Engineered cell-based therapies comprise a promising emerging strategy for treating diverse diseases. Realizing this promise requires new tools for engineering cells to sense and respond to soluble extracellular factors, which provide information about both physiological state and the local environment. Here, we report such a biosensor engineering strategy, leveraging a self-contained receptor-signal transduction system termed modular extracellular sensor architecture (MESA). We developed MESA receptors that enable cells to sense vascular endothelial growth factor (VEGF) and, in response, secrete interleukin 2 (IL-2). By implementing these receptors in human T cells, we created a customized function not observed in nature-an immune cell that responds to a normally immunosuppressive cue (VEGF) by producing an immunostimulatory factor (IL-2). Because this platform utilizes modular, engineerable domains for ligand binding (antibodies) and output (programmable transcription factors based upon Cas9), this approach may be readily extended to novel inputs and outputs. This generalizable approach for rewiring cellular functions could enable both translational applications and fundamental biological research.

Splitting bioactive proteins into conditionally reconstituting fragments is a powerful strategy for building tools to study and control biological systems. However, split proteins often exhibit a high propensity to reconstitute even without the conditional trigger, limiting their utility. Current approaches for tuning reconstitution propensity are laborious, context-specific, or often ineffective. Here, we report a computational design strategy grounded in fundamental protein biophysics to guide experimental evaluation of a sparse set of mutants to identify an optimal functional window. We hypothesized that testing a limited set of mutants would direct subsequent mutagenesis efforts by predicting desirable mutant combinations from a vast mutational landscape. This strategy varies the degree of interfacial destabilization while preserving stability and catalytic activity. We validate our method by solving two distinct split protein design challenges, generating both design and mechanistic insights. This new technology will streamline the generation and use of split protein systems for diverse applications.

Synthetic receptors are powerful tools for engineering mammalian cell-based devices. These biosensors enable cell-based therapies to perform complex tasks such as regulating therapeutic gene expression in response to sensing physiological cues. Although multiple synthetic receptor systems now exist, many aspects of receptor performance are poorly understood. In general, it would be useful to understand how receptor design choices influence performance characteristics. In this study, we examined the modular extracellular sensor architecture (MESA) and systematically evaluated previously unexamined design choices, yielding substantially improved receptors. A key finding that might extend to other receptor systems is that the choice of transmembrane domain (TMD) is important for generating high-performing receptors. To provide mechanistic insights, we adopted and employed a Förster resonance energy transfer (FRET)-based assay to elucidate how TMDs affect receptor complex formation and connected these observations to functional performance. To build further insight into these phenomena, we developed a library of new MESA receptors that sense an expanded set of ligands. Based upon these explorations, we conclude that TMDs affect signaling primarily by modulating intracellular domain geometry. Finally, to guide the design of future receptors, we propose general principles for linking design choices to biophysical mechanisms and performance characteristics.

25Splitting bioactive proteins, such as enzymes or fluorescent reporters, into conditionally reconstituting 26 fragments is a powerful strategy for building tools to study and control biochemical systems. However, split 27 proteins often exhibit a high propensity to reconstitute even in the absence of the conditional trigger, which 28 limits their utility. Current approaches for tuning reconstitution propensity are laborious, context-specific, or 29 often ineffective. Here, we report a computational design-driven strategy that is grounded in fundamental 30 protein biophysics and which guides the experimental evaluation of a focused, sparse set of mutants-31 which vary in the degree of interfacial destabilization while preserving features such as stability and catalytic 32 activity-to identify an optimal functional window. We validate our method by solving two distinct split 33 protein design challenges, generating both broad insights and new technology platforms. This method will 34 streamline the generation and use of split protein systems for diverse applications. 35 36 KEYWORDS: synthetic biology, split proteins, computational protein design, protein engineering 37 38

Synthetic receptors are powerful tools for engineering mammalian cell-based devices. These biosensors confer unique capabilities for detecting environmental ligands and transducing signals to control downstream gene expression events such as therapeutic protein production. For many applications, it would be useful to understand how receptor design choices impart desirable performance metrics and trade-offs. Towards this goal, we employed the existing modular extracellular sensor architecture (MESA) and systematically characterized biosensors with previously unexamined protein domain choices. A key finding that might extend to other receptor systems is that choice of transmembrane domain (TMD) is highly consequential. To provide mechanistic insights, we adopted and employed a FRET-based assay to elucidate how TMDs affect receptor complex formation and connected these observations to functional performance. To build further insight into these phenomena, we developed a library of new MESA receptors that sense an expanded set of ligands. Based upon these explorations, we conclude that TMDs affect signaling primarily by modulating intracellular domain geometry, and we apply this understanding to rationally tune receptors. Finally, to guide the design of future receptors, we propose general principles for linking design choices to biophysical mechanisms and performance characteristics.