Function-guided proximity mapping unveils electrophilic-metabolite sensing by proteins not present in their canonical locales

Significance The cell is a hive of information flow in which metabolite signals and proteins translocate across subcompartments. Peering into this submicroscopic world is hugely challenging. Using a strategy termed Localis-rex, we identify subcellular locale-specific sensors of a native reactive electrophilic metabolite. Surprisingly, several proteins sense electrophiles in locales where they do not canonically reside. One example is the nuclear protein, CDK9. In the cytosol, the electrophilic-metabolite–modified CDK9 has no negative function. However, following nuclear translocation, the electrophilic-metabolite–modified CDK9 downregulates the transcriptional activator, RNA-Polymerase-II. This exquisitely-nuanced signaling modality highlights the need to assign means, motive, and opportunity to proteins involved in reactive metabolite signaling.