Next generation approaches toward engineering therapeutic proteases
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Engineering protease substrate specificity and selectivity has the potential to yield entirely new possibilities in the analytical, biotechnological, and therapeutic domains. For example, therapeutic applications can be envisioned in which engineered proteases could replace antibodies by irreversibly inactivating a large excess of disease-associated target proteins in a catalytic fashion. Technological advances in molecular biology have made laboratory-based evolution techniques for protein engineering readily accessible. However, the ability to interrogate the activities and substrate preference of large numbers of protease variants is predicated on the availability of quantitative high-throughput assays that maintain the essential link between genotype and phenotype. In this work we have investigated a variety of novel single cell fluorescence assays and selections for engineering protease substrate specificity and selectivity, and demonstrated the utility of some of these systems for the engineering of novel enzymes. The second chapter of this dissertation reports the isolation of a highly active ([chemical formula]) variant of the Escherichia coli endopeptidase OmpT that selectively hydrolyzes peptides after 3-nitrotyrosine while effectively discriminating against similar peptides containing unmodified tyrosine, sulfotyrosine, phosphotyrosine and phosphoserine. The isolation of protease variants that can discriminate between substrates based on the posttranslational modification of Tyr was made possible by implementing a multi-color flow cytometric assay using multiple simultaneous counter-selection substrates for the screening of large mutant libraries. While primary sequence recognition may suffice for proteomic applications, many therapeutic applications of engineered proteases will require the cleavage of folded protein targets. Unfortunately, we have found that engineered proteases that can cleave peptides very efficiently are often unable to digest the same sequences inserted into the loop regions of a folded protein. The logical conclusion, then, is that an entire target protein or at least a protein domain, rather than peptide segments, must be incorporated into protease engineering screening assays. As a critical first step toward the development of next generation, single cell screening systems for therapeutic protease engineering we have developed novel assays that exploit cell surface capture of exogenous protein substrates. One assay (Chapter 3) relies on an autoinhibited protein fusion that capitalizes on the p53 antagonist MDM2 as a detector of protease activity in addition to its utility as a counter-selection substrate. Using this system we successfully isolated OmpT variants that selectively cleave a designed site within our autoinhibited substrate. A second high-throughput screen (Chapter 4) monitors native protein cleavage. Target proteins are captured at the cell surface using a polycationic tail, incorporating counter-selection, and the proteolytic state of the substrate can be monitored using epitope tags fused to the N-and C-termini and fluorescently labeled anti-epitope tag antibodies.