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In the model plant Arabidopsis, a protein named MYB30 is one transcription factor that is able to promote disease resistance. Previous research identified some proteins that can reduce the activity of this transcription factor to avoid triggering a response when it is not needed, for example, when no infectious microbes are present. However, it was likely that other proteins were also involved in the process.

Now, Serrano et al. report that an enzyme called SBT5.2 is an additional negative regulator of MYB30 activity. SBT5.2 belongs to a family of protein-degrading enzymes called subtilases, which are typically localized outside cells. As such, it was unclear how SBT5.2 could interact and regulate a transcription factor that is found inside the nucleus of plant cells. Nevertheless, Serrano et al. found that the gene that encodes SBT5.2 actually gives rise to two distinct proteins. The first is a classical subtilase that is indeed located outside of the cell, and so cannot interact with MYB30 and does not affect its activity. The second protein is an atypical subtilase that localises to bubble-like compartments called vesicles within the cell and is able to highjack MYB30 on its way to the nucleus. When the atypical subtilase interacts with MYB30 at vesicles, it stops MYB30 from entering the nucleus. As a result, MYB30 cannot bind to the DNA nor activate its target genes. This means that the defensive response that normally depends on MYB30 is weakened.

The work of Serrano et al. uncovers a new way to regulate the expression of defence-related genes. Further unravelling the molecular mechanisms involved in the fine-tuning of gene expression represents a challenging task for future research.