Antlions are insects, relatives of the lacewings, that snare ants by making conical pits in loose sand. An unwary ant or other insect slips down the treacherous slope, to be met by a partially buried antlion larva that is eager and ready to suck it dry. To ensure that the prey does not escape, the antlion injects it with a paralyzing neurotoxin. Surprise! The toxin was found to be a homologue of the otherwise-benign bacterial protein GroEL. A highly conserved protein, GroEL is a chaperonin that, together with GroES and ATP, protects bacterial cells during stress by ensuring that other proteins are folded correctly.
GroEL is present in the saliva of the antlion. How does it get there? It turns out that it is made by a bacterial symbiont that lives there, Enterobacter aerogenes (otherwise known as a soil member of the enteric bacteria and an occasional pathogen of vertebrates). The amino acids of GroEL needed for the toxic action are far removed from the domains used in protein folding, so GroEL could serve both as a toxin for the insect and a chaperonin for the bacteria.
Not all GroELs are toxic to insects. In a cockroach assay, the toxicity of GroEL from E. coli was 1000 times less than that of its E. aerogenes homolog. The two proteins differ by only a few amino acids; changing those amino acids in the E. coli protein to match those of E. aerogenes did indeed increase toxicity. Neither protein is toxic to mice. This is far from being the first known effect of bacterial chaperonins on animals. In vertebrates, chaperonins of bacterial pathogens function as signaling molecules. They stimulate various cells to release compounds that elicit inflammation (proinflammatory cytokines). But determining that GroEL can also be a toxin is news, indeed.
Nature’s tools are often endowed with multiple functions. When you reach into its toolkit for a screwdriver, you may pull out a Swiss Army Knife, complete with several blades, a saw, and more.