Djoko, K. | bioRxiv Science

bioRxiv2026

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 metal binding for their function. However, controlling trace nutrients such as transition 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 may bind 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 we propose for abaecin-2 indicate that this previously under-characterized HDP may be used by attine ants to regulate both harmful and beneficial symbionts. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=69 SRC="FIGDIR/small/719391v2_ufig1.gif" ALT="Figure 1"> View larger version (17K): org.highwire.dtl.DTLVardef@1298f41org.highwire.dtl.DTLVardef@17c16d9org.highwire.dtl.DTLVardef@1c03081org.highwire.dtl.DTLVardef@b0a5a6_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗

bioRxiv2022

The salivary, metal-binding peptide histatin-5 buffers extracellular copper availability

Antimicrobial peptides (AMPs) are key components of diverse host innate immune systems. The family of human salivary AMPs known as histatins bind Zn and Cu. Fluctuations in Zn and Cu availability play significant roles in the host innate immune response (so-called "nutritional immunity"). Thus, we hypothesised that histatins contribute to nutritional immunity by influencing host Zn and/or Cu availability. We posited that histatins limit Zn availability (promote bacterial Zn starvation) and/or raise Cu availability (promote bacterial Cu poisoning). To test this hypothesis, we examined the interactions between histatin-5 (Hst5) and Group A Streptococcus (GAS), which colonises the human oropharynx. Our results showed that Hst5 does not strongly influence Zn availability. Hst5 did not induce expression of Zn-responsive genes in GAS, nor did it suppress growth of mutant strains that are impaired in Zn transport. Biochemical examination of purified peptides confirmed that Hst5 binds Zn only weakly. By contrast, Hst5 bound Cu tightly and it strongly influenced Cu availability. However, Hst5 did not promote Cu toxicity. Instead, Hst5 suppressed expression of Cu-inducible genes, stopped intracellular accumulation of Cu, and rescued growth of a {Delta}copA mutant strain that is impaired in Cu efflux. We thus proposed a new role for salivary histatins as major Cu buffers in saliva that contribute to microbial homeostasis in the oral cavity and oropharynx by reducing the potential negative effects of Cu exposure (e.g. from food) to microbes. Our results raise broad questions regarding the physiological roles of diverse metal-binding AMPs and the management of host metal availability during host-microbe interactions.

microbiology↗