Bacteria rely on a previously unknown trick to disarm a protein that is crucial for defending plants and animals from disease.

Found in a wide variety of living things, heat shock protein 90 (HSP90) helps to fold and assemble molecules that detect pathogen invasion and fight off infection. But Vincent Tagliabracci, at the University of Texas Southwestern Medical Center in Dallas, and his colleagues have found that bacteria can undermine this defence.

The researchers infected plants with the bacterium Pseudomonas syringae, a major plant pathogen that secretes a virulence protein called HopBF1. The team found that this protein mimics plant molecules that bind to HSP90. Thanks to this deception, HSP90 can be deactivated by HopBF1, which dampens immunity and results in severe disease.

Compounds that inhibit HopBF1 might help to control crop pathogens, the authors say. HSP90 also promotes the growth and survival of many cancer cells, raising the tantalizing prospect that a therapeutic form of HopBF1 might block a wide range of tumour types.

Correction: An earlier picture caption incorrectly said that the bacterium Pseudomonas syringae had damaged the pictured leaf. An insect had damaged the leaf.