-terminal tail. The OB fold resembles those in RPA, whilst the tail is reminiscent of bacterial SSBs and mediates interaction with other proteins. One paradigm in the field is that SSBs bind specifically to ssDNA and much less strongly to RNA, ensuring that their functions are restricted to DNA metabolism. Here, we use a combination of biochemical and biophysical approaches to demonstrate that the binding properties of S. solfataricus SSB are essentially identical for ssDNA and ssRNA. These features may represent an adaptation to a hyperthermophilic lifestyle, where DNA and RNA damage is a more frequent event.