We have developed a sensitive and highly efficient whole-cell methodology for quantitative analysis and screening of protease activity in vivo. The method is based on the ability of a genetically encoded protease to rescue a coexpressed short-lived fluorescent substrate reporter from cytoplasmic degradation and thereby confer increased whole-cell fluorescence in proportion to the protease's apparent activity in the Escherichia coli cytoplasm. We demonstrated that this system can reveal differences in the efficiency with which tobacco etch virus (TEV) protease processes different substrate peptides. In addition, when analyzing E. coli cells expressing TEV protease variants that differed in terms of their in vivo solubility, cells containing the most-soluble protease variant exhibited the highest fluorescence intensity. Furthermore, flow cytometry screening allowed for enrichment and subsequent identification of an optimal substrate peptide and protease variant from a large excess of cells expressing suboptimal variants (1:100,000). Two rounds of cell sorting resulted in a 69,000-fold enrichment and a 22,000-fold enrichment of the superior substrate peptide and protease variant, respectively. Our approach presents a new promising path forward for high-throughput substrate profiling of proteases, engineering of novel protease variants with desired properties (e.g., altered substrate specificity and improved solubility and activity), and identification of protease inhibitors.Proteases constitute a group of enzymes that irreversibly catalyze the cleavage of peptide bonds and represent approximately 2% of all protein-encoding genes in living organisms (39). Besides acting as virulence factors for many pathogens (16), proteases are crucial for the regulation of numerous biological processes that influence the life and death of a cell (4). These enzymes also underlie several pathological conditions, such as cancer (13) and neurodegenerative (20) and cardiovascular (8) diseases. A key issue for increasing our knowledge about such complex biological processes, and thereby hopefully also providing possibilities for new therapeutic strategies, is to deduce the proteases' substrate repertoires. Consequently, a lot of efforts around the world are dedicated to the characterization of proteases and their substrates (2, 31). In addition to their biological importance, proteases have attracted much interest in several biotechnological and industrial applications, such as removal of "fusion tags" from recombinant target proteins (38), as supplements in dishwashing and laundry detergents, or for bating of hides and skin in the leather industry (41, 44). Sometimes, however, their use is hindered due to limitations inherent to a specific protease: for example, low solubility, poor enzyme stability and specificity, or limited activity. It would therefore be of great aid to have powerful and straightforward methods available that facilitate the engineering of novel protease variants not suffering from such limitations.Traditionally, pr...