Application of a novel FAM-conjugated activity-based probe to determine cathepsin G activity intracellularly

Cathepsin G (CatG) is responsible for several distinct immune processes of adaptive and innate immunity depending on extra- or intracellular occurrence of CatG. Recently, we established a method to detect CatG activity at the cell surface of natural killer cells by using the activity-based probe MARS116-Bt in flow cytometry. MARS116-Bt consists of biotin, spacer, amino acid sequence, and a phosphonate warhead which binds covalently to the serine amino acid residue within the active center of CatG. Herein, MARS116 was conjugated to 5(6)-carboxyfluorescein (FAM) in order to limit non-specific signal-to-noise ratio generally resulting from binding of fluorescein-labelled avidin (avidin-FAM) to biotinylated, intracellular proteins; since MARS116-Bt is incubated with avidin-FAM in a second labelling step. MARS116-FAM was capable to detect intracellular CatG activity, in contrast to the control compound MARS116*-FAM which lacks the functional phosphonate warhead crucial for binding to the active-site of CatG and contains a carboxyl group instead. Furthermore, intracellular CatG activity was determined in CD4⁺ T cells, CD8⁺ T cells as well as in T regulatory cell (Treg) subsets. Thus, MARS116-FAM is a convenient activity-based probe to detect intracellular CatG activity in a flow cytometry approach.