Autonomous Synthesis of Functional, Permanently Phosphorylated Proteins for Defining the Interactome of Monomeric 14-3-3 zeta

14-3-3 proteins are dimeric hubs that bind hundreds of phosphorylated "clients" to regulate their function. Installing stable, functional mimics of phosphorylated amino acids into proteins offers a powerful strategy to study 14-3-3 function in cellular-like environments, but a previous genetic code expansion (GCE) system to translationally install nonhydrolyzable phosphoserine (nhpSer), with the.-oxygen replaced with CH2, site-specifically into proteins has seen limited usage. Here, we achieve a 40-fold improvement in this system by engineering into Escherichia coli a six-step biosynthetic pathway that produces nhpSer from phosphoenolpyruvate. Using this autonomous "PermaPhos" expression system, we produce three biologically relevant proteins with nhpSer and confirm that nhpSer mimics the effects of phosphoserine for activating GSK3 beta phosphorylation of the SARS-CoV-2 nucleocapsid protein, promoting 14-3-3/client complexation, and monomerizing 14-3-3 dimers. Then, to understand the biological function of these phosphorylated 14-3-3 zeta monomers (containing nhpSer at Ser58), we isolate its interactome from HEK293T lysates and compare it with that of wild-type 14-3-3 zeta. These data identify two new subsets of 14-3-3 client proteins: (i) those that selectively bind dimeric 14-3-3 zeta and (ii) those that selectively bind monomeric 14-3-3 zeta. We discover that monomeric-but not dimeric-14-3-3 zeta interacts with cereblon, an E3 ubiquitin-ligase adaptor protein of pharmacological interest.