LYST deficiency impairs autophagic lysosome reformation in neurons and alters lysosome number and size

Chediak-Higashi syndrome (CHS) is a rare, autosomal recessive disorder caused by biallelic mutations in the lysosomal trafficking regulator (LYST) gene. Even though enlarged lysosomes and/or lysosome-related organelles (LROs) are the typical cellular hallmarks of CHS, they have not been investigated in human neuronal models. Moreover, how and why the loss of LYST function causes a lysosome phenotype in cells has not been elucidated. We report that the LYST-deficient human neuronal model exhibits lysosome depletion accompanied by hyperelongated tubules extruding from enlarged autolysosomes. These results have also been recapitulated in neurons differentiated from CHS patients' induced pluripotent stem cells (iPSCs), validating our model system. We propose that LYST ensures the correct fission/scission of the autolysosome tubules during autophagic lysosome reformation (ALR), a crucial process to restore the number of free lysosomes after autophagy. We further demonstrate that LYST is recruited to the lysosome membrane, likely to facilitate the fission of autolysosome tubules. Together, our results highlight the key role of LYST in maintaining lysosomal homeostasis following autophagy and suggest that ALR dysregulation is likely associated with the neurodegenerative CHS phenotype.

Increased Mitochondrial Biogenesis and Reactive Oxygen Species Production Accompany Prolonged CD4+ T Cell Activation

Activation of CD4⁺ T cells to proliferate drives cells toward aerobic glycolysis for energy production while using mitochondria primarily for macromolecular synthesis. In addition, the mitochondria of activated T cells increase production of reactive oxygen species, providing an important second messenger for intracellular signaling pathways. To better understand the critical changes in mitochondria that accompany prolonged T cell activation, we carried out an extensive analysis of mitochondrial remodeling using a combination of conventional strategies and a novel high-resolution imaging method. We show that for 4 d following activation, mouse CD4⁺ T cells sustained their commitment to glycolysis facilitated by increased glucose uptake through increased expression of GLUT transporters. Despite their limited contribution to energy production, mitochondria were active and showed increased reactive oxygen species production. Moreover, prolonged activation of CD4⁺ T cells led to increases in mitochondrial content and volume, in the number of mitochondria per cell and in mitochondrial biogenesis. Thus, during prolonged activation, CD4⁺ T cells continue to obtain energy predominantly from glycolysis but also undergo extensive mitochondrial remodeling, resulting in increased mitochondrial activity.

Imaging Protein-Protein Interactions by Förster Resonance Energy Transfer (FRET) Microscopy in Live Cells

This updated unit compares three methods to acquire Förster Resonance Energy Transfer (FRET) data in living cells using a confocal microscope: Acceptor photobleaching, Acceptor-sensitized emission FRET, and Donor fluorescence lifetime imaging. Detailed protocols for live cell husbandry, image acquisition, and data analysis are provided. In addition to providing instructions for manufacturer's analysis tool sets, we provide an easy-to-use, MATLAB-based code to calculate FRET efficiency from data obtained using the Acceptor photobleaching or Acceptor-sensitized emission method, which can be freely downloaded. © 2018 by John Wiley & Sons, Inc.

The Plasmodium falciparum rhoptry protein RhopH3 plays essential roles in host cell invasion and nutrient uptake

Merozoites of the protozoan parasite responsible for the most virulent form of malaria, Plasmodium falciparum, invade erythrocytes. Invasion involves discharge of rhoptries, specialized secretory organelles. Once intracellular, parasites induce increased nutrient uptake by generating new permeability pathways (NPP) including a Plasmodium surface anion channel (PSAC). RhopH1/Clag3, one member of the three-protein RhopH complex, is important for PSAC/NPP activity. However, the roles of the other members of the RhopH complex in PSAC/NPP establishment are unknown and it is unclear whether any of the RhopH proteins play a role in invasion. Here we demonstrate that RhopH3, the smallest component of the complex, is essential for parasite survival. Conditional truncation of RhopH3 substantially reduces invasive capacity. Those mutant parasites that do invade are defective in nutrient import and die. Our results identify a dual role for RhopH3 that links erythrocyte invasion to formation of the PSAC/NPP essential for parasite survival within host erythrocytes.

DOI:http://dx.doi.org/10.7554/eLife.23239.001

Malaria is a life-threatening disease that affects millions of people around the world. The parasites that cause malaria have a complex life cycle that involves infecting both mosquitoes and mammals, including humans. In humans, the parasites spend part of their life cycle inside red blood cells, which causes the symptoms of the disease. In order to survive and multiply, malaria parasites need to make the red blood cell more permeable so that it can absorb nutrients from the blood stream and get rid of the toxic waste products they generate.

It remains unclear how the parasites do this, but previous research has shown that the parasites produce channel-like proteins that make red blood cells more permeable to nutrients. One of the proteins involved in this process forms part of a complex with two other proteins, called RhopH2 and RhopH3. It is not known what these other two proteins do, and whether they are necessary for creating the new nutrient channels.

Sherling et al. studied the RhopH3 protein to see if it is required to make red blood cells more permeable. The experiments used a genetically modified version of the parasite, in which RhopH3 no longer interacted with the two other proteins. The findings show that RhopH3 has two important roles: first, parasites need it to invade the red blood cells, and second, parasites cannot get nutrients into the red blood cell without RhopH3.

Most antimalarial drugs work by preventing parasite replication in red blood cells, but parasites are becoming increasingly resistant to these drugs. Understanding which proteins allow parasites to invade and grow within blood cells will further the development of new malaria medication. The next step will be to understand the molecular mechanisms by which RhopH3 promotes invasion and subsequently facilitates nutrient uptake, and will help researchers to explore its potential as a drug target.

DOI:http://dx.doi.org/10.7554/eLife.23239.002

Arrestins function in cAR1 GPCR-mediated signaling and cAR1 internalization in the development of Dictyostelium discoideum

Evolutionarily conserved arrestin-like proteins are key components of the cAR1-mediated ERK2 activation that controls cAMP cell–cell signaling during Dictyostelium aggregation. They are also involved in ligand-induced cAR1 internalization, which is required for the switch of cAMP receptors during multicellular development.

Oscillation of chemical signals is a common biological phenomenon, but its regulation is poorly understood. At the aggregation stage of Dictyostelium discoideum development, the chemoattractant cAMP is synthesized and released at 6-min intervals, directing cell migration. Although the G protein–coupled cAMP receptor cAR1 and ERK2 are both implicated in regulating the oscillation, the signaling circuit remains unknown. Here we report that D. discoideum arrestins regulate the frequency of cAMP oscillation and may link cAR1 signaling to oscillatory ERK2 activity. Cells lacking arrestins (adcB⁻C⁻) display cAMP oscillations during the aggregation stage that are twice as frequent as for wild- type cells. The adcB⁻C⁻ cells also have a shorter period of transient ERK2 activity and precociously reactivate ERK2 in response to cAMP stimulation. We show that arrestin domain–containing protein C (AdcC) associates with ERK2 and that activation of cAR1 promotes the transient membrane recruitment of AdcC and interaction with cAR1, indicating that arrestins function in cAR1-controlled periodic ERK2 activation and oscillatory cAMP signaling in the aggregation stage of D. discoideum development. In addition, ligand-induced cAR1 internalization is compromised in adcB⁻C⁻ cells, suggesting that arrestins are involved in elimination of high-affinity cAR1 receptors from cell surface after the aggregation stage of multicellular development.

Phosphorylation of chemoattractant receptors regulates chemotaxis, actin reorganization and signal relay

Migratory cells, including mammalian leukocytes and Dictyostelium, use G-protein-coupled receptor (GPCR) signaling to regulate MAPK/ERK, PI3K, TORC2/AKT, adenylyl cyclase and actin polymerization, which collectively direct chemotaxis. Upon ligand binding, mammalian GPCRs are phosphorylated at cytoplasmic residues, uncoupling G-protein pathways, but activating other pathways. However, connections between GPCR phosphorylation and chemotaxis are unclear. In developing Dictyostelium, secreted cAMP serves as a chemoattractant, with extracellular cAMP propagated as oscillating waves to ensure directional migratory signals. cAMP oscillations derive from transient excitatory responses of adenylyl cyclase, which then rapidly adapts. We have studied chemotactic signaling in Dictyostelium that express non-phosphorylatable cAMP receptors and show through chemotaxis modeling, single-cell FRET imaging, pure and chimeric population wavelet quantification, biochemical analyses and TIRF microscopy, that receptor phosphorylation is required to regulate adenylyl cyclase adaptation, long-range oscillatory cAMP wave production and cytoskeletal actin response. Phosphorylation defects thus promote hyperactive actin polymerization at the cell periphery, misdirected pseudopodia and the loss of directional chemotaxis. Our data indicate that chemoattractant receptor phosphorylation is required to co-regulate essential pathways for migratory cell polarization and chemotaxis. Our results significantly extend the understanding of the function of GPCR phosphorylation, providing strong evidence that this evolutionarily conserved mechanism is required in a signal attenuation pathway that is necessary to maintain persistent directional movement of Dictyostelium, neutrophils and other migratory cells.

The Elmo family forms an ancient group of actin-regulating proteins

The Elmo protein family members are important mediators of small G protein activity, regulating actin-mediated processes such as chemotaxis and engulfment. Until recently,1 Elmo function has not been explored in professional phagocytes such as Dictyostelium discoideum. We discuss the significance of this family with respect to pathways that regulate Rac signaling, we present a comparison of Elmo proteins between representative taxa, and discuss our findings on ElmoA, one of six Elmo proteins found in D. discoideum.

Cutting edge: NKG2D-dependent cytotoxicity is controlled by ligand distribution in the target cell membrane

Although the importance of membrane microdomains in receptor-mediated activation of lymphocytes has been established, much less is known about the role of receptor ligand distribution on APC and target cells. Detergent-resistant membrane domains, into which GPI-linked proteins partition, are enriched in cholesterol and glycosphingolipids. ULBP1 is a GPI-linked ligand for natural cytotoxicity receptor NKG2D. To investigate how ULBP1 distribution on target cells affects NKG2D-dependent NK cell activation, we fused the extracellular domain of ULBP1 to the transmembrane domain of CD45. Introduction of this transmembrane domain eliminated the association of ULBP1 with the detergent-resistant membrane fraction and caused a significant reduction of cytotoxicity and degranulation by NK cells. Clustering and lateral diffusion of ULBP1 was not affected by changes in the membrane anchor. These results show that the partitioning of receptor ligands in discrete membrane domains of target cells is an important determinant of NK cell activation.

Coupling mechanism of a GPCR and a heterotrimeric G protein during chemoattractant gradient sensing in Dictyostelium

The coupling of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) with G proteins is fundamental for GPCR signaling; however, the mechanism of coupling is still debated. Moreover, how the proposed mechanisms affect the dynamics of downstream signaling remains unclear. Here, through experiments involving fluorescence recovery after photobleaching and single-molecule imaging, we directly measured the mobilities of cyclic adenosine monophosphate (cAMP) receptor 1 (cAR1), a chemoattractant receptor, and a G protein βγ subunit in live cells. We found that cAR1 diffused more slowly in the plasma membrane than did Gβγ. Upon binding of ligand to the receptor, the mobility of cAR1 was unchanged, whereas the speed of a fraction of the faster-moving Gβγ subunits decreased. Our measurements showed that cAR1 was relatively immobile and Gβγ diffused freely, suggesting that chemoattractant-bound cAR1 transiently interacted with G proteins. Using models of possible coupling mechanisms, we computed the temporal kinetics of G protein activation. Our fluorescence resonance energy transfer imaging data showed that fully activated cAR1 induced the sustained dissociation of G protein α and βγ subunits, which indicated that ligand-bound cAR1 activated G proteins continuously. Finally, simulations indicated that a high-affinity coupling of ligand-bound receptors and G proteins was essential for cAR1 to translate extracellular gradient signals into directional cellular responses. We suggest that chemoattractant receptors use a ligand-induced coupling rather than a precoupled mechanism to control the activation of G proteins during chemotaxis.

Tethering of intercellular adhesion molecule on target cells is required for LFA-1-dependent NK cell adhesion and granule polarization

Alpha(L)beta(2) integrin (LFA-1) has an important role in the formation of T cell and NK cell cytotoxic immunological synapses and in target cell killing. Binding of LFA-1 to ICAM on target cells promotes not only adhesion but also polarization of cytolytic granules in NK cells. In this study, we tested whether LFA-1-dependent NK cell responses are regulated by the distribution and mobility of ICAM at the surface of target cells. We show that depolymerization of F-actin in NK-sensitive target cells abrogated LFA-1-dependent conjugate formation and granule polarization in primary NK cells. Degranulation, which is not controlled by LFA-1, was not impaired. Fluorescence recovery after photobleaching experiments and particle tracking by total internal reflection fluorescence microscopy revealed that ICAM-1 and ICAM-2 were distributed in largely immobile clusters. ICAM clusters were maintained and became highly mobile after actin depolymerization. Moreover, reducing ICAM-2 mobility on an NK-resistant target cell through expression of ezrin, an adaptor molecule that tethers proteins to the actin cytoskeleton, enhanced LFA-1-dependent adhesion and granule polarization. Finally, although NK cells kept moving over freely diffusible ICAM-1 on a lipid bilayer, they bound and spread over solid-phase ICAM-1. We conclude that tethering, rather than clustering of ICAM, promotes proper signaling by LFA-1 in NK cells. Our findings suggest that the lateral diffusion of integrin ligands on cells may be an important determinant of susceptibility to lysis by cytotoxic lymphocytes.

Imaging protein-protein interactions by Förster resonance energy transfer (FRET) microscopy in live cells

This unit describes an acceptor-sensitized emission FRET method using a confocal microscope for image acquisition. In contrast to acceptor photobleaching, which is an end-point assay that destroys the acceptor fluorophore, the sensitized emission method is amenable for FRET measurements in live cells and can be used to measure changes in FRET efficiency over time. The purpose of this unit is to provide a basic starting point for understanding and performing the sensitized emission method with a simple teaching tool for live-cell imaging. References that discuss the vagaries of acquiring and analyzing FRET between individually tagged molecules are provided.

Monitoring dynamic GPCR signaling events using fluorescence microscopy, FRET imaging, and single-molecule imaging

How a eukaryotic cell translates a small concentration difference of a chemoattractant across the length of its surface into highly polarized intracellular responses is a fundamental question in chemotaxis. Chemoattractants are detected by G-protein-coupled receptors (GPCRs). Binding of chemoattractants to GPCRs induces the dissociation of heterotrimeric G-proteins into G alpha and G betagamma subunits, which in turn, activate downstream signaling networks. To fully understand the molecular mechanisms of chemotaxis, it is essential to quantitatively measure the dynamic changes of chemoattractant concentrations around cells, activation of heterotrimeric G-proteins, and the mobility of GPCR and G-protein subunits in the cell membrane. Here, we outline fluorescence imaging methods including Förster resonance energy transfer (FRET) imaging and a single-molecule analysis that allow us to measure the dynamic properties of GPCR signaling in single live cells.

A vesicle surface tyrosine kinase regulates phagosome maturation

Phagocytosis is crucial for host defense against microbial pathogens and for obtaining nutrients in Dictyostelium discoideum. Phagocytosed particles are delivered via a complex route from phagosomes to lysosomes for degradation, but the molecular mechanisms involved in the phagosome maturation process are not well understood. Here, we identify a novel vesicle-associated receptor tyrosine kinase-like protein, VSK3, in D. discoideum. We demonstrate how VSK3 is involved in phagosome maturation. VSK3 resides on the membrane of late endosomes/lysosomes with its C-terminal kinase domain facing the cytoplasm. Inactivation of VSK3 by gene disruption reduces the rate of phagocytosis in cells, which is rescued by re-expression of VSK3. We found that the in vivo function of VSK3 depends on the presence of the kinase domain and vesicle localization. Furthermore, VSK3 is not essential for engulfment, but instead, is required for the fusion of phagosomes with late endosomes/lysosomes. Our findings suggest that localized tyrosine kinase signaling on the surface of endosome/lysosomes represents a control mechanism for phagosome maturation.

Using quantitative fluorescence microscopy and FRET imaging to measure spatiotemporal signaling events in single living cells

The mechanisms that mediate how migratory eukaryotic cells amplify a shallow, extracellular chemoattractant gradient into a steep intracellular gradient of signaling components to guide chemotaxis remains unknown. To unravel these mechanisms, it is essential to quantitatively measure the spatiotemporal patterns of chemoattractant gradients, the dynamic movement of intracellular signaling pathway molecules, and the localized activation of these molecules in single living cells. Recent developments in live-cell fluorescence microscopy have permitted direct visualization and quantitative measurement of signal transduction events with high temporal and spatial resolution. Here, we outline fluorescence imaging methods to simultaneously visualize and quantitatively measure spatiotemporal changes in chemoattractant concentration by using the fluorescent tracer dye Alexa 594. Next, we provide a method to correlate the dynamic changes in ligand to the spatiotemporal changes in the second messenger phosphatidylinositol 3,4,5-triphosphate (PIP3) along the inner surface of the plasma membrane in live cells. Finally, we describe a fluorescence resonance energy transfer (FRET) method to determine the extent of heterotrimeric G protein activation in single living cells in response to various chemoattractant fields.

Nonadaptive regulation of ERK2 in Dictyostelium: implications for mechanisms of cAMP relay

It is assumed that ERK2 in Dictyostelium is subject to adaptive regulation in response to constant extracellular ligand stimulation. We now show, to the contrary, that ERK2 remains active under continuous stimulation, differing from most ligand-activated pathways in chemotactically competent Dictyostelium and other cells. We show that the upstream phosphorylation pathway, responsible for ERK2 activation, transiently responds to receptor stimulation, whereas ERK2 dephosphorylation (deactivation) is inhibited by continuous stimulation. We argue that the net result of these two regulatory actions is a persistently active ERK2 pathway when the extracellular ligand (i.e., cAMP) concentration is held constant and that oscillatory production/destruction of secreted cAMP in chemotaxing cells accounts for the observed oscillatory activity of ERK2. We also show that pathways controlling seven-transmembrane receptor (7-TMR) ERK2 activation/deactivation function independently of G proteins and ligand-induced production of intracellular cAMP and the consequent activation of PKA. Finally, we propose that this regulation enables ERK2 to function both in an oscillatory manner, critical for chemotaxis, and in a persistent manner, necessary for gene expression, as secreted ligand concentration increases during later development. This work redefines mechanisms of ERK2 regulation by 7-TMR signaling in Dictyostelium and establishes new implications for control of signal relay during chemotaxis.

A Rab21/LIM-only/CH-LIM complex regulates phagocytosis via both activating and inhibitory mechanisms

We have identified two LIM domain proteins, LimF and ChLim, from Dictyostelium that interact with each other and with the small, Rab5-related, Rab21 GTPase to collectively regulate phagocytosis. To investigate in vivo functions, we generated cell lines that lack or overexpress LimF and ChLim and strains that express activating or inhibiting variants of Rab21. Overexpression of LimF, loss of ChLim, or expression of constitutively active Rab21 increases the rate of phagocytosis above that of wild type. Conversely, loss of LimF, overexpression of ChLim, or expression of a dominant-negative Rab21 inhibits phagocytosis. Our studies using cells carrying multiple mutations in these genes further indicate that ChLim antagonizes the activating function of Rab21-GTP during phagocytosis; in turn, LimF is required for Rab21-GTP function. Finally, we demonstrate that ChLim and LimF localize to the phagocytic cup and phago-lysosomal vesicles. We suggest that LimF, ChLim, and activated Rab21-GTP participate as a novel signaling complex that regulates phagocytic activity.

Dictyostelium discoideum expresses a malaria chloroquine resistance mechanism upon transfection with mutant, but not wild-type, Plasmodium falciparum transporter PfCRT

Chloroquine resistance in Plasmodium falciparum malaria results from mutations in PfCRT, a member of a unique family of transporters present in apicomplexan parasites and Dictyostelium discoideum. Mechanisms that have been proposed to explain chloroquine resistance are difficult to evaluate within malaria parasites. Here we report on the targeted expression of wild-type and mutant forms of PfCRT to acidic vesicles in D. discoideum. We show that wild-type PfCRT has minimal effect on the accumulation of chloroquine by D. discoideum, whereas forms of PfCRT carrying a key charge-loss mutation of lysine 76 (e.g. K76T) enable D. discoideum to expel chloroquine. As in P. falciparum, the chloroquine resistance phenotype conferred on transformed D. discoideum can be reversed by the channel-blocking agent verapamil. Although intravesicular pH levels in D. discoideum show small acidic changes with the expression of different forms of PfCRT, these changes would tend to promote intravesicular trapping of chloroquine (a weak base) and do not account for reduced drug accumulation in transformed D. discoideum. Our results instead support outward-directed chloroquine efflux for the mechanism of chloroquine resistance by mutant PfCRT. This mechanism shows structural specificity as D. discoideum transformants that expel chloroquine do not expel piperaquine, a bisquinoline analog of chloroquine used frequently against chloroquine-resistant parasites in Southeast Asia. PfCRT, nevertheless, may have some ability to act on quinine and quinidine. Transformed D. discoideum will be useful for further studies of the chloroquine resistance mechanism and may assist in the development and evaluation of new antimalarial drugs.

A G alpha-dependent pathway that antagonizes multiple chemoattractant responses that regulate directional cell movement

Chemotactic cells, including neutrophils and Dictyostelium discoideum, orient and move directionally in very shallow chemical gradients. As cells polarize, distinct structural and signaling components become spatially constrained to the leading edge or rear of the cell. It has been suggested that complex feedback loops that function downstream of receptor signaling integrate activating and inhibiting pathways to establish cell polarity within such gradients. Much effort has focused on defining activating pathways, whereas inhibitory networks have remained largely unexplored. We have identified a novel signaling function in Dictyostelium involving a Galpha subunit (Galpha9) that antagonizes broad chemotactic response. Mechanistically, Galpha9 functions rapidly following receptor stimulation to negatively regulate PI3K/PTEN, adenylyl cyclase, and guanylyl cyclase pathways. The coordinated activation of these pathways is required to establish the asymmetric mobilization of actin and myosin that typifies polarity and ultimately directs chemotaxis. Most dramatically, cells lacking Galpha9 have extended PI(3,4,5)P(3), cAMP, and cGMP responses and are hyperpolarized. In contrast, cells expressing constitutively activated Galpha9 exhibit a reciprocal phenotype. Their second message pathways are attenuated, and they have lost the ability to suppress lateral pseudopod formation. Potentially, functionally similar Galpha-mediated inhibitory signaling may exist in other eukaryotic cells to regulate chemoattractant response.

Galpha-mediated inhibition of developmental signal response

BACKGROUND

Seven-transmembrane receptor (7-TMR)-G protein networks are molecular sensors of extracellular signals in all eukarya. These pathways cycle through activated (sensitized) and inhibited (desensitized) states, and, while many of the molecular components for signal activation have been described, inhibitory mechanisms are not well characterized. In Dictyostelium, 7-TM cAMP receptors direct chemotaxis and development but also regulate the periodic synthesis of their own ligand, the chemoattractant/morphogen cAMP. We now demonstrate through loss-of-function/gain-of-function studies that the novel heterotrimeric Galpha9 protein subunit regulates an inhibitory pathway during early Dictyostelium development for the cAMP signal response.

RESULTS

galpha9 null cells form more cAMP signaling centers, are more resistant to compounds that inhibit cAMP signaling, and complete aggregation sooner and at lower cell densities than wild-type cells. These phentoypes are consistent with the loss of an inhibitory signaling pathway during development of galpha9 null cells. Cells expressing constitutively activated Galpha9 are defective in cAMP signaling center formation and development at low cell density and display an increased sensitivity to cAMP signal inhibition that is characteristic of enhanced suppression of the cAMP signal response. Finally, we demonstrate that galpha9 null cells, which have been codeveloped with a majority of wild-type cells, primarily establish cAMP signaling centers and are able to non-autonomously direct wild-type cells to adopt a galpha9 null-like phenotype.

CONCLUSIONS

We suggest that Galpha9 functions in an inhibitory-feedback pathway that regulates cAMP signaling center formation and propagation. Galpha9 may be part of the mechanism that regulates lateral signal inhibition or that modulates receptor desensitization.

Signaling at zero G: G-protein-independent functions for 7-TM receptors

Eukaryotic cells, whether free-living, single-celled microbes or components of complex metazoa, can sense environmental cues through specialized seven-transmembrane (7-TM) receptors (also called heptahelical or G-protein-coupled receptors). 7-TM receptors detect "inputs" such as light, peptide hormones, neurotransmitters, pheromones, odorants, morphogens and chemoattractants, linking extracellular stimuli to intracellular signaling networks via heterotrimeric G proteins. Recently, this obligatory paradigm has been challenged. A growing body of evidence indicates that 7-TM receptors can also transmit extracellular signals through mechanisms that function independently of G-protein coupling. This review discusses pathways and protein interactions for 7-TM receptors signaling "at zero G" in Dictyostelium and mammalian cells.