(A) Schematic representation of the SILAC-based quantitative proteomics experiment in B. subtilis. Cells overexpressing active YqgP (BS50, Table EV2) or its catalytically dead mutant YqgP.S288A (BS51), both auxotrophic for lysine, were grown in parallel in "heavy" (containing 13C615N2- Lysine isotope) or "light" (containing stable 12C614N2- Lysine) M9 minimal medium, respectively. After mixing the cell cultures in the 1:1 ratio (based on OD600), cell suspension was lysed and the fraction enriched for transmembrane proteome was analysed in a GeLC-MS/MS experiment. Using bioinformatic analysis of the MS data, highest-scoring candidate substrates were further evaluated. 