Li, G. et al. Nat. Commun. 8, 1775 (2017).

Measuring the activity of proteins using chemical probes and mass spectrometry requires sufficient amounts of the sample, is limited to abundant proteins and does not preserve cellular resolution. In a twist of the well-established proximity ligation assay, Li et al. measure the activity of proteins of interest in single cells. Their activity-dependent proximity ligation (ADPL) assay relies on tagging of active enzymes with chemical probes that are specific for protein families such as serine hydrolases. The researchers then apply antibodies specific to the chemical probe and the protein of interest, followed by a proximity ligation assay. This technology allows them to visualize even low-abundance proteins in single cells. The researchers used the ADPL assay to measure the differential activity of a cholesterol ester hydrolase in different cancer cell lines and in patient tissue samples.