Technologies that convert transient protein-protein interactions (PPIs) into stable expression of a reporter gene are useful for genetic selections, high-throughput screening, and multiplexing with omics technologies. We previously reported SPARK (Kim et al., 2017), a transcription factor that is activated by the coincidence of blue light and a PPI. Here, we report an improved, second-generation SPARK2 that incorporates a luciferase moiety to control the light-sensitive LOV domain. SPARK2 can be temporally gated by either external light or addition of a small-molecule luciferin, which causes luciferase to open LOV via proximity-dependent BRET. Furthermore, the nested 'AND' gate design of SPARK2-in which both protease recruitment to the membrane-anchored transcription factor and LOV domain opening are regulated by the PPI of interest-yields a lower-background system and improved PPI specificity. We apply SPARK2 to high-throughput screening for GPCR agonists and for the detection of trans-cellular contacts, all with versatile transcriptional readout.

A key step in nutrient sensing is activation of the master growth regulator, mTORC1 kinase, on the lysosomal membrane. Nutrients enable mTORC1 scaffolding by a complex composed of the Rag GTPases (Rags) and Ragulator, but the underlying mechanism of mTORC1 capture is poorly understood. Combining dynamic imaging in cells and reconstituted systems, we uncover an affinity switch that controls mTORC1 lifetime and activation at the lysosome. Nutrients destabilize the Rag-Ragulator interface, causing cycling of the Rags between lysosome-bound Ragulator and the cytoplasm, and rendering mTORC1 capture contingent on simultaneous engagement of two Rag-binding interfaces. Rag GTPase domains trigger cycling by coordinately weakening binding of the C-terminal domains to Ragulator in a nucleotide-controlled manner. Cancer-specific Rag mutants override release from Ragulator and enhance mTORC1 recruitment and signalling output. Cycling in the active state sets the Rags apart from most signalling GTPases, and provides a mechanism to attenuate mTORC1 signalling.