SARS-CoV-2 nucleocapsid protein (N protein) binds nucleic acids and packages viral RNA. DNA aptamers that specifically bind the N protein have been used in antigen-based COVID-19 detection and have potential clinical applications for preventing SARS-CoV-2 infection. However, the complex structures of the N protein with DNA aptamers and the mechanisms by which aptamers recognize the N protein remain unclear. Here, we report the NMR-derived complex structure of the N-terminal domain of the N protein (N-NTD) with a 58 nt DNA aptamer, A48. The complex structure reveals a distinct topology with a large contact area between A48 and N-NTD. The N-terminal intrinsically disordered region (IDR) of N-NTD forms close contact with A48, primarily stabilized by hydrophilic interactions. Deletion of the N-terminal IDR or substitution of positively charged arginine residues with negatively charged glutamate residues in the IDR region substantially reduced the binding affinity for A48. Because most previously determined N protein structures were obtained using constructs lacking the N-terminal IDR, this study reveals a topology of the N protein-nucleic acid complex and highlights the importance of the N-terminal IDR in nucleic acid binding.

Colorectal carcinoma (CRC) is characterized by mutations in p53 and the Wnt signaling pathway, and immunotherapy has shown limited efficacy in microsatellite-stable CRC. Here, CEABP1, a binding protein for the CRC biomarker carcinoembryonic antigen (CEA), was designed de novo through the AI-based computational generation methods RFDiffusion/ProteinMPNN and stringent in silico selection, for targeted delivery of purified p53 protein and transcription factor T-cell factor (TCF)/lymphoid enhancer-binding factor (LEF) transcription factor decoy (TFD) DNA into CRC cells. The cell-penetrating peptide (CPP) p28 was employed to deliver the p28-p53-CEABP1 protein, which significantly enhanced p53’s inhibition of CRC cell proliferation and xenograft tumor growth. Codelivery of the p14ARF protein together with p53 prolonged the effective antitumor duration of p53. In addition, the DNA binding domain of Max was fused with CPP and CEABP1 to deliver TCF/LEF TFD DNA, comprising concatenated consensus binding motifs for TCF/LEF and Max, into CRC cells to inhibit Wnt target gene transcription, leading to marked suppression of CRC cell proliferation and xenograft tumor growth. These findings paved the way for the development of precision anticancer therapeutics using designed binding proteins of tumor biomarkers for targeted delivery of tumor suppressor proteins and TFD DNA.

In a vast variety of prokaryotes such as Escherichia coli and Streptomyces lividans, the DNA degradation (Dnd) CDE protein complex (consisting of DndC, DndD, and DndE), together with the DndA/IscS protein and the DndFGH complex, function as a defense barrier to prevent the invasion of non-self-DNA. The DndCDE complex introduces phosphorothioation (PT) modifications into DNA, and the DndFGH complex specifically cleaves non-PT DNA and, thus, restricts horizontal gene transfer and phage invasion. Despite the central importance of the DndCDE complex in DNA PT modification, which catalyzes the oxygen–sulfur swap on DNA, our understanding of this key complex remains poor. Here, we employed protein structure prediction to provide a reasonably reliable prediction of the structure of the DndCDE complex and a 23 bp DNA-DndCDE complex. We found that among the three proteins in the DndCDE complex, DndC, especially its “specificity loop”, plays a key role in recognizing the consensus PT modification sequence. In addition, the DndD protein is found to possess a highly conserved structural surface on its globular domain, presumably mediating the dimerization of DndD as well as the DndCDE complex. Furthermore, our normal mode analysis showed that there exists a dynamic transition between a closed and an open state for the DndCDE complex, facilitating its association and release of DNA. Our conclusions were corroborated by biochemical assays using purified proteins. On the whole, we provide molecular insights into the assembly and DNA-recognition mechanism of a central protein complex involved in DNA phosphorothioation.