Spatiotemporal Control of Intracellular Phase Transitions Using Light-Activated optoDroplets.

Phase transitions driven by intrinsically disordered protein regions (IDRs) have emerged as a ubiquitous mechanism for assembling liquid-like RNA/protein (RNP) bodies and other membrane-less organelles. However, a lack of tools to control intracellular phase transitions limits our ability to understand their role in cell physiology and disease. Here, we introduce an optogenetic platform that uses light to activate IDR-mediated phase transitions in living cells. We use this "optoDroplet" system to study condensed phases driven by the IDRs of various RNP body proteins, including FUS, DDX4, and HNRNPA1. Above a concentration threshold, these constructs undergo light-activated phase separation, forming spatiotemporally definable liquid optoDroplets. FUS optoDroplet assembly is fully reversible even after multiple activation cycles. However, cells driven deep within the phase boundary form solid-like gels that undergo aging into irreversible aggregates. This system can thus elucidate not only physiological phase transitions but also their link to pathological aggregates.

Light-based control of metabolic flux through assembly of synthetic organelles.

To maximize a desired product, metabolic engineers typically express enzymes to high, constant levels. Yet, permanent pathway activation can have undesirable consequences including competition with essential pathways and accumulation of toxic intermediates. Faced with similar challenges, natural metabolic systems compartmentalize enzymes into organelles or post-translationally induce activity under certain conditions. Here we report that optogenetic control can be used to extend compartmentalization and dynamic control to engineered metabolisms in yeast. We describe a suite of optogenetic tools to trigger assembly and disassembly of metabolically active enzyme clusters. Using the deoxyviolacein biosynthesis pathway as a model system, we find that light-switchable clustering can enhance product formation six-fold and product specificity 18-fold by decreasing the concentration of intermediate metabolites and reducing flux through competing pathways. Inducible compartmentalization of enzymes into synthetic organelles can thus be used to control engineered metabolic pathways, limit intermediates and favor the formation of desired products.

Ubiquitin-mediated interaction of p210 BCR/ABL with ß-catenin supports disease progression in a murine model for chronic myelogenous leukemia.

We have identified a ubiquitin-binding domain within the NH2-terminal sequences of p210 BCR/ABL and determined that the binding site co-localizes with the binding site for ß-catenin. The domain does not support the auto- or trans-kinase activity of p210 BCR/ABL or its ability to interact with GRB2 and activate ERK1/2 signaling. Expression of p210 BCR/ABL, but not a ß-catenin-binding mutant, in hematopoietic cells is associated with the accumulation of p-ß-catenin (Tyr654) and increased TCF/LEF-mediated transcription. In a bone marrow transplantation model, the interaction between ß-catenin and p-ß-catenin (Tyr654) is detectable in mice transplanted with p210 BCR/ABL, but not the mutant. Whereas mice transplanted with p210 BCR/ABL exhibit myeloid disease with expansion of monocytes and neutrophils, mice transplanted with the mutant predominantly exhibit expansion of neutrophils, polycythemia, and increased lifespan. The increased disease latency is associated with expansion of megakaryocyte-erythrocyte progenitors, a decrease in common myeloid progenitors, and reduced ß-catenin signaling in the bone marrow of the diseased mice. These observations support a model in which p210 BCR/ABL may influence lineage-specific leukemic expansion by directly binding and phosphorylating ß-catenin and altering its transcriptional activity. They further suggest that the interaction may play a role in chronic phase disease progression.

The Spatiotemporal Limits of Developmental Erk Signaling.

Animal development is characterized by signaling events that occur at precise locations and times within the embryo, but determining when and where such precision is needed for proper embryogenesis has been a long-standing challenge. Here we address this question for extracellular signal regulated kinase (Erk) signaling, a key developmental patterning cue. We describe an optogenetic system for activating Erk with high spatiotemporal precision in vivo. Implementing this system in Drosophila, we find that embryogenesis is remarkably robust to ectopic Erk signaling, except from 1 to 4 hr post-fertilization, when perturbing the spatial extent of Erk pathway activation leads to dramatic disruptions of patterning and morphogenesis. Later in development, the effects of ectopic signaling are buffered, at least in part, by combinatorial mechanisms. Our approach can be used to systematically probe the differential contributions of the Ras/Erk pathway and concurrent signals, leading to a more quantitative understanding of developmental signaling.