Directed Evolution of the BpsA Carrier Protein Domain for Recognition by Non-cognate 4'-Phosphopantetheinyl Transferases to Enable Inhibitor Screening.

4'-Phosphopantetheinyl transferases (PPTases) play an essential role in activating the carrier protein domains of mega-synthases involved in primary and secondary metabolism and have been validated as promising drug targets in multiple pathogens. Monitoring phosphopantetheinylation of the non-ribosomal peptidase synthetase BpsA, which produces blue indigoidine pigment upon activation, is a useful strategy to screen chemical collections for inhibitors of a target PPTase. However, PPTases can exhibit carrier protein specificity and some medically important PPTases do not activate BpsA. Here, we describe how to conduct a directed evolution campaign to evolve the BpsA carrier protein domain for improved recognition by a candidate PPTase, as exemplified for the human Sfp-like PPTase. This method can be applied to other non-cognate PPTases for discovery of new drug candidates or chemical probes, or to enable development of next-generation biosensors that utilize BpsA as a reporter.