Showing 1 - 25 of 146 results
LILAC: Enhanced actin imaging with an optogenetic Lifeact.
We have designed an improved Lifeact variant that binds to actin under the control of light using the LOV2 protein. Light control enables one to subtract the pre-illumination signal of the unbound label, yielding an enhanced view of F-actin dynamics in cells. Furthermore, the tool eliminates actin network perturbations and cell sickness caused by Lifeact overexpression.
Optical control of protein delivery and partitioning in the nucleolus.
The nucleolus is a subnuclear membraneless compartment intimately involved in ribosomal RNA synthesis, ribosome biogenesis and stress response. Multiple optogenetic devices have been developed to manipulate nuclear protein import and export, but molecular tools tailored for remote control over selective targeting or partitioning of cargo proteins into subnuclear compartments capable of phase separation are still limited. Here, we report a set of single-component photoinducible nucleolus-targeting tools, designated pNUTs, to enable rapid and reversible nucleoplasm-to-nucleolus shuttling, with the half-lives ranging from milliseconds to minutes. pNUTs allow both global protein infiltration into nucleoli and local delivery of cargoes into the outermost layer of the nucleolus, the granular component. When coupled with the amyotrophic lateral sclerosis (ALS)-associated C9ORF72 proline/arginine-rich dipeptide repeats, pNUTs allow us to photomanipulate poly-proline-arginine nucleolar localization, perturb nucleolar protein nucleophosmin 1 and suppress nascent protein synthesis. pNUTs thus expand the optogenetic toolbox by permitting light-controllable interrogation of nucleolar functions and precise induction of ALS-associated toxicity in cellular models.
Local temporal Rac1-GTP nadirs and peaks restrict cell protrusions and retractions.
Cells probe their microenvironment using membrane protrusion-retraction cycles. Spatiotemporal coordination of Rac1 and RhoA GTP-binding activities initiates and reinforces protrusions and retractions, but the control of their finite lifetime remains unclear. We examined the relations of Rac1 and RhoA GTP-binding levels to key protrusion and retraction events, as well as to cell-ECM traction forces at physiologically relevant ECM stiffness. High RhoA-GTP preceded retractions and Rac1-GTP elevation before protrusions. Notable temporal Rac1-GTP nadirs and peaks occurred at the maximal edge velocity of local membrane protrusions and retractions, respectively, followed by declined edge velocity. Moreover, altered local Rac1-GTP consistently preceded similarly altered traction force. Local optogenetic Rac1-GTP perturbations defined a function of Rac1 in restricting protrusions and retractions and in promoting local traction force. Together, we show that Rac1 plays a fundamental role in restricting the size and durability of protrusions and retractions, plausibly in part through controlling traction forces.
Optogenetic control of the Bicoid morphogen reveals fast and slow modes of gap gene regulation.
Developmental patterning networks are regulated by multiple inputs and feedback connections that rapidly reshape gene expression, limiting the information that can be gained solely from slow genetic perturbations. Here we show that fast optogenetic stimuli, real-time transcriptional reporters, and a simplified genetic background can be combined to reveal the kinetics of gene expression downstream of a developmental transcription factor in vivo. We engineer light-controlled versions of the Bicoid transcription factor and study their effects on downstream gap genes in embryos. Our results recapitulate known relationships, including rapid Bicoid-dependent transcription of giant and hunchback and delayed repression of Krüppel. In addition, we find that the posterior pattern of knirps exhibits a quick but inverted response to Bicoid perturbation, suggesting a noncanonical role for Bicoid in directly suppressing knirps transcription. Acute modulation of transcription factor concentration while recording output gene activity represents a powerful approach for studying developmental gene networks in vivo.
MYC amplifies gene expression through global changes in transcription factor dynamics.
The MYC oncogene has been studied for decades, yet there is still intense debate over how this transcription factor controls gene expression. Here, we seek to answer these questions with an in vivo readout of discrete events of gene expression in single cells. We engineered an optogenetic variant of MYC (Pi-MYC) and combined this tool with single-molecule RNA and protein imaging techniques to investigate the role of MYC in modulating transcriptional bursting and transcription factor binding dynamics in human cells. We find that the immediate consequence of MYC overexpression is an increase in the duration rather than in the frequency of bursts, a functional role that is different from the majority of human transcription factors. We further propose that the mechanism by which MYC exerts global effects on the active period of genes is by altering the binding dynamics of transcription factors involved in RNA polymerase II complex assembly and productive elongation.
Quantification of nuclear transport inhibition by SARS-CoV-2 ORF6 using a broadly applicable live-cell dose-response pipeline.
SARS coronavirus ORF6 inhibits the classical nuclear import pathway to antagonize host antiviral responses. Several models were proposed to explain its inhibitory function, but quantitative measurement is needed for model evaluation and refinement. We report a broadly applicable live-cell method for calibrated dose-response characterization of the nuclear transport alteration by a protein of interest. Using this method, we found that SARS-CoV-2 ORF6 is ∼5 times more potent than SARS-CoV-1 ORF6 in inhibiting bidirectional nuclear transport, due to differences in the NUP98-binding C-terminal region that is required for the inhibition. The N-terminal region was also required, but its membrane binding function was dispensable, since loss of the inhibitory function due to N-terminal truncation was rescued by forced oligomerization using a soluble construct. Based on these data, we propose that the hydrophobic N-terminal region drives oligomerization of ORF6 to multivalently cross-link the FG domains of NUP98 at the nuclear pore complex.
Analysis of Three Architectures for Controlling PTP1B with Light.
Photosensory domains are powerful tools for placing proteins under optical control, but their integration into light-sensitive chimeras is often challenging. Many designs require structural iterations, and direct comparisons of alternative approaches are rare. This study uses protein tyrosine phosphatase 1B (PTP1B), an influential regulatory enzyme, to compare three architectures for controlling PTPs with light: a protein fusion, an insertion chimera, and a split construct. All three designs permitted optical control of PTP1B activity in vitro (i.e., kinetic assays of purified enzyme) and in mammalian cells; photoresponses measured under both conditions, while different in magnitude, were linearly correlated. The fusion- and insertion-based architectures exhibited the highest dynamic range and maintained native localization patterns in mammalian cells. A single insertion architecture enabled optical control of both PTP1B and TCPTP, but not SHP2, where the analogous chimera was active but not photoswitchable. Findings suggest that PTPs are highly tolerant of domain insertions and support the use of in vitro screens to evaluate different optogenetic designs.
Two-input protein logic gate for computation in living cells.
Advances in protein design have brought us within reach of developing a nanoscale programming language, in which molecules serve as operands and their conformational states function as logic gates with precise input and output behaviors. Combining these nanoscale computing agents into larger molecules and molecular complexes will allow us to write and execute "code". Here, in an important step toward this goal, we report an engineered, single protein design that is allosterically regulated to function as a 'two-input logic OR gate'. Our system is based on chemo- and optogenetic regulation of focal adhesion kinase. In the engineered FAK, all of FAK domain architecture is retained and key intramolecular interactions between the kinase and the FERM domains are externally controlled through a rapamycin-inducible uniRapR module in the kinase domain and a light-inducible LOV2 module in the FERM domain. Orthogonal regulation of protein function was possible using the chemo- and optogenetic switches. We demonstrate that dynamic FAK activation profoundly increased cell multiaxial complexity in the fibrous extracellular matrix microenvironment and decreased cell motility. This work provides proof-of-principle for fine multimodal control of protein function and paves the way for construction of complex nanoscale computing agents.
Spatial and temporal control of expression with light-gated LOV-LexA.
The ability to drive expression of exogenous genes in different tissues and cell types, under control of specific enhancers, has catapulted discovery in biology. While many enhancers drive expression broadly, several genetic tricks have been developed to obtain access to isolated cell types. However, studies of topographically organized neuropiles, such as the optic lobe in fruit flies, have raised the need for a system that can access subsets of cells within a single neuron type, a feat currently dependent on stochastic flip-out methods. To access the same subsets of cells consistently across flies, we developed LOV-LexA, a light-gated expression system based on the bacterial LexA transcription factor and the plant-derived LOV photosensitive domain. Expression of LOV-Lex in larval fat body as well as pupal and adult neurons enables spatial and temporal control of expression of transgenes under LexAop sequences with blue light. The LOV-LexA tool thus provides another layer of intersectional genetics, allowing for light-controlled genetic access to the same subsets of cells within an expression pattern across individual flies.
Requirements for mammalian promoters to decode transcription factor dynamics.
In response to different stimuli many transcription factors (TFs) display different activation dynamics that trigger the expression of specific sets of target genes, suggesting that promoters have a way to decode them. Combining optogenetics, deep learning-based image analysis and mathematical modeling, we find that decoding of TF dynamics occurs only when the coupling between TF binding and transcription pre-initiation complex formation is inefficient and that the ability of a promoter to decode TF dynamics gets amplified by inefficient translation initiation. Furthermore, we propose a theoretical mechanism based on phase separation that would allow a promoter to be activated better by pulsatile than sustained TF signals. These results provide an understanding on how TF dynamics are decoded in mammalian cells, which is important to develop optimal strategies to counteract disease conditions, and suggest ways to achieve multiplexing in synthetic pathways.
Optogenetic control of the Bicoid morphogen reveals fast and slow modes of gap gene regulation.
Developmental patterning networks are regulated by multiple inputs and feedback connections that rapidly reshape gene expression, limiting the information that can be gained solely from slow genetic perturbations. Here we show that fast optogenetic stimuli, real-time transcriptional reporters, and a simplified genetic background can be combined to reveal quantitative regulatory dynamics from a complex genetic network in vivo. We engineer light-controlled variants of the Bicoid transcription factor and study their effects on downstream gap genes in embryos. Our results recapitulate known relationships, including rapid Bicoid-dependent expression of giant and hunchback and delayed repression of Krüppel. In contrast, we find that the posterior pattern of knirps exhibits a quick but inverted response to Bicoid perturbation, suggesting a previously unreported role for Bicoid in suppressing knirps expression. Acute modulation of transcription factor concentration while simultaneously recording output gene activity represents a powerful approach for studying how gene circuit elements are coupled to cell identification and complex body pattern formation in vivo.
Desensitisation of Notch signalling through dynamic adaptation in the nucleus.
During embryonic development, signalling pathways orchestrate organogenesis by controlling tissue-specific gene expression programmes and differentiation. Although the molecular components of many common developmental signalling systems are known, our current understanding of how signalling inputs are translated into gene expression outputs in real-time is limited. Here we employ optogenetics to control the activation of Notch signalling during Drosophila embryogenesis with minute accuracy and follow target gene expression by quantitative live imaging. Light-induced nuclear translocation of the Notch Intracellular Domain (NICD) causes a rapid activation of target mRNA expression. However, target gene transcription gradually decays over time despite continuous photo-activation and nuclear NICD accumulation, indicating dynamic adaptation to the signalling input. Using mathematical modelling and molecular perturbations, we show that this adaptive transcriptional response fits to known motifs capable of generating near-perfect adaptation and can be best explained by state-dependent inactivation at the target cis-regulatory region. Taken together, our results reveal dynamic nuclear adaptation as a novel mechanism controlling Notch signalling output during tissue differentiation.
Mechanical worrying drives cell migration in crowded environments.
Migratory cells navigate through crowded 3D microenvironments in vivo. Amoeboid cells, such as immune cells and some cancer cells, are thought to do so by deforming their bodies to squeeze through tight spaces.1 Yet large populations of nearly spherical amoeboid cells migrate2–4 in microenvironments too dense5,6 to move through without extensive shape deformations. How they do so is unknown. We used high-resolution light-sheet microscopy to visualize metastatic melanoma cells in dense environments, finding that cells maintain a round morphology as they migrate and create a path through which to move via bleb-driven mechanical degradation and subsequent macropinocytosis of extracellular matrix components. Proteolytic degradation of the extracellular matrix via matrix metalloproteinases is not required. Membrane blebs are short-lived relative to the timescale of migration, and thus persistence in their polarization is critical for productive ablation of the extracellular matrix. Interactions between small but long-lived cortical adhesions and collagen at the cell front induce PI-3 Kinase signaling that drive bleb enlargement via branched actin polymerization. Large blebs in turn abrade collagen, creating a feedback between extracellular matrix structure, cell morphology, and cell polarization that results in both path generation and persistent cell movement.
Extremely rapid and reversible optogenetic perturbation of nuclear proteins in living embryos.
Many developmental regulators have complex and context-specific roles in different tissues and stages, making the dissection of their function extremely challenging. As regulatory processes often occur within minutes, perturbation methods that match these dynamics are needed. Here, we present the improved light-inducible nuclear export system (iLEXY), an optogenetic loss-of-function approach that triggers translocation of proteins from the nucleus to the cytoplasm. By introducing a series of mutations, we substantially increased LEXY's efficiency and generated variants with different recovery times. iLEXY enables rapid (t1/2 < 30 s), efficient, and reversible nuclear protein depletion in embryos, and is generalizable to proteins of diverse sizes and functions. Applying iLEXY to the Drosophila master regulator Twist, we phenocopy loss-of-function mutants, precisely map the Twist-sensitive embryonic stages, and investigate the effects of timed Twist depletions. Our results demonstrate the power of iLEXY to dissect the function of pleiotropic factors during embryogenesis with unprecedented temporal precision.
Mechanosensitivity of nucleocytoplasmic transport.
Mechanical force controls fundamental cellular processes in health and disease, and increasing evidence shows that the nucleus both experiences and senses applied forces. Here we show that nuclear forces differentially control both passive and facilitated nucleocytoplasmic transport, setting the rules for the mechanosensitivity of shuttling proteins. We demonstrate that nuclear force increases permeability across nuclear pore complexes, with a dependence on molecular weight that is stronger for passive than facilitated diffusion. Due to this differential effect, force leads to the translocation into or out of the nucleus of cargoes within a given range of molecular weight and affinity for nuclear transport receptors. Further, we show that the mechanosensitivity of several transcriptional regulators can be both explained by this mechanism, and engineered exogenously by introducing appropriate nuclear localization signals. Our work sets a novel framework to understand mechanically induced signalling, with potential general applicability across signalling pathways and pathophysiological scenarios.
Circularly permuted AsLOV2 as an optogenetic module for engineering photoswitchable peptides.
We re-engineered a commonly-used light-sensing protein, AsLOV2, using a circular permutation strategy to allow photoswitchable control of the C-terminus of a peptide. We demonstrate that the circularly permuted AsLOV2 can be used on its own or together with the original AsLOV2 for enhanced caging. In summary, circularly permuted AsLOV2 could expand the engineering capabilities of optogenetic tools.
Reliably Engineering and Controlling Stable Optogenetic Gene Circuits in Mammalian Cells.
Reliable gene expression control in mammalian cells requires tools with high fold change, low noise, and determined input-to-output transfer functions, regardless of the method used. Toward this goal, optogenetic gene expression systems have gained much attention over the past decade for spatiotemporal control of protein levels in mammalian cells. However, most existing circuits controlling light-induced gene expression vary in architecture, are expressed from plasmids, and utilize variable optogenetic equipment, creating a need to explore characterization and standardization of optogenetic components in stable cell lines. Here, the study provides an experimental pipeline of reliable gene circuit construction, integration, and characterization for controlling light-inducible gene expression in mammalian cells, using a negative feedback optogenetic circuit as a case example. The protocols also illustrate how standardizing optogenetic equipment and light regimes can reliably reveal gene circuit features such as gene expression noise and protein expression magnitude. Lastly, this paper may be of use for laboratories unfamiliar with optogenetics who wish to adopt such technology. The pipeline described here should apply for other optogenetic circuits in mammalian cells, allowing for more reliable, detailed characterization and control of gene expression at the transcriptional, proteomic, and ultimately phenotypic level in mammalian cells.
O-GlcNAc modification of nuclear pore complexes accelerates bidirectional transport.
Macromolecular transport across the nuclear envelope depends on facilitated diffusion through nuclear pore complexes (NPCs). The interior of NPCs contains a permeability barrier made of phenylalanine-glycine (FG) repeat domains that selectively facilitates the permeation of cargoes bound to nuclear transport receptors (NTRs). FG-repeat domains in NPCs are a major site of O-linked N-acetylglucosamine (O-GlcNAc) modification, but the functional role of this modification in nucleocytoplasmic transport is unclear. We developed high-throughput assays based on optogenetic probes to quantify the kinetics of nuclear import and export in living human cells. We found that increasing O-GlcNAc modification of the NPC accelerated NTR-facilitated transport of proteins in both directions, and decreasing modification slowed transport. Superresolution imaging revealed strong enrichment of O-GlcNAc at the FG-repeat barrier. O-GlcNAc modification also accelerated passive permeation of a small, inert protein through NPCs. We conclude that O-GlcNAc modification accelerates nucleocytoplasmic transport by enhancing the nonspecific permeability of the FG-repeat barrier, perhaps by steric inhibition of interactions between FG repeats.
SPARK: A Transcriptional Assay for Recording Protein-Protein Interactions in a Defined Time Window.
Protein-protein interactions (PPIs) are ubiquitously involved in cellular processes such as gene expression, enzymatic catalysis, and signal transduction. To study dynamic PPIs, real-time methods such as Förster resonance energy transfer and bioluminescence resonance energy transfer can provide high temporal resolution, but they only allow PPI detection in a limited area at a time and do not permit post-PPI analysis or manipulation of the cells. Integration methods such as the yeast two-hybrid system and split protein systems integrate PPI signals over time and allow subsequent analysis, but they lose information on dynamics. To address some of these limitations, an assay named SPARK (Specific Protein Association tool giving transcriptional Readout with rapid Kinetics) has recently been published. Similar to many existing integrators, SPARK converts PPIs into a transcriptional signal. SPARK, however, also adds blue light as a co-stimulus to achieve temporal gating; SPARK only records PPIs during light stimulation. Here, we describe the procedures for using SPARK assays to study a dynamic PPI of interest, including designing DNA constructs and optimization in HEK293T/17 cell cultures. These protocols are generally applicable to various PPI partners and can be used in different biological contexts. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Designing DNA constructs for SPARK Basic Protocol 2: Performing the SPARK assay in HEK293T/17 cell cultures Support Protocol 1: Lentivirus preparation Support Protocol 2: Immunostaining of SPARK components.
Engineering a Blue Light Inducible SpyTag System (BLISS).
The SpyCatcher/SpyTag protein conjugation system has recently exploded in popularity due to its fast kinetics and high yield under biologically favorable conditions in both in vitro and intracellular settings. The utility of this system could be expanded by introducing the ability to spatially and temporally control the conjugation event. Taking inspiration from photoreceptor proteins in nature, we designed a method to integrate light dependency into the protein conjugation reaction. The light-oxygen-voltage domain 2 of Avena sativa (AsLOV2) undergoes a dramatic conformational change in its c-terminal Jα-helix in response to blue light. By inserting SpyTag into the different locations of the Jα-helix, we created a blue light inducible SpyTag system (BLISS). In this design, the SpyTag is blocked from reacting with the SpyCatcher in the dark, but upon irradiation with blue light, the Jα-helix of the AsLOV2 undocks to expose the SpyTag. We tested several insertion sites and characterized the kinetics. We found three variants with dynamic ranges over 15, which were active within different concentration ranges. These could be tuned using SpyCatcher variants with different reaction kinetics. Further, the reaction could be instantaneously quenched by removing light. We demonstrated the spatial aspect of this light control mechanism through photopatterning of two fluorescent proteins. This system offers opportunities for many other biofabrication and optogenetics applications.
Rab10-Positive Tubular Structures Represent a Novel Endocytic Pathway That Diverges From Canonical Macropinocytosis in RAW264 Macrophages.
Using the optogenetic photo-manipulation of photoactivatable (PA)-Rac1, remarkable cell surface ruffling and the formation of a macropinocytic cup (premacropinosome) could be induced in the region of RAW264 macrophages irradiated with blue light due to the activation of PA-Rac1. However, the completion of macropinosome formation did not occur until Rac1 was deactivated by the removal of the light stimulus. Following PA-Rac1 deactivation, some premacropinosomes closed into intracellular macropinosomes, whereas many others transformed into long Rab10-positive tubules without forming typical macropinosomes. These Rab10-positive tubules moved centripetally towards the perinuclear Golgi region along microtubules. Surprisingly, these Rab10-positive tubules did not contain any endosome/lysosome compartment markers, such as Rab5, Rab7, or LAMP1, suggesting that the Rab10-positive tubules were not part of the degradation pathway for lysosomes. These Rab10-positive tubules were distinct from recycling endosomal compartments, which are labeled with Rab4, Rab11, or SNX1. These findings suggested that these Rab10-positive tubules may be a part of non-degradative endocytic pathway that has never been known. The formation of Rab10-positive tubules from premacropinosomes was also observed in control and phorbol myristate acetate (PMA)-stimulated macrophages, although their frequencies were low. Interestingly, the formation of Rab10-positive premacropinosomes and tubules was not inhibited by phosphoinositide 3-kinase (PI3K) inhibitors, while the classical macropinosome formation requires PI3K activity. Thus, this study provides evidence to support the existence of Rab10-positive tubules as a novel endocytic pathway that diverges from canonical macropinocytosis.
A photo-switchable yeast isocitrate dehydrogenase to control metabolic flux through the citric acid cycle.
For various research questions in metabolism, it is highly desirable to have means available, with which the flux through specific pathways can be perturbed dynamically, in a reversible manner, and at a timescale that is consistent with the fast turnover rates of metabolism. Optogenetics, in principle, offers such possibility. Here, we developed an initial version of a photo-switchable isocitrate dehydrogenase (IDH) aimed at controlling the metabolic flux through the citric acid cycle in budding yeast. By inserting a protein-based light switch (LOV2) into computationally identified active/regulatory-coupled sites of IDH and by using in vivo screening in Saccharomyces cerevisiae, we obtained a number of IDH enzymes whose activity can be switched by light. Subsequent in-vivo characterization and optimization resulted in an initial version of photo-switchable (PS) IDH. While further improvements of the enzyme are necessary, our study demonstrates the efficacy of the overall approach from computational design, via in vivo screening and characterization. It also represents one of the first few examples, where optogenetics were used to control the activity of a metabolic enzyme.
Light-dependent N-end rule-mediated disruption of protein function in Saccharomyces cerevisiae and Drosophila melanogaster.
Here we describe the development and characterization of the photo-N-degron, a peptide tag that can be used in optogenetic studies of protein function in vivo. The photo-N-degron can be expressed as a genetic fusion to the amino termini of other proteins, where it undergoes a blue light-dependent conformational change that exposes a signal for the class of ubiquitin ligases, the N-recognins, which mediate the N-end rule mechanism of proteasomal degradation. We demonstrate that the photo-N-degron can be used to direct light-mediated degradation of proteins in Saccharomyces cerevisiae and Drosophila melanogaster with fine temporal control. In addition, we compare the effectiveness of the photo-N-degron with that of two other light-dependent degrons that have been developed in their abilities to mediate the loss of function of Cactus, a component of the dorsal-ventral patterning system in the Drosophila embryo. We find that like the photo-N-degron, the blue light-inducible degradation (B-LID) domain, a light-activated degron that must be placed at the carboxy terminus of targeted proteins, is also effective in eliciting light-dependent loss of Cactus function, as determined by embryonic dorsal-ventral patterning phenotypes. In contrast, another previously described photosensitive degron (psd), which also must be located at the carboxy terminus of associated proteins, has little effect on Cactus-dependent phenotypes in response to illumination of developing embryos. These and other observations indicate that care must be taken in the selection and application of light-dependent and other inducible degrons for use in studies of protein function in vivo, but importantly demonstrate that N- and C-terminal fusions to the photo-N-degron and the B-LID domain, respectively, support light-dependent degradation in vivo.
Circularly permuted LOV2 as a modular photoswitch for optogenetic engineering.
Plant-based photosensors, such as the light-oxygen-voltage sensing domain 2 (LOV2) from oat phototropin 1, can be modularly wired into cell signaling networks to remotely control protein activity and physiological processes. However, the applicability of LOV2 is hampered by the limited choice of available caging surfaces and its preference to accommodate the effector domains downstream of the C-terminal Jα helix. Here, we engineered a set of LOV2 circular permutants (cpLOV2) with additional caging capabilities, thereby expanding the repertoire of genetically encoded photoswitches to accelerate the design of optogenetic devices. We demonstrate the use of cpLOV2-based optogenetic tools to reversibly gate ion channels, antagonize CRISPR-Cas9-mediated genome engineering, control protein subcellular localization, reprogram transcriptional outputs, elicit cell suicide and generate photoactivatable chimeric antigen receptor T cells for inducible tumor cell killing. Our approach is widely applicable for engineering other photoreceptors to meet the growing need of optogenetic tools tailored for biomedical and biotechnological applications.
Rac1 activation can generate untemplated, lamellar membrane ruffles.
Membrane protrusions that occur on the dorsal surface of a cell are an excellent experimental system to study actin machinery at work in a living cell. Small GTPase Rac1 controls the membrane protrusions that form and encapsulate extracellular volumes to perform pinocytic or phagocytic functions.