The role of metal binding in the function of the human salivary antimicrobial peptide histatin-5

Abstract

The family of human salivary histidine-rich peptides known as histatins bind zinc (Zn) and copper (Cu), but whether they contribute to nutritional immunity by influencing Zn and/or Cu availability has not been examined. We hypothesised that histatin-5 (Hst5) limits Zn availability (and promotes bacterial Zn starvation) and/or raises Cu availability (and promotes bacterial Cu poisoning). To test this hypothesis, Group A Streptococcus (GAS), which colonises the human oropharynx, was used as a model bacterium. Contrary to our hypothesis, Hst5 did not strongly influence Zn availability. This peptide did not induce expression of Zn uptake genes in GAS, nor did it suppress growth of an ΔadcAI mutant strain that is impaired in Zn uptake. Equilibrium competition measurements confirmed that Hst5 binds Zn weakly and does not compete with the high-affinity Zn uptake protein AdcAI for binding Zn. By contrast, Hst5 bound Cu with a high affinity and strongly influenced Cu availability. However, contrary to our hypothesis, Hst5 did not promote Cu toxicity. Instead, this peptide suppressed expression of Cu-inducible genes in GAS, stopped intracellular accumulation of Cu, and rescued growth of a ΔcopA mutant strain that is impaired in Cu efflux in the presence of added Cu. These findings led us to propose a new role for Hst5 and salivary histatins as major Cu buffers in saliva that reduce the potential negative effects of Cu exposure to microbes. We speculate that histatins promote oral and oropharyngeal health by contributing to microbial homeostasis in these host niches.

Zinc Binding Inhibits Cellular Uptake and Antifungal Activity of Histatin-5 in Candida albicans

“…In a healthy individual, is Hist-5 actively entering and killing commensal microbial cells or is there a surveillance mechanism that triggers the antifungal response when the microbial environment is disrupted and infection is initiated? Here, we add to the growing body of literature suggesting that Hist-5 may participate in interactions with cells that are not solely for antifungal purposes but rather promote and maintain microbial homeostasis for oral microbial health. , …”

“…Here, we add to the growing body of literature suggesting that Hist-5 may participate in interactions with cells that are not solely for antifungal purposes but rather promote and maintain microbial homeostasis for oral microbial health. 29,53 We propose a working model for the role of Zn 2+ in Hist-5 antifungal activity and commensalism where modulation of exchangeable Zn 2+ concentration in the growth environment acts as a dial to tune Hist-5 uptake and activity in C. albicans (Figure 9). When the host is healthy, commensal C. albicans are continuously exposed to sublethal concentrations of Hist-5 and Zn 2+ .…”

Zinc binding inhibits cellular uptake and antifungal activity of Histatin-5 in Candida albicans

“…Here, we add to the growing body of literature suggesting that Hist-5 may participate in interactions with cells that are not solely for antifungal purposes but rather promote and maintain microbial homeostasis for oral microbial health. 29, 53…”

The Synergy Between Zinc and Antimicrobial Peptides: An Insight into Unique Bioinorganic Interactions

“…The effects of Zn(II) on the antibacterial activity of Hst5 are less defined. A recent preprint indicates that Hst5 does not influence growth or survival of multiple Streptococcus species that typically colonize the oral cavity and thus interact with salivary fluids [70]. While the addition of Zn(II) neither enhanced nor suppressed this effect.…”

“…Given the weak affinity of Hst5 to Zn(II), it has been argued that a Zn(II)-Hst5 complex is not likely to form in vivo or even in in vitro conditions in which potential competing Zn(II) ligands such as phosphates or amino acids are present [70]. Therefore, Hst5 is not likely to act as a Class I Zn-AMP.…”