Binds and inhibits cysteine proteinases. Inhibits most strongly papain and cathepsin L, more weakly bromelain and cathepsin B while it is completely ineffective against cathepsin H. Homodimer. Not secreted. Uniformly expressed throughout the mature fruiting body (at mRNA and protein level). Promising candidate for a biopesticide. Slows growth and reduces survival of Colorado potato beetle (Leptinotarsa decemlineata) larvae in a concentration dependent manner. Younger larvae are more susceptible to the action of clitocypin. Expressed at low levels by transgenic potato reduced larval weight gain and delayed development. The inhibition of digestive cysteine proteases, intestains, by clitocypin was shown to be the underlying mode of action. Belongs to the protease inhibitor I48 family.