Showing 1 - 25 of 28 results
Composition dependent phase separation underlies directional flux through the nucleolus.
Intracellular bodies such as nucleoli, Cajal bodies, and various signaling assemblies, represent membraneless organelles, or condensates, that form via liquid-liquid phase separation (LLPS)1,2. Biomolecular interactions, particularly homotypic interactions mediated by self-associating intrinsically disordered protein regions (IDRs), are thought to underlie the thermodynamic driving forces for LLPS, forming condensates that can facilitate the assembly and processing of biochemically active complexes, such as ribosomal subunits within the nucleolus. Simplified model systems3–6 have led to the concept that a single fixed saturation concentration (Csat) is a defining feature of endogenous LLPS7–9, and has been suggested as a mechanism for intracellular concentration buffering2,7,8,10. However, the assumption of a fixed Csat remains largely untested within living cells, where the richly multicomponent nature of condensates could complicate this simple picture. Here we show that heterotypic multicomponent interactions dominate endogenous LLPS, and give rise to nucleoli and other condensates that do not exhibit a fixed Csat. As the concentration of individual components is varied, their partition coefficients change, in a manner that can be used to extract thermodynamic interaction energies, that we interpret within a framework we term polyphasic interaction thermodynamic analysis (PITA). We find that heterotypic interactions between protein and RNA components stabilize a variety of archetypal intracellular condensates, including the nucleolus, Cajal bodies, stress granules, and P bodies. These findings imply that the composition of condensates is finely tuned by the thermodynamics of the underlying biomolecular interaction network. In the context of RNA processing condensates such as the nucleolus, this stoichiometric self-tuning manifests in selective exclusion of fully-assembled RNP complexes, providing a thermodynamic basis for vectorial ribosomal RNA (rRNA) flux out of the nucleolus. The PITA methodology is conceptually straightforward and readily implemented, and it can be broadly utilized to extract thermodynamic parameters from microscopy images. These approaches pave the way for a deep understanding of the thermodynamics of multi-component intracellular phase behavior and its interplay with nonequilibrium activity characteristic of endogenous condensates.
Flotillins promote T cell receptor sorting through a fast Rab5-Rab11 endocytic recycling axis.
The targeted endocytic recycling of the T cell receptor (TCR) to the immunological synapse is essential for T cell activation. Despite this, the mechanisms that underlie the sorting of internalised receptors into recycling endosomes remain poorly understood. To build a comprehensive picture of TCR recycling during T cell activation, we developed a suite of new imaging and quantification tools centred on photoactivation of fluorescent proteins. We show that the membrane-organising proteins, flotillin-1 and -2, are required for TCR to reach Rab5-positive endosomes immediately after endocytosis and for transfer from Rab5- to Rab11a-positive compartments. We further observe that after sorting into in Rab11a-positive vesicles, TCR recycles to the plasma membrane independent of flotillin expression. Our data suggest a mechanism whereby flotillins delineate a fast Rab5-Rab11a endocytic recycling axis and functionally contribute to regulate the spatial organisation of these endosomes.
Nucleated transcriptional condensates amplify gene expression.
Liquid-liquid phase separation is thought to underly gene transcription, through the condensation of the large-scale nucleolus, or in smaller assemblies known as transcriptional hubs or condensates. However, phase separation has not yet been directly linked with transcriptional output, and our biophysical understanding of transcription dynamics is poor. Here, we utilize an optogenetic approach to control condensation of key FET-family transcriptional regulators, particularly TAF15. We show that amino acid sequence-dependent phase separation of TAF15 is enhanced significantly due to strong nuclear interactions with the C-terminal domain (CTD) of RNA Pol II. Nascent CTD clusters at primed genomic loci lower the energetic barrier for nucleation of TAF15 condensates, which in turn further recruit RNA Pol II to drive transcriptional output. These results suggest a model in which positive feedback between key transcriptional components drives intermittent dynamics of localized phase separation, to amplify gene expression.
Degradation of integral membrane proteins modified with the photosensitive degron module requires the cytosolic endoplasmic reticulum-associated degradation pathway.
Protein quality mechanisms are fundamental for proteostasis of eukaryotic cells. Endoplasmic reticulum-associated degradation (ERAD) is a well-studied pathway that ensures quality control of secretory and endoplasmic reticulum (ER)-resident proteins. Different branches of ERAD are involved in degradation of malfolded secretory proteins, depending on the localization of the misfolded part, the ER lumen (ERAD-L), the ER membrane (ERAD-M), and the cytosol (ERAD-C). Here we report that modification of several ER transmembrane proteins with the photosensitive degron (psd) module resulted in light-dependent degradation of the membrane proteins via the ERAD-C pathway. We found dependency on the ubiquitylation machinery including the ubiquitin-activating enzyme Uba1, the ubiquitin--conjugating enzymes Ubc6 and Ubc7, and the ubiquitin-protein ligase Doa10. Moreover, we found involvement of the Cdc48 AAA-ATPase complex members Ufd1 and Npl4, as well as the proteasome, in degradation of Sec62-myc-psd. Thus, our work shows that ERAD-C substrates can be systematically generated via synthetic degron constructs, which facilitates future investigations of the ERAD-C pathway.
Controlling the material properties and rRNA processing function of the nucleolus using light.
The nucleolus is a prominent nuclear condensate that plays a central role in ribosome biogenesis by facilitating the transcription and processing of nascent ribosomal RNA (rRNA). A number of studies have highlighted the active viscoelastic nature of the nucleolus, whose material properties and phase behavior are a consequence of underlying molecular interactions. However, the ways in which the material properties of the nucleolus impact its function in rRNA biogenesis are not understood. Here we utilize the Cry2olig optogenetic system to modulate the viscoelastic properties of the nucleolus. We show that above a threshold concentration of Cry2olig protein, the nucleolus can be gelled into a tightly linked, low mobility meshwork. Gelled nucleoli no longer coalesce and relax into spheres but nonetheless permit continued internal molecular mobility of small proteins. These changes in nucleolar material properties manifest in specific alterations in rRNA processing steps, including a buildup of larger rRNA precursors and a depletion of smaller rRNA precursors. We propose that the flux of processed rRNA may be actively tuned by the cell through modulating nucleolar material properties, which suggests the potential of materials-based approaches for therapeutic intervention in ribosomopathies.
ESCRT-mediated phagophore sealing during mitophagy.
Inactivation of the endosomal sorting complex required for transport (ESCRT) machinery has been reported to cause autophagic defects, but the exact functions of ESCRT proteins in macroautophagy/autophagy remain incompletely understood. Using live-cell fluorescence microscopy we found that the filament-forming ESCRT-III subunit CHMP4B was recruited transiently to nascent autophagosomes during starvation-induced autophagy and mitophagy, with residence times of about 1 and 2 min, respectively. Correlative light microscopy and electron tomography revealed CHMP4B recruitment at a late step in mitophagosome formation. The autophagosomal dwell time of CHMP4B was strongly increased by depletion of the regulatory ESCRT-III subunit CHMP2A. Using a novel optogenetic closure assay we observed that depletion of CHMP2A inhibited phagophore sealing during mitophagy. Consistent with this, depletion of CHMP2A and other ESCRT-III subunits inhibited both PRKN/PARKIN-dependent and -independent mitophagy. We conclude that the ESCRT machinery mediates phagophore closure, and that this is essential for mitophagic flux. Abbreviations: BSA: bovine serum albumin; CHMP: chromatin-modifying protein; CLEM: correlative light and electron microscopy; EGFP: enhanced green fluorescent protein; ESCRT: endosomal sorting complex required for transport; HEPES: 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid; HRP: horseradish peroxidase; ILV: intralumenal vesicle; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; LOV2: light oxygen voltage 2; MLS: mitochondrial localization sequence; MT-CO2: mitochondrially encoded cytochrome c oxidase II; O+A: oligomycin and antimycin A; PBS: phosphate-buffered saline; PIPES: piperazine-N,N-bis(2-ethanesulfonic acid); PRKN/PARKIN: parkin RBR E3 ubiquitin protein ligase; RAB: RAS-related in brain; SD: standard deviation; SEM: standard error of the mean; TOMM20: TOMM20: translocase of outer mitochondrial membrane 20; VCL: vinculin; VPS4: vacuolar protein sorting protein 4; Zdk1: Zdark 1; TUBG: Tubulin gamma chain.
Phase separation of 53BP1 determines liquid-like behavior of DNA repair compartments.
The DNA damage response (DDR) generates transient repair compartments to concentrate repair proteins and activate signaling factors. The physicochemical properties of these spatially confined compartments and their function remain poorly understood. Here, we establish, based on live cell microscopy and CRISPR/Cas9-mediated endogenous protein tagging, that 53BP1-marked repair compartments are dynamic, show droplet-like behavior, and undergo frequent fusion and fission events. 53BP1 assembly, but not the upstream accumulation of γH2AX and MDC1, is highly sensitive to changes in osmotic pressure, temperature, salt concentration and to disruption of hydrophobic interactions. Phase separation of 53BP1 is substantiated by optoDroplet experiments, which further allowed dissection of the 53BP1 sequence elements that cooperate for light-induced clustering. Moreover, we found the tumor suppressor protein p53 to be enriched within 53BP1 optoDroplets, and conditions that disrupt 53BP1 phase separation impair 53BP1-dependent induction of p53 and diminish p53 target gene expression. We thus suggest that 53BP1 phase separation integrates localized DNA damage recognition and repair factor assembly with global p53-dependent gene activation and cell fate decisions.
Nuclear actin regulates inducible transcription by enhancing RNA polymerase II clustering.
Gene expression in response to external stimuli underlies a variety of fundamental cellular processes. However, how the transcription machinery is regulated under these scenarios is largely unknown. Here, we discover a novel role of nuclear actin in inducible transcriptional regulation using next-generation transcriptome sequencing and super-resolution microscopy. The RNA-seq data reveal that nuclear actin is required for the establishment of the serum-induced transcriptional program. Using super-resolution imaging, we found a remarkable enhancement of RNA polymerase II (Pol II) clustering upon serum stimulation and this enhancement requires the presence of nuclear actin. To study the molecular mechanisms, we firstly observed that Pol II clusters co-localized with the serum-response genes and nuclear actin polymerized in adjacent to Pol II clusters upon serum stimulation. Furthermore, N-WASP and Arp2/3 are reported to interact with Pol II, and we demonstrated N-WASP is required for serum-enhanced Pol II clustering. Importantly, using an optogenetic tool, we revealed that N-WASP phase-separated with the carboxy-terminal domain of Pol II and nuclear actin. In addition to serum stimulation, we found nuclear actin also essential in enhancing Pol II clustering upon interferon-γ treatment. Taken together, our work unveils nuclear actin promotes the formation of transcription factory on inducible genes, acting as a general mechanism underlying the rapid response to environmental cues.
The Glycolytic Protein Phosphofructokinase Dynamically Relocalizes into Subcellular Compartments with Liquid-like Properties in vivo.
While much is known about the biochemical regulation of glycolytic enzymes, less is understood about how they are organized inside cells. Here we built a hybrid microfluidic-hydrogel device for use in Caenorhabditis elegans to systematically examine and quantify the dynamic subcellular localization of the rate-limiting enzyme of glycolysis, phosphofructokinase-1/PFK-1.1. We determine that endogenous PFK-1.1 localizes to distinct, tissue-specific subcellular compartments in vivo. In neurons, PFK-1.1 is diffusely localized in the cytosol, but capable of dynamically forming phase-separated condensates near synapses in response to energy stress from transient hypoxia. Restoring animals to normoxic conditions results in the dispersion of PFK-1.1 in the cytosol, indicating that PFK-1.1 reversibly organizes into biomolecular condensates in response to cues within the cellular environment. PFK-1.1 condensates exhibit liquid-like properties, including spheroid shapes due to surface tension, fluidity due to deformations, and fast internal molecular rearrangements. Prolonged conditions of energy stress during sustained hypoxia alter the biophysical properties of PFK-1.1 in vivo, affecting its viscosity and mobility within phase-separated condensates. PFK-1.1’s ability to form tetramers is critical for its capacity to form condensates in vivo, and heterologous self-association domain such as cryptochrome 2 (CRY2) is sufficient to constitutively induce the formation of PFK-1.1 condensates. PFK-1.1 condensates do not correspond to stress granules and might represent novel metabolic subcompartments. Our studies indicate that glycolytic protein PFK-1.1 can dynamically compartmentalize in vivo to specific subcellular compartments in response to acute energy stress via multivalency as phase-separated condensates.
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.
Self-Organized Nuclear Positioning Synchronizes the Cell Cycle in Drosophila Embryos.
The synchronous cleavage divisions of early embryogenesis require coordination of the cell-cycle oscillator, the dynamics of the cytoskeleton, and the cytoplasm. Yet, it remains unclear how spatially restricted biochemical signals are integrated with physical properties of the embryo to generate collective dynamics. Here, we show that synchronization of the cell cycle in Drosophila embryos requires accurate nuclear positioning, which is regulated by the cell-cycle oscillator through cortical contractility and cytoplasmic flows. We demonstrate that biochemical oscillations are initiated by local Cdk1 inactivation and spread through the activity of phosphatase PP1 to generate cortical myosin II gradients. These gradients cause cortical and cytoplasmic flows that control proper nuclear positioning. Perturbations of PP1 activity and optogenetic manipulations of cortical actomyosin disrupt nuclear spreading, resulting in loss of cell-cycle synchrony. We conclude that mitotic synchrony is established by a self-organized mechanism that integrates the cell-cycle oscillator and embryo mechanics.
Reversible induction of mitophagy by an optogenetic bimodular system.
Autophagy-mediated degradation of mitochondria (mitophagy) is a key process in cellular quality control. Although mitophagy impairment is involved in several patho-physiological conditions, valuable methods to induce mitophagy with low toxicity in vivo are still lacking. Herein, we describe a new optogenetic tool to stimulate mitophagy, based on light-dependent recruitment of pro-autophagy protein AMBRA1 to mitochondrial surface. Upon illumination, AMBRA1-RFP-sspB is efficiently relocated from the cytosol to mitochondria, where it reversibly mediates mito-aggresome formation and reduction of mitochondrial mass. Finally, as a proof of concept of the biomedical relevance of this method, we induced mitophagy in an in vitro model of neurotoxicity, fully preventing cell death, as well as in human T lymphocytes and in zebrafish in vivo. Given the unique features of this tool, we think it may turn out to be very useful for a wide range of both therapeutic and research applications.
Cortical mitochondria regulate insulin secretion by local Ca2+ buffering.
Mitochondria play an essential role in regulating insulin secretion from beta cells by providing ATP needed for the membrane depolarization that results in voltage-dependent Ca2+ influx and subsequent insulin granule exocytosis. Ca2+, in turn, is also rapidly taken up by the mitochondria and exerts important feedback regulation of metabolism. The aim of this study was to determine if the distribution of mitochondria within beta cells is important for the secretory capacity of these cells. We find that cortically localized mitochondria are abundant in beta cells, and that these mitochondria redistribute towards the cell interior following depolarization. The redistribution requires Ca2+-induced remodeling of the cortical F-actin network. Using light-regulated motor proteins, we increased the cortical density of mitochondria 2-fold and found that this blunted the voltage-dependent increase in cytosolic Ca2+ concentration and suppressed insulin secretion. The activity-dependent changes in mitochondria distribution are likely important for the generation of Ca2+ microdomains required for efficient insulin granule release.
Chronic optogenetic induction of stress granules is cytotoxic and reveals the evolution of ALS-FTD pathology.
Stress granules (SGs) are non-membrane-bound RNA-protein granules that assemble through phase separation in response to cellular stress. Disturbances in SG dynamics have been implicated as a primary driver of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), suggesting the hypothesis that these diseases reflect an underlying disturbance in the dynamics and material properties of SGs. However, this concept has remained largely untestable in available models of SG assembly, which require the confounding variable of exogenous stressors. Here we introduce a light-inducible SG system, termed OptoGranules, based on optogenetic multimerization of G3BP1, which is an essential scaffold protein for SG assembly. In this system, which permits experimental control of SGs in living cells in the absence of exogenous stressors, we demonstrate that persistent or repetitive assembly of SGs is cytotoxic and is accompanied by the evolution of SGs to cytoplasmic inclusions that recapitulate the pathology of ALS-FTD.
RNA Binding Antagonizes Neurotoxic Phase Transitions of TDP-43.
TDP-43 proteinopathy is a pathological hallmark of amyotrophic lateral sclerosis and frontotemporal dementia where cytoplasmic TDP-43 inclusions are observed within degenerating regions of patient postmortem tissue. The mechanism by which TDP-43 aggregates has remained elusive due to technological limitations, which prevent the analysis of specific TDP-43 interactions in live cells. We present an optogenetic approach to reliably induce TDP-43 proteinopathy under spatiotemporal control. We show that the formation of pathologically relevant inclusions is driven by aberrant interactions between low-complexity domains of TDP-43 that are antagonized by RNA binding. Although stress granules are hypothesized to be a conduit for seeding TDP-43 proteinopathy, we demonstrate pathological inclusions outside these RNA-rich structures. Furthermore, we show that aberrant phase transitions of cytoplasmic TDP-43 are neurotoxic and that treatment with oligonucleotides composed of TDP-43 target sequences prevent inclusions and rescue neurotoxicity. Collectively, these studies provide insight into the mechanisms that underlie TDP-43 proteinopathy and present a potential avenue for therapeutic intervention.
Mapping Local and Global Liquid Phase Behavior in Living Cells Using Photo-Oligomerizable Seeds.
Liquid-liquid phase separation plays a key role in the
assembly of diverse intracellular structures. However,
the biophysical principles by which phase separation
can be precisely localized within subregions
of the cell are still largely unclear, particularly for
low-abundance proteins. Here, we introduce an oligomerizing
biomimetic system, ‘‘Corelets,’’ and utilize
its rapid and quantitative light-controlled
tunability to map full intracellular phase diagrams,
which dictate the concentrations at which phase
separation occurs and the transition mechanism, in
a protein sequence dependent manner. Surprisingly,
both experiments and simulations show that while
intracellular concentrations may be insufficient for
global phase separation, sequestering protein ligands
to slowly diffusing nucleation centers can
move the cell into a different region of the phase diagram,
resulting in localized phase separation. This
diffusive capture mechanism liberates the cell from
the constraints of global protein abundance and is
likely exploited to pattern condensates associated
with diverse biological processes.
Liquid Nuclear Condensates Mechanically Sense and Restructure the Genome.
Phase transitions involving biomolecular liquids are a
fundamental mechanism underlying intracellular organization.
In the cell nucleus, liquid-liquid phase
separation of intrinsically disordered proteins (IDPs)
is implicated in assembly of the nucleolus, as well
as transcriptional clusters, and other nuclear bodies.
However, it remains unclear whether and how physical
forces associated with nucleation, growth, and
wetting of liquid condensates can directly restructure
chromatin. Here, we use CasDrop, a novel
CRISPR-Cas9-based optogenetic technology, to
show that various IDPs phase separate into liquid
condensates that mechanically exclude chromatin
as they grow and preferentially form in low-density,
largely euchromatic regions. A minimal physical
model explains how this stiffness sensitivity arises
from lower mechanical energy associated with deforming
softer genomic regions. Targeted genomic
loci can nonetheless be mechanically pulled together
through surface tension-driven coalescence. Nuclear
condensates may thus function as mechanoactive
chromatin filters, physically pulling in targeted
genomic loci while pushing out non-targeted regions
of the neighboring genome.
Engaging myosin VI tunes motility, morphology, and identity in endocytosis.
While unconventional myosins interact with different stages of the endocytic pathway, they are ascribed a transport function that is secondary to the protein complexes that control organelle identity. Endosomes are subject to a dynamic, continuous flux of proteins that control their characteristic properties, including their motility within the cell. Efforts to describe the changes in identity of this compartment have largely focused on the adaptors present on the compartment and not on the motile properties of the compartment itself. In this study, we use a combination of optogenetic and chemical-dimerization strategies to target exogenous myosin VI to early endosomes, and probe its influence on organelle motility, morphology, and identity. Our analysis across time scales suggests a model wherein the artificial engagement of myosin VI motility on early endosomes restricts microtubule-based motion, followed by morphological changes characterized by the rapid condensation and disintegration of organelles, ultimately leading to the enhanced overlap of markers that demarcate endosomal compartments. Together, our findings show that synthetic engagement of myosin VI motility is sufficient to alter organelle homeostasis in the endocytic pathway. This article is protected by copyright. All rights reserved.
Protein Phase Separation Provides Long-Term Memory of Transient Spatial Stimuli.
Protein/RNA clusters arise frequently in spatially regulated biological processes, from the asymmetric distribution of P granules and PAR proteins in developing embryos to localized receptor oligomers in migratory cells. This co-occurrence suggests that protein clusters might possess intrinsic properties that make them a useful substrate for spatial regulation. Here, we demonstrate that protein droplets show a robust form of spatial memory, maintaining the spatial pattern of an inhibitor of droplet formation long after it has been removed. Despite this persistence, droplets can be highly dynamic, continuously exchanging monomers with the diffuse phase. We investigate the principles of biophysical spatial memory in three contexts: a computational model of phase separation; a novel optogenetic system where light can drive rapid, localized dissociation of liquid-like protein droplets; and membrane-localized signal transduction from clusters of receptor tyrosine kinases. Our results suggest that the persistent polarization underlying many cellular and developmental processes could arise through a simple biophysical process, without any additional biochemical feedback loops.
Optogenetic Control of Endoplasmic Reticulum-Mitochondria Tethering.
The organelle interface emerges as a dynamic platform for a variety of biological responses. However, their study has been limited by the lack of tools to manipulate their occurrence in live cells spatiotemporally. Here, we report the development of a genetically encoded light-inducible tethering (LIT) system allowing the induction of contacts between endoplasmic reticulum (ER) and mitochondria, taking advantage of a pair of light-dependent heterodimerization called an iLID system. We demonstrate that the iLID-based LIT approach enables control of ER-mitochondria tethering with high spatiotemporal precision in various cell types including primary neurons, which will facilitate the functional study of ER-mitochondrial contacts.
Intracellular production of hydrogels and synthetic RNA granules by multivalent molecular interactions.
Some protein components of intracellular non-membrane-bound entities, such as RNA granules, are known to form hydrogels in vitro. The physico-chemical properties and functional role of these intracellular hydrogels are difficult to study, primarily due to technical challenges in probing these materials in situ. Here, we present iPOLYMER, a strategy for a rapid induction of protein-based hydrogels inside living cells that explores the chemically inducible dimerization paradigm. Biochemical and biophysical characterizations aided by computational modelling show that the polymer network formed in the cytosol resembles a physiological hydrogel-like entity that acts as a size-dependent molecular sieve. We functionalize these polymers with RNA-binding motifs that sequester polyadenine-containing nucleotides to synthetically mimic RNA granules. These results show that iPOLYMER can be used to synthetically reconstitute the nucleation of biologically functional entities, including RNA granules in intact cells.
Optical control of membrane tethering and interorganellar communication at nanoscales.
Endoplasmic reticulum (ER) forms an extensive intracellular membranous network in eukaryotes that dynamically connects and communicates with diverse subcellular compartments such as plasma membrane (PM) through membrane contact sites (MCSs), with the inter-membrane gaps separated by a distance of 10-40 nm. Phosphoinositides (PI) constitute an important class of cell membrane phospholipids shared by many MCSs to regulate a myriad of cellular events, including membrane trafficking, calcium homeostasis and lipid metabolism. By installing photosensitivity into a series of engineered PI-binding domains with minimal sizes, we have created an optogenetic toolkit (designated as 'OptoPB') to enable rapid and reversible control of protein translocation and inter-membrane tethering at MCSs. These genetically-encoded, single-component tools can be used as scaffolds for grafting lipid-binding domains to dissect molecular determinants that govern protein-lipid interactions in living cells. Furthermore, we have demonstrated the use of OptoPB as a versatile fusion tag to photomanipulate protein translocation toward PM for reprogramming of PI metabolism. When tethered to the ER membrane with the insertion of flexible spacers, OptoPB can be applied to reversibly photo-tune the gap distances at nanometer scales between the two organellar membranes at MCSs, and to gauge the distance requirement for the free diffusion of protein complexes into MCSs. Our modular optical tools will find broad applications in non-invasive and remote control of protein subcellular localization and interorganellar contact sites that are critical for cell signaling.
A Phytochrome-Derived Photoswitch for Intracellular Transport.
Cells depend on the proper positioning of their organelles, suggesting that active manipulation of organelle positions can be used to explore spatial cell biology and to restore cellular defects caused by organelle misplacement. Recently, blue-light dependent recruitment of specific motors to selected organelles has been shown to alter organelle motility and positioning, but these approaches lack rapid and active reversibility. The light-dependent interaction of phytochrome B with its interacting factors has been shown to function as a photoswitch, dimerizing under red light and dissociating under far-red light. Here we engineer phytochrome domains into photoswitches for intracellular transport that enable the reversible interaction between organelles and motor proteins. Using patterned illumination and live-cell imaging, we demonstrate that this system provides unprecedented spatiotemporal control. We also demonstrate that it can be used in combination with a blue-light dependent system to independently control the positioning of two different organelles. Precise optogenetic control of organelle motility and positioning will provide a better understanding of and control over the spatial biology of cells.
Investigations of human myosin VI targeting using optogenetically controlled cargo loading.
Myosins play countless critical roles in the cell, each requiring it to be activated at a specific location and time. To control myosin VI with this specificity, we created an optogenetic tool for activating myosin VI by fusing the light-sensitive Avena sativa phototropin1 LOV2 domain to a peptide from Dab2 (LOVDab), a myosin VI cargo protein. Our approach harnesses the native targeting and activation mechanism of myosin VI, allowing direct inferences on myosin VI function. LOVDab robustly recruits human full-length myosin VI to various organelles in vivo and hinders peroxisome motion in a light-controllable manner. LOVDab also activates myosin VI in an in vitro gliding filament assay. Our data suggest that protein and lipid cargoes cooperate to activate myosin VI, allowing myosin VI to integrate Ca(2+), lipid, and protein cargo signals in the cell to deploy in a site-specific manner.
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.