LAUSR

ABSTRACT Host‐defense peptides and proteins are vital for first line protection against bacteria. Most host‐defense peptides and proteins common in vertebrates have been studied primarily in mammals, while their orthologues in non‐mammalian vertebrates received less attention. We found that the European Common Frog Rana temporaria expresses a protein in its skin that is evolutionarily related to the host‐defense protein S100A7. This prompted us to test if the encoded protein, which is an important microbicidal protein in human skin, shows similar activity in frogs. The R. temporaria protein lacks the zinc‐binding sites that are key to the antimicrobial activity of human S100A7 at neutral pH. However, despite being less potent, the R. temporaria protein does compromise bacterial membranes at low pH, similar to its human counterpart. We postulate that, while amphibian S100A7 likely serves other functions, the capacity to compromise bacterial cell membranes evolved early in tetrapod evolution. HighlightsA skin‐expressed protein in Rana temporaria is orthologous to human psoriasin/S100A7.Unlike human S100A7, frog S100A7 exerts only zinc‐independent antimicrobial activity.Rana temporaria S100A7 disrupts bacterial membranes at low pH.Evolution of cytolytic activity in S100 proteins started early in tetrapod evolution.