Defensin‐DNA Supramolecular Complexes as a New pH‐Sensing Nanomaterial

Defensins are a diverse class of host defence peptides (HDPs) that harbour both direct antimicrobial and immunomodulatory activities. These cysteine‐rich HDPs are ubiquitously expressed throughout nature; however, they are classified into two evolutionarily independent superfamilies based on their ancestral lineage: the cis‐ defensins (expressed by plants, fungi and invertebrates) and trans‐ defensins (expressed by vertebrates and some invertebrates). A conserved attribute amongst defensins belonging to both the cis ‐ and trans‐ superfamilies is the ability to self‐assemble into multimeric complexes when interacting directly with target pathogens. These oligomeric complexes vary greatly in morphology and the required complementary ligands, ranging from phospholipid‐induced fibrils to bacteria‐ensnaring nanonets. This study now reports that the saltwater crocodile defensin CpoBD13 can bind short sequences of single‐stranded and double‐stranded DNA to form supramolecular spherical oligomeric complexes. These complexes, named defensin‐induced spherical complexes (DISCs), form in a pH‐sensitive and reversible process, whereby the complexes assemble in acidic conditions and disassemble when the pH is basic. This innate ability of CpoBD13 DISCs to disassemble upon a change in pH was used to pH‐dependently release captured doxorubicin to induce cell death in osteosarcoma KRIB cells, demonstrating proof of concept for the potential use of DISCs as a bioactive pH‐sensing nanomaterial. Furthermore, mutagenesis of CpoBD13 (H21R/H35R) generated DISCs that remained stable for at least 3 h in vivo within the vasculature of zebrafish embryos and retained morphological characteristics comparable to the wild‐type, underscoring the potential of protein engineering to enhance DISC stability.