Impact of C-terminal domains of paralogous single-stranded DNA binding proteins from Streptomyces coelicolor on their biophysical properties and biological functions.

Single-stranded DNA-binding proteins (SSB) are crucial in DNA metabolism. While Escherichia coli SSB is extensively studied, the significance of its C-terminal domain has only recently emerged. This study explored the significance of C-domains of two paralogous Ssb proteins in S. coelicolor. Mutational analyses of C-domains uncovered a novel role of SsbA during sporulation-specific cell division and demonstrated that the C-tip is non-essential for survival. In vitro methods revealed altered biophysical and biochemical properties of Ssb proteins with modified C-domains. Determined hydrodynamic properties suggested that the C-domains of SsbA and SsbB occupy a globular position proposed to mediate cooperative binding. Only SsbA was found to form biomolecular condensates independent of the C-tip. Interestingly, the truncated C-domain of SsbA increased the molar enthalpy of unfolding. Additionally, calorimetric titrations revealed that C-domain mutations affected ssDNA binding. Moreover, this analysis showed that the SsbA C-tip aids binding most likely by regulating the position of the flexible C-domain. It also highlighted ssDNA-induced conformational mobility restrictions of all Ssb variants. Finally, the gel mobility shift assay confirmed that the intrinsically disordered linker is essential for cooperative binding of SsbA. These findings highlight the important role of the C-domain in the functioning of SsbA and SsbB proteins.

Interdisciplinary Study of the Effects of Dipeptidyl-Peptidase III Cancer Mutations on the KEAP1-NRF2 Signaling Pathway.

Dipeptidyl peptidase III (DPP III) is associated with cancer progression via interaction with KEAP1, leading to upregulation of the KEAP1-NRF2 oxidative stress pathway. Numerous DPP III mutations have been found in human tumor genomes, and it is suggested that some of them may alter affinity for KEAP1. One such example is the DPP III-R623W variant, which in our previous study showed much higher affinity for the Kelch domain of KEAP1 than the wild-type protein. In this work, we have investigated the effects of this mutation in cultured cells and the effects of several other DPP III mutations on the stability of KEAP1-DPP III complex using an interdisciplinary approach combining biochemical, biophysical and molecular biology methods with computational studies. We determined the affinity of the DPP III variants for the Kelch domain experimentally and by molecular modeling, as well as the effects of the R623W on the expression of several NRF2-controlled genes. We confirmed that the R623W variant upregulates NQO1 expression at the transcriptional level. This supports the hypothesis from our previous study that the increased affinity of the R623W variant for KEAP1 leads to upregulation of the KEAP1-NRF2 pathway. These results provide a new perspective on the involvement of DPP III in cancer progression and prognosis.