Ant abaecin-2 is a context-dependent copper-binding effector that can be either inhibitory or protective

Host defense peptides (HDPs) are important components of the innate immune system that are used to combat pathogens and often rely on binding trace nutrient metals for their function. However, controlling nutrient metals may have other roles in host-symbiont interactions beyond poisoning harmful pathogens. This study characterizes the evolution, structural properties, and biochemical activity of the novel hymenopteran HDP abaecin-2. In myrmicine ants such as the fungus-growing tribe Attini, abaecin-2 has evolved to include an Amino-Terminal Cu(ii) and Ni(ii)-binding (ATCUN) motif, which we hypothesize binds copper, a trace nutrient that is enriched in attine ant colonies. Combined results from mass spectrometry, competitive binding assays, circular dichroism, and NMR indicate that the abaecin-2 peptide lacks a defined secondary structure and can associate with up to 2 Cu(ii) ions, one strongly bound at the ATCUN motif and another weakly bound, likely at a conserved histidine residue. Despite its copper-binding activity, abaecin-2 alone does not exhibit antibacterial activity against Escherichia coli or Bacillus subtilis (models for bacteria that live in ant fungus gardens). However, it synergizes with a model pore-forming peptide cecropin A to inhibit the growth of E. coli, similar to the related peptide abaecin-1. The copper-binding activity conferred by the ATCUN motif also protects copper-sensitive E. coli from excess copper toxicity. The dual context-dependent inhibitory and protective roles for abaecin-2 indicate that this previously under-characterized HDP may be used by attine ants to regulate both harmful and beneficial symbionts.