The pleckstrin homology domain of phospholipase C-beta2 as an effector site for Rac

Gαq signalling from endosomes: A new conundrum

G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors, and are involved in the transmission of a variety of extracellular stimuli such as hormones, neurotransmitters, light and odorants into intracellular responses. They regulate every aspect of physiology and, for this reason, about one third of all marketed drugs target these receptors. Classically, upon binding to their agonist, GPCRs are thought to activate G-proteins from the plasma membrane and to stop signalling by subsequent desensitisation and endocytosis. However, accumulating evidence indicates that, upon internalisation, some GPCRs can continue to activate G-proteins in endosomes. Importantly, this signalling from endomembranes mediates alternative cellular responses other than signalling at the plasma membrane. Endosomal G-protein signalling and its physiological relevance have been abundantly documented for Gαs - and Gαi -coupled receptors. Recently, some Gαq -coupled receptors have been reported to activate Gαq on endosomes and mediate important cellular processes. However, several questions relative to the series of cellular events required to translate endosomal Gαq activation into cellular responses remain unanswered and constitute a new conundrum. How are these responses in endosomes mediated in the quasi absence of the substrate for the canonical Gαq -activated effector? Is there another effector? Is there another substrate? If so, how does this alternative endosomal effector or substrate produce a downstream signal? This review aims to unravel and discuss these important questions, and proposes possible routes of investigation.

Structure and regulation of phospholipase Cβ and ε at the membrane

Phospholipase C (PLC) β and ε enzymes hydrolyze phosphatidylinositol (PI) lipids in response to direct interactions with heterotrimeric G protein subunits and small GTPases, which are activated downstream of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). PI hydrolysis generates second messengers that increase the intracellular Ca²⁺ concentration and activate protein kinase C (PKC), thereby regulating numerous physiological processes. PLCβ and PLCε share a highly conserved core required for lipase activity, but use different strategies and structural elements to autoinhibit basal activity, bind membranes, and engage G protein activators. In this review, we discuss recent structural insights into these enzymes and the implications for how they engage membranes alone or in complex with their G protein regulators.

l-Glutamate stimulates cholecystokinin secretion via the T1R1/T1R3 mediated PLC/TRPM5 transduction pathway

BACKGROUND

It is known that cholecystokinin (CCK) plays an essential role in reducing food intake and driving weight loss. Previous studies demonstrated that amino acids were capable of triggering CCK release through G protein-coupled receptors, but the sensing mechanism remains obscure, especially the intracellular signaling pathway.

RESULTS

l-Glu, rather than its d-isomer, robustly stimulated CCK secretion in a porcine duodenal model, and the secretory response was augmented by incubation with the allosteric ligand of T1R1, while T1R3 antagonist attenuated it. Upon inhibiting phospholipase C (PLC) or transient receptor potential M5 (TRPM5) activity, l-Glu failed to increase CCK release. Oral administration of monosodium glutamate in rats also suppressed food intake and increased plasma CCK levels, accompanied by elevated expression of T1R1, PLCβ2 and TRPM5 in the duodenum.

CONCLUSION

These data demonstrated that l-Glu stimulated CCK secretion through the activation of T1R1/T1R3 in a PLC/TRPM5-dependent manner. © 2020 Society of Chemical Industry.

Phosphoinositide-specific phospholipase C isoforms are conveyed by osteosarcoma-derived extracellular vesicles

Cancer cells are able to release high amounts of extracellular vesicles, thereby conditioning the normal cells in the surrounding tissue and/or in distant target organs. In the context of bone cancers, previous studies suggested that osteosarcoma cancer cells produce transforming extracellular vesicles able to induce a tumour-like phenotype in normal recipient cells. Indeed, phosphoinositide-specific phospholipase C (PI-PLC) enzymes are differentially expressed in osteosarcoma cell lines with increasing aggressiveness, thus providing helpful insights to better define their role and functions in this bone tumour. By confocal microscopy analysis, we demonstrated that osteosarcoma-derived extracellular vesicles convey all the assessed PI-PLC isoforms, and that they localize into cell membrane bubble-like structures, resembling extracellular vesicles about to be released, as conveyed and/or membrane protein. Cytofluorimetric analysis confirmed the presence of PI-PLC isoforms in the extracellular vesicles collected from conditioned media of osteosarcoma cells. These findings suggest the feasibility to use circulating extracellular vesicles as biomarkers of osteosarcoma progression and/or the monitoring of this distressing disease.

Actin dynamics in cell migration

Cell migration is an essential process, both in unicellular organisms such as amoeba and as individual or collective motility in highly developed multicellular organisms like mammals. It is controlled by a variety of activities combining protrusive and contractile forces, normally generated by actin filaments. Here, we summarize actin filament assembly and turnover processes, and how respective biochemical activities translate into different protrusion types engaged in migration. These actin-based plasma membrane protrusions include actin-related protein 2/3 complex-dependent structures such as lamellipodia and membrane ruffles, filopodia as well as plasma membrane blebs. We also address observed antagonisms between these protrusion types, and propose a model - also inspired by previous literature - in which a complex balance between specific Rho GTPase signaling pathways dictates the protrusion mechanism employed by cells. Furthermore, we revisit published work regarding the fascinating antagonism between Rac and Rho GTPases, and how this intricate signaling network can define cell behavior and modes of migration. Finally, we discuss how the assembly of actin filament networks can feed back onto their regulators, as exemplified for the lamellipodial factor WAVE regulatory complex, tightly controlling accumulation of this complex at specific subcellular locations as well as its turnover.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Interaction of the N terminus of ADP-ribosylation factor with the PH domain of the GTPase-activating protein ASAP1 requires phosphatidylinositol 4,5-bisphosphate

Arf GAP with Src homology 3 domain, ankyrin repeat, and pleckstrin homology (PH) domain 1 (ASAP1) is a multidomain GTPase-activating protein (GAP) for ADP-ribosylation factor (ARF)-type GTPases. ASAP1 affects integrin adhesions, the actin cytoskeleton, and invasion and metastasis of cancer cells. ASAP1's cellular function depends on its highly-regulated and robust ARF GAP activity, requiring both the PH and the ARF GAP domains of ASAP1, and is modulated by phosphatidylinositol 4,5-bisphosphate (PIP₂). The mechanistic basis of PIP₂-stimulated GAP activity is incompletely understood. Here, we investigated whether PIP₂ controls binding of the N-terminal extension of ARF1 to ASAP1's PH domain and thereby regulates its GAP activity. Using [Δ17]ARF1, lacking the N terminus, we found that PIP₂ has little effect on ASAP1's activity. A soluble PIP₂ analog, dioctanoyl-PIP₂ (diC8PIP₂), stimulated GAP activity on an N terminus-containing variant, [L8K]ARF1, but only marginally affected activity on [Δ17]ARF1. A peptide comprising residues 2-17 of ARF1 ([2-17]ARF1) inhibited GAP activity, and PIP₂-dependently bound to a protein containing the PH domain and a 17-amino acid-long interdomain linker immediately N-terminal to the first β-strand of the PH domain. Point mutations in either the linker or the C-terminal α-helix of the PH domain decreased [2-17]ARF1 binding and GAP activity. Mutations that reduced ARF1 N-terminal binding to the PH domain also reduced the effect of ASAP1 on cellular actin remodeling. Mutations in the ARF N terminus that reduced binding also reduced GAP activity. We conclude that PIP₂ regulates binding of ASAP1's PH domain to the ARF1 N terminus, which may partially regulate GAP activity.

The role of phospholipase Cβ on the plasma membrane and in the cytosol: How modular domains enable novel functions

Phospholipase Cβ (PLCβ) is a signaling enzyme activated by G proteins to generate calcium signals. The catalytic core of PLCβ is surrounded by modular domains that mediate the interaction of the enzyme with known protein partners on the plasma membrane. The C-terminal region PLCβ contains a novel coiled-coil domain that is required for Gαq binding and activation. Recent work has shown that this domain also binds a number of cytosolic proteins that regulate protein translation, and that these proteins compete with Gαq for PLCβ binding. The ability of PLCβ to shuttle between the cytosol to impact protein translation and the plasma membrane to mediate calcium signals puts PLCβ in a central role in cell function.

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

K-Ras G-domain binding with signaling lipid phosphatidylinositol (4,5)-phosphate (PIP2): membrane association, protein orientation, and function

Ras genes potently drive human cancers, with mutated proto-oncogene GTPase KRAS4B (K-Ras4B) being the most abundant isoform. Targeted inhibition of oncogenic gene products is considered the "holy grail" of present-day cancer therapy, and recent discoveries of small-molecule KRas4B inhibitors were made thanks to a deeper understanding of the structure and dynamics of this GTPase. Because interactions with biological membranes are key for Ras function, Ras-lipid interactions have become a major focus, especially because such interactions evidently involve both the Ras C terminus for lipid anchoring and its G-protein domain. Here, using NMR spectroscopy and molecular dynamics simulations complemented by biophysical- and cell-biology assays, we investigated the interaction between K-Ras4B with the signaling lipid phosphatidylinositol (4,5)-phosphate (PIP2). We discovered that the β2 and β3 strands as well as helices 4 and 5 of the GTPase G-domain bind to PIP2 and identified the specific residues in these structural elements employed in these interactions, likely occurring in two K-Ras4B orientation states relative to the membrane. Importantly, we found that some of these residues known to be oncogenic when mutated (D47K, D92N, K104M, and D126N) are critical for K-Ras-mediated transformation of fibroblast cells, but do not substantially affect basal and assisted nucleotide hydrolysis and exchange. Moreover, the K104M substitution abolished localization of K-Ras to the plasma membrane. The findings suggest that specific G-domain residues can critically regulate Ras function by mediating interactions with membrane-associated PIP2 lipids; these insights that may inform the future design of therapeutic reagents targeting Ras activity.

Distinct Roles of Small GTPases Rac1 and Rac2 in Histamine H4 Receptor-Mediated Chemotaxis of Mast Cells

Histamine induces chemotaxis of mast cells through the H4 receptor. However, little is known about the precise intracellular signaling pathway that mediates this process. In this study, we identified small GTPases Rac1 and Rac2 as intracellular binding partners of the H4 receptor and characterized their roles in H4 receptor signaling. We showed that histamine induced Rac GTPase activation via the H4 receptor. A Rac inhibitor NSC23766 attenuated chemotaxis of mast cells toward histamine, as well as histamine-induced calcium mobilization and extracellular signal-regulated kinase (ERK) activation. Histamine-induced migration of mast cells was also sensitive to PD98059, an inhibitor of the mitogen-activated protein kinase kinase, indicating that the Rac-ERK pathway was involved in chemotaxis through the H4 receptor. Inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) by LY294002 suppressed the histamine-induced chemotaxis and activation of Rac GTPases, suggesting that PI3K regulates chemotaxis upstream of Rac activation. Specific knockdown of Rac1 and Rac2 by short-hairpin RNA revealed that both Rac GTPases are necessary for histamine-induced migration. Downregulation of Rac1 and Rac2 led to attenuated response in calcium mobilization and ERK activation, respectively. These observations suggested that Rac1 and Rac2 have distinct and essential roles in intracellular signaling downstream of H4 receptor-PI3K in histamine-induced chemotaxis of mast cells.

Targeting of Rac1 prevents bronchoconstriction and airway hyperresponsiveness

BACKGROUND

The molecular mechanisms responsible for airway smooth muscle cells' (aSMCs) contraction and proliferation in airway hyperresponsiveness (AHR) associated with asthma are still largely unknown. The small GTPases of the Rho family (RhoA, Rac1, and Cdc42) play a central role in SMC functions including migration, proliferation, and contraction.

OBJECTIVE

The objective of this study was to identify the role of Rac1 in aSMC contraction and to investigate its involvement in AHR associated with allergic asthma.

METHODS

To define the role of Rac1 in aSMC, ex and in vitro analyses of bronchial reactivity were performed on bronchi from smooth muscle (SM)-specific Rac1 knockout mice and human individuals. In addition, this murine model was exposed to allergens (ovalbumin or house dust mite extract) to decipher in vivo the implication of Rac1 in AHR.

RESULTS

The specific SMC deletion or pharmacological inhibition of Rac1 in mice prevented the bronchoconstrictor response to methacholine. In human bronchi, a similar role of Rac1 was observed during bronchoconstriction. We further demonstrated that Rac1 activation is responsible for bronchoconstrictor-induced increase in intracellular Ca²⁺ concentration and contraction both in murine and in human bronchial aSMCs, through its association with phospholipase C β2 and the stimulation of inositol 1,4,5-trisphosphate production. In vivo, Rac1 deletion in SMCs or pharmacological Rac1 inhibition by nebulization of NSC23766 prevented AHR in murine models of allergic asthma. Moreover, nebulization of NSC23766 decreased eosinophil and neutrophil populations in bronchoalveolar lavages from mice with asthma.

CONCLUSIONS

Our data reveal an unexpected and essential role of Rac1 in the regulation of intracellular Ca²⁺ and contraction of aSMCs, and the development of AHR. Rac1 thus appears as an attractive therapeutic target in asthma, with a combined beneficial action on both bronchoconstriction and pulmonary inflammation.

The binding of activated Gαq to phospholipase C-β exhibits anomalous affinity

Upon activation by the G_q family of Gα subunits, Gβγ subunits, and some Rho family GTPases, phospholipase C-β (PLC-β) isoforms hydrolyze phosphatidylinositol 4,5-bisphosphate to the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. PLC-β isoforms also function as GTPase-activating proteins, potentiating G_q deactivation. To elucidate the mechanism of this mutual regulation, we measured the thermodynamics and kinetics of PLC-β3 binding to Gα_q FRET and fluorescence correlation spectroscopy, two physically distinct methods, both yielded K_d values of about 200 nm for PLC-β3-Gα_q binding. This K_d is 50-100 times greater than the EC₅₀ for Gα_q-mediated PLC-β3 activation and for the Gα_q GTPase-activating protein activity of PLC-β. The measured K_d was not altered either by the presence of phospholipid vesicles, phosphatidylinositol 4,5-bisphosphate and Ca²⁺, or by the identity of the fluorescent labels. FRET-based kinetic measurements were also consistent with a K_d of 200 nm We determined that PLC-β3 hysteresis, whereby PLC-β3 remains active for some time following either Gα_q-PLC-β3 dissociation or PLC-β3-potentiated Gα_q deactivation, is not sufficient to explain the observed discrepancy between EC₅₀ and K_d These results indicate that the mechanism by which Gα_q and PLC-β3 mutually regulate each other is far more complex than a simple, two-state allosteric model and instead is probably kinetically determined.

Decoding Gαq signaling

Gαq signals with phospholipase C-β (PLC-β) to modify behavior in response to an agonist-bound GPCR. While the fundamental steps which prime Gαq to interact with PLC-β have been identified, questions remain concerning signal strength with PLC-β and other effectors. Gαq is generally viewed to function as a simple ON and OFF switch for its effector, dependent on the binding of GTP or GDP. However, Gαq does not have a single effector, Gαq has many different effectors. Furthermore, select effectors also regulate Gαq activity. PLC-β is a lipase and a GTPase activating protein (GAP) selective for Gαq. The contribution of G protein regulating activity to signal amplitude remains unclear. The unique PLC-β coiled-coil domain is essential for maximum Gαq response, both lipase and GAP. Nonetheless, coiled-coil domain associations necessary to maximum response have not been revealed by the structural approach. This review discusses progress towards understanding the basis for signal strength with PLC-β and other effectors. Shared and effector-specific interactions have been identified. Finally, the evidence for allosteric regulation of lipase stimulation by protein kinase C, the membrane, phosphatidic acid, phosphatidylinositol-4, 5-bisphosphate and GPCR is explored. Endogenous allosteric regulators can suppress or enhance maximum lipase stimulation dependent on the PLC-β coiled-coil domain. A better understanding of allosteric modulation may therefore identify a wealth of new targets to regulate signal strength and behavior.

Intrinsic Pleckstrin Homology (PH) Domain Motion in Phospholipase C-β Exposes a Gβγ Protein Binding Site

Mammalian phospholipase C-β (PLC-β) isoforms are stimulated by heterotrimeric G protein subunits and members of the Rho GTPase family of small G proteins. Although recent structural studies showed how Gαq and Rac1 bind PLC-β, there is a lack of consensus regarding the Gβγ binding site in PLC-β. Using FRET between cerulean fluorescent protein-labeled Gβγ and the Alexa Fluor 594-labeled PLC-β pleckstrin homology (PH) domain, we demonstrate that the PH domain is the minimal Gβγ binding region in PLC-β3. We show that the isolated PH domain can compete with full-length PLC-β3 for binding Gβγ but not Gαq, Using sequence conservation, structural analyses, and mutagenesis, we identify a hydrophobic face of the PLC-β PH domain as the Gβγ binding interface. This PH domain surface is not solvent-exposed in crystal structures of PLC-β, necessitating conformational rearrangement to allow Gβγ binding. Blocking PH domain motion in PLC-β by cross-linking it to the EF hand domain inhibits stimulation by Gβγ without altering basal activity or Gαq response. The fraction of PLC-β cross-linked is proportional to the fractional loss of Gβγ response. Cross-linked PLC-β does not bind Gβγ in a FRET-based Gβγ-PLC-β binding assay. We propose that unliganded PLC-β exists in equilibrium between a closed conformation observed in crystal structures and an open conformation where the PH domain moves away from the EF hands. Therefore, intrinsic movement of the PH domain in PLC-β modulates Gβγ access to its binding site.

Gnb isoforms control a signaling pathway comprising Rac1, Plcβ2, and Plcβ3 leading to LFA-1 activation and neutrophil arrest in vivo

Chemokines are required for leukocyte recruitment and appropriate host defense and act through G protein-coupled receptors (GPCRs), which induce downstream signaling leading to integrin activation. Although the α and β subunits of the GPCRs are the first intracellular molecules that transduce signals after ligand binding and are therefore indispensable for downstream signaling, relatively little is known about their contribution to lymphocyte function-associated antigen 1 (LFA-1) activation and leukocyte recruitment. We used knockout mice and short hairpin RNA to knock down guanine nucleotide binding protein (GNB) isoforms (GNB1, GNB2, GNB4, and GNB5) in HL60 cells and primary murine hematopoietic cells. Neutrophil function was assessed by using intravital microscopy, flow chamber assays, and chemotaxis and biochemistry studies. We unexpectedly discovered that all expressed GNB isoforms are required for LFA-1 activation. Their downregulation led to a significant impairment of LFA-1 activation, which was demonstrated in vitro and in vivo. Furthermore, we showed that GPCR activation leads to Ras-related C3 botulinum toxin substrate 1 (Rac1)-dependent activation of both phospholipase C β2 (Plcβ2) and Plcβ3. They act nonredundantly to produce inositol triphosphate-mediated intracellular Ca(2+) flux and LFA-1 activation that support chemokine-induced arrest in vivo. In a complex inflammatory disease model, Plcβ2-, Plcβ3-, or Rac1-deficient mice were protected from lipopolysaccharide-induced lung injury. Taken together, we demonstrated that all Gnb isoforms are required for chemokine-induced downstream signaling, and Rac1, Plcβ2, and Plcβ3 are critically involved in integrin activation and leukocyte arrest.

Structures of Ras superfamily effector complexes: What have we learnt in two decades?

The Ras superfamily small G proteins are master regulators of a diverse range of cellular processes and act via downstream effector molecules. The first structure of a small G protein-effector complex, that of Rap1A with c-Raf1, was published 20 years ago. Since then, the structures of more than 60 small G proteins in complex with their effectors have been published. These effectors utilize a diverse array of structural motifs to interact with the G protein fold, which we have divided into four structural classes: intermolecular β-sheets, helical pairs, other interactions, and pleckstrin homology (PH) domains. These classes and their representative structures are discussed and a contact analysis of the interactions is presented, which highlights the common effector-binding regions between and within the small G protein families.

Ezrin-related Phosphoinositide pathway modifies RhoA and Rac1 in human osteosarcoma cell lines

Selected Phosphoinositide-specific Phospholipase C (PI-PLC) enzymes occupy the convergence point of the broad range of pathways that promote Rho and Ras GTPase mediated signalling, which also regulate the activation of ezrin, a member of the ezrin-radixin-moesin (ERM) proteins family involved in the metastatic osteosarcoma spread. Previous studies described that in distinct human osteosarcoma cell lines ezrin networks the PI-PLC with complex interplay controlling the expression of the PLC genes, which codify for PI-PLC enzymes. In the present study, we analyzed the expression and the sub-cellular distribution of RhoA and Rac1 respectively after ezrin silencing and after PI-PLC ε silencing, in order to investigate whether ezrin-RhoGTPAses signalling might involve one or more specific PI-PLC isoforms in cultured 143B and Hs888 human osteosarcoma cell lines. In the present experiments, both ezrin and PLCE gene silencing had different effects upon RhoA and Rac1 expression and sub-cellular localization. Displacements of Ezrin and of RhoA localization were observed, probably playing functional roles.

Membrane-induced allosteric control of phospholipase C-β isozymes

All peripheral membrane proteins must negotiate unique constraints intrinsic to the biological interface of lipid bilayers and the cytosol. Phospholipase C-β (PLC-β) isozymes hydrolyze the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) to propagate diverse intracellular responses that underlie the physiological action of many hormones, neurotransmitters, and growth factors. PLC-β isozymes are autoinhibited, and several proteins, including Gαq, Gβγ, and Rac1, directly engage distinct regions of these phospholipases to release autoinhibition. To understand this process, we used a novel, soluble analog of PIP2 that increases in fluorescence upon cleavage to monitor phospholipase activity in real time in the absence of membranes or detergents. High concentrations of Gαq or Gβ1γ2 did not activate purified PLC-β3 under these conditions despite their robust capacity to activate PLC-β3 at membranes. In addition, mutants of PLC-β3 with crippled autoinhibition dramatically accelerated the hydrolysis of PIP2 in membranes without an equivalent acceleration in the hydrolysis of the soluble analog. Our results illustrate that membranes are integral for the activation of PLC-β isozymes by diverse modulators, and we propose a model describing membrane-mediated allosterism within PLC-β isozymes.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

Interaction of Fapp1 with Arf1 and PI4P at a membrane surface: an example of coincidence detection

Interactions among ADP-ribosylation factors (ARFs), various adaptor proteins, and membrane lipids are essential for intracellular vesicle transport of a variety of cellular materials. Here, we present nuclear magnetic resonance (NMR)-based information on the nature of the interaction of yeast Arf1 (yArf1) and the pleckstrin homology (PH) domain of four-phosphate-adaptor protein 1 (Fapp1) as it occurs at a model membrane surface. Interactions favor a model in which Fapp1 is partially embedded in the membrane and interacts with a membrane-associated Arf1 molecule primarily through contacts between residues in switch I of Arf1 and regions near and under the solution exposed C-terminal extension of the PH domain. The Arf1 binding site on Fapp1-PH is distinct from a positively charged phosphatidylinositol-4-phosphate (PI4P) binding site. A structural model is constructed that supports coincidence detection of both activated ARF and PI4P as a mechanism facilitating Fapp1 recruitment to membranes.

Immune regulation by phospholipase C-β isoforms

Rapid progress has recently been made regarding how phospholipase C (PLC)-β functions downstream of G protein-coupled receptors and how PLC-β functions in the nucleus. PLC-β has also been shown to interplay with tyrosine kinase-based signaling pathways, specifically to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1. In this regard, a new multimolecular signaling platform, named SPS complex, has been identified. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation. Furthermore, a growing body of work suggests that PLC-β also participates in the differentiation and activation of immune cells that control both the innate and adaptive immune systems.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Distinct phospholipase C-β isozymes mediate lysophosphatidic acid receptor 1 effects on intestinal epithelial homeostasis and wound closure

Maintenance of the epithelial barrier in the intestinal tract is necessary to protect the host from the hostile luminal environment. Phospholipase C-β (PLC-β) has been implicated to control myriad signaling cascades. However, the biological effects of selective PLC-β isozymes are poorly understood. We describe novel findings that lysophosphatidic acid (LPA) regulates PLC-β1 and PLC-β2 via two distinct pathways to enhance intestinal epithelial cell (IEC) proliferation and migration that facilitate wound closure and recovery of the intestinal epithelial barrier. LPA acting on the LPA1 receptor promotes IEC migration by facilitating the interaction of Gαq with PLC-β2. LPA-induced cell proliferation is PLC-β1 dependent and involves translocation of Gαq to the nucleus, where it interacts with PLC-β1 to induce cell cycle progression. An in vivo study using LPA1-deficient mice (Lpar1(-/-)) shows a decreased number of proliferating IECs and migration along the crypt-luminal axis. Additionally, LPA enhances migration and proliferation of IECs in an LPA1-dependent manner, and Lpar1(-/-) mice display defective mucosal wound repair that requires cell proliferation and migration. These findings delineate novel LPA1-dependent lipid signaling that facilitates mucosal wound repair via spatial targeting of distinct PLC-βs within the cell.

Mammalian phospholipase C

Phospholipase C (PLC) converts phosphatidylinositol 4,5-bisphosphate (PIP(2)) to inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol (DAG). DAG and IP(3) each control diverse cellular processes and are also substrates for synthesis of other important signaling molecules. PLC is thus central to many important interlocking regulatory networks. Mammals express six families of PLCs, each with both unique and overlapping controls over expression and subcellular distribution. Each PLC also responds acutely to its own spectrum of activators that includes heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca(2+), and phospholipids. Mammalian PLCs are autoinhibited by a region in the catalytic TIM barrel domain that is the target of much of their acute regulation. In combination, the PLCs act as a signaling nexus that integrates numerous signaling inputs, critically governs PIP(2) levels, and regulates production of important second messengers to determine cell behavior over the millisecond to hour timescale.

Pleckstrin homology (PH) like domains - versatile modules in protein-protein interaction platforms

The initial reports on pleckstrin homology (PH) domains almost 20 years ago described them as sequence feature of proteins involved in signal transduction processes. Investigated at first along the phospholipid binding properties of a small subset of PH representatives, the PH fold turned out to appear as mediator of phosphotyrosine and polyproline peptide binding to other signaling proteins. While phospholipid binding now seems rather the exception among PH-like domains, protein-protein interactions established as more and more important feature of these modules. In this review we focus on the PH superfold as a versatile protein-protein interaction platform and its three-dimensional integration in an increasing number of available multidomain structures.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

The correlation between multidomain enzymes and multiple activation mechanisms--the case of phospholipase Cβ and its membrane interactions

Phospholipase Cβ2 (PLCβ2) is a large, multidomain enzyme that catalyzes the hydrolysis of the signaling lipid phosphoinositol 4,5 bisphosphate (PIP2) to promote mitogenic and proliferative changes in the cell. PLCβ2 is activated by Gα and Gβγ subunits of heterotrimeric G proteins, as well as small G proteins and specific peptides. Activation depends on the nature of the membrane surface. Recent crystal structures suggest one model of activation involving the movement of a small autoinhibitory loop upon membrane binding of the enzyme. Additionally, solution studies indicate multiple levels of activation that involve changes in the membrane orientation as well as interdomain movement. Here, we review the wealth of biochemical studies of PLCβ2-G protein activation and propose a comprehensive model that accounts for both the crystallographic and solution results.

Molecular basis of phosphatidylinositol 4-phosphate and ARF1 GTPase recognition by the FAPP1 pleckstrin homology (PH) domain

Four-phosphate-adaptor protein 1 (FAPP1) regulates secretory transport from the trans-Golgi network (TGN) to the plasma membrane. FAPP1 is recruited to the Golgi through binding of its pleckstrin homology (PH) domain to phosphatidylinositol 4-phosphate (PtdIns(4)P) and a small GTPase ADP-ribosylation factor 1 (ARF1). Despite the critical role of FAPP1 in membrane trafficking, the molecular basis of its dual function remains unclear. Here, we report a 1.9 Å resolution crystal structure of the FAPP1 PH domain and detail the molecular mechanisms of the PtdIns(4)P and ARF1 recognition. The FAPP1 PH domain folds into a seven-stranded β-barrel capped by an α-helix at one edge, whereas the opposite edge is flanked by three loops and the β4 and β7 strands that form a lipid-binding pocket within the β-barrel. The ARF1-binding site is located on the outer side of the β-barrel as determined by NMR resonance perturbation analysis, mutagenesis, and measurements of binding affinities. The two binding sites have little overlap, allowing FAPP1 PH to associate with both ligands simultaneously and independently. Binding to PtdIns(4)P is enhanced in an acidic environment and is required for membrane penetration and tubulation activity of FAPP1, whereas the GTP-bound conformation of the GTPase is necessary for the interaction with ARF1. Together, these findings provide structural and biochemical insight into the multivalent membrane anchoring by the PH domain that may augment affinity and selectivity of FAPP1 toward the TGN membranes enriched in both PtdIns(4)P and GTP-bound ARF1.

Subtype-specific roles of phospholipase C-β via differential interactions with PDZ domain proteins

Since we first identified the PLC-β isozyme, enormous studies have been conducted to investigate the functional roles of this protein (Min et al., 1993; Suh et al.,1988). It is now well-known that the four PLC-β subtypes are major effector molecules in GPCR-mediated signaling, especially for intracellular Ca2+ signaling. Nonetheless, it is still poorly understood why multiple PLC-β subtype exist. Most cells express multiple subtypes of PLC-β in different combinations, and each subtype is involved in somewhat different signaling pathways. Therefore, studying the differential roles of each PLC-β subtype is a very interesting issue. In this regard, we focus here on PDZ domain proteins which are novel PLC-β interacting proteins. As scaffolders, PDZ domain proteins recruit various target proteins ranging from membrane receptors to cytoskeletal proteins to assemble highly organized signaling complexes; this can give rise to efficiency and diversity in cellular signaling. Because PLC-β subtypes have different PDZ-binding motifs, it is possible that they are engaged with different PDZ domain proteins, and in turn participate in distinct physiological responses. To date, several PDZ domain proteins, such as the NHERF family, Shank2, and Par-3, have been reported to selectively interact with certain PLC-β subtypes and GPCRs. Systematic predictions of potential binding partners also suggests differential binding properties between PLC-β subtypes. Furthermore, we elucidated parallel signaling processes for multiple PLC-β subtypes, which still perform distinct functions resulting from differential interactions with PDZ domain proteins within a single cell. Therefore, these results highlight the novel function of PDZ domain proteins as intermediaries in subtype-specific role of PLC-β in GPCR-mediated signaling. Future studies will focus on the physiological meanings of this signaling complex formation by different PDZ domain proteins and PLC-β subtypes. It has been observed for a long time that the expression of certain PLC-β subtype fluctuates during diverse physiological conditions. For example, the expression of PLC-β1 is selectively increased during myoblast and adipocyte differentiation (Faenza et al., 2004; O'Carroll et al., 2009). Likewise, PLC-β2 is highly up-regulated during breast cancer progression and plays a critical role in cell migration and mitosis (Bertagnolo et al., 2007). Although PLC-β3 is selectively down-regulated in neuroendocrine tumors, the expression of PLC-β1 is increased in small cell lung carcinoma (Stalberg et al., 2003; Strassheim et al., 2000). In our hypothetical model, it is most likely that up- and down regulation of certain PLC-β subtypes are due to their selective coupling with specific GPCR-mediated signaling, implicated in these pathophysiologic conditions. Therefore, better understanding of selective coupling between PLC-β subtypes, PDZ domain proteins, and GPCRs will shed light on new prognosis and therapy of diverse diseases, and provide potential targets for drug development.

The small G protein Rac1 activates phospholipase Cdelta1 through phospholipase Cbeta2

Rac1, which is associated with cytoskeletal pathways, can activate phospholipase Cbeta2 (PLCbeta2) to increase intracellular Ca(2+) levels. This increased Ca(2+) can in turn activate the very robust PLCdelta1 to synergize Ca(2+) signals. We have previously found that PLCbeta2 will bind to and inhibit PLCdelta1 in solution by an unknown mechanism and that PLCbeta2.PLCdelta1 complexes can be disrupted by Gbetagamma subunits. However, because the major populations of PLCbeta2 and PLCdelta1 are cytosolic, their regulation by Gbetagamma subunits is not clear. Here, we have found that the pleckstrin homology (PH) domains of PLCbeta2 and PLCbeta3 are the regions that result in PLCdelta1 binding and inhibition. In cells, PLCbeta2.PLCdelta1 form complexes as seen by Förster resonance energy transfer and co-immunoprecipitation, and microinjection of PHbeta2 dissociates the complex. Using PHbeta2 as a tool to assess the contribution of PLCbeta inhibition of PLCdelta1 to Ca(2+) release, we found that, although PHbeta2 only results in a 25% inhibition of PLCdelta1 in solution, in cells the presence of PHbeta2 appears to eliminates Ca(2+) release suggesting a large threshold effect. We found that the small plasma membrane population of PLCbeta2.PLCdelta1 is disrupted by activation of heterotrimeric G proteins, and that the major cytosolic population of the complexes are disrupted by Rac1 activation. Thus, the activity of PLCdelta1 is controlled by the amount of bound PLCbeta2 that changes with displacement of the enzyme by heterotrimeric or small G proteins. Through PLCbeta2, PLCdelta1 activation is linked to surface receptors as well as signals that mediate cytoskeletal pathways.

Clostridium difficile toxin B differentially affects GPCR-stimulated Ca2+ responses in macrophages: independent roles for Rho and PLA2

Clostridium difficile toxins cause acute colitis by disrupting the enterocyte barrier and promoting inflammation. ToxB from C. difficile inactivates Rho family GTPases and causes release of cytokines and eicosanoids by macrophages. We studied the effects of ToxB on GPCR signaling in murine RAW264.7 macrophages and found that ToxB elevated Ca(2+) responses to Galphai-linked receptors, including the C5aR, but reduced responses to Galphaq-linked receptors, including the UDP receptors. Other Rho inhibitors also reduced UDP Ca(2+) responses, but they did not affect C5a responses, suggesting that ToxB inhibited UDP responses by inhibiting Rho but enhanced C5a responses by other mechanisms. By using PLCbeta isoform-deficient BMDM, we found that ToxB inhibited Ca(2+) signaling through PLCbeta4 but enhanced signaling through PLCbeta3. Effects of ToxB on GPCR Ca(2+) responses correlated with GPCR use of PLCbeta3 versus PLCbeta4. ToxB inhibited UDP Ca(2+) signaling without reducing InsP3 production or the sensitivity of cellular Ca(2+) stores to exogenous InsP3, suggesting that ToxB impairs UDP signaling at the level of InsP3/Ca(2+)coupling. In contrast, ToxB elevated InsP3 production by C5a, and the enhancement of Ca(2+) signaling by C5a was prevented by inhibition of PLA(2) or 5-LOX but not COX, implicating LTs but not prostanoids in the mechanism. In sum, ToxB has opposing, independently regulated effects on Ca(2+) signaling by different GPCR-linked PLCbeta isoforms in macrophages.

Differential regulation of phospholipase C-beta2 activity and membrane interaction by Galphaq, Gbeta1gamma2, and Rac2

We combined fluorescence recovery after photobleaching (FRAP) beam-size analysis with biochemical assays to investigate the mechanisms of membrane recruitment and activation of phospholipase C-beta(2) (PLCbeta(2)) by G protein alpha(q) and betagamma dimers. We show that activation by alpha(q) and betagamma differ from activation by Rac2 and from each other. Stimulation by alpha(q) enhanced the plasma membrane association of PLCbeta(2), but not of PLCbeta(2)Delta, which lacks the alpha(q)-interacting region. Although alpha(q) resembled Rac2 in increasing the contribution of exchange to the FRAP of PLCbeta(2) and in enhancing its membrane association, the latter effect was weaker than with Rac2. Moreover, the membrane recruitment of PLCbeta(2) by alpha(q) occurred by enhancing PLCbeta(2) association with fast-diffusing (lipid-like) membrane components, whereas stimulation by Rac2 led to interactions with slow diffusing membrane sites. On the other hand, activation by betagamma shifted the FRAP of PLCbeta(2) and PLCbeta(2)Delta to pure lateral diffusion 3- to 5-fold faster than lipids, suggesting surfing-like diffusion along the membrane. We propose that these different modes of PLCbeta(2) membrane recruitment may accommodate contrasting functional needs to hydrolyze phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) in localized versus dispersed populations. PLCbeta(2) activation by Rac2, which leads to slow lateral diffusion and much faster exchange, recruits PLCbeta(2) to act locally on PtdInsP(2) at specific domains. Activation by alpha(q) leads to lipid-like diffusion of PLCbeta(2) accompanied by exchange, enabling the sampling of larger, yet limited, areas prior to dissociation. Finally, activation by betagamma recruits PLCbeta(2) to the membrane by transient interactions, leading to fast "surfing" diffusion along the membrane, sampling large regions for dispersed PtdInsP(2) populations.

Rac1 modulates stimulus-evoked Ca(2+) release in neuronal growth cones via parallel effects on microtubule/endoplasmic reticulum dynamics and reactive oxygen species production

The small G protein Rac regulates cytoskeletal protein dynamics in neuronal growth cones and has been implicated in axon growth, guidance, and branching. Intracellular Ca(2+) is another well known regulator of growth cone function; however, effects of Rac activity on intracellular Ca(2+) metabolism have not been well characterized. Here, we investigate how Rac1 activity affects release of Ca(2+) from intracellular endoplasmic reticulum (ER) stores stimulated by application of serotonin (5-hydroxytriptamine). We also address how Rac1 effects on microtubule assembly dynamics affect distribution of Ca(2+) release sites. Multimode fluorescent microscopy was used to correlate microtubule and ER behavior, and ratiometric imaging was used to assess intracellular Ca(2+) dynamics. We report that Rac1 activity both promotes Ca(2+) release and affects its spatial distribution in neuronal growth cones. The underlying mechanism involves synergistic Rac1 effects on microtubule assembly and reactive oxygen species (ROS) production. Rac1 activity modulates Ca(2+) by 1) enhancing microtubule assembly which in turn promotes spread of the ER-based Ca(2+) release machinery into the growth cone periphery, and 2) by increasing ROS production which facilitated inositol 1,4,5-trisphosphate-dependent Ca(2+) release. These results cast Rac1 as a key modulator of intracellular Ca(2+) function in the neuronal growth cone.

In vitro reconstitution of activation of PLCepsilon by Ras and Rho GTPases

Phosphatidylinositol-specific phospholipase C (PLC) enzymes catalyze the hydrolysis of phophatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to diacylglycerol (DAG) and inositol 1,4,5-triphosphate [Ins(1,4,5)P3]. PLCepsilon is a recently discovered isoform that has been shown to be activated by members of the Ras and Rho families of guanosine trisphosphatases (GTPases) as well as subunits of heterotrimeric G-proteins. We describe a method for expressing a truncated PLCepsilon variant as an MBP fusion protein in E. coli. Subsequently, we describe the methodology necessary to reconstitute this protein with K-Ras-4B and RhoA GTPases and measure its activation.

Phospholipase C isozymes as effectors of Ras superfamily GTPases

The physiological effects of many extracellular stimuli are initiated through receptor-promoted activation of phospholipase C and inositol lipid signaling pathways. The historical view that phospholipase C-promoted signaling primarily occurs through activation of heterotrimeric G proteins or tyrosine kinases has expanded in recent years with the realization that at least three different mammalian phospholipase C isozymes are directly activated by members of the Ras superfamily of GTPases. Thus, Ras, Rap, Rac, and Rho GTPases all specifically regulate certain phospholipase C isozymes, and insight into the physiological significance of these signaling responses is beginning to accrue. High resolution three-dimensional structures of phospholipase C isozymes also are beginning to shed light on their mechanism of activation.

rac regulates its effector phospholipase Cgamma2 through interaction with a split pleckstrin homology domain

Several isoforms of phospholipase C (PLC) are regulated through interactions with Ras superfamily GTPases, including Rac proteins. Interestingly, of two closely related PLCgamma isoforms, only PLCgamma(2) has previously been shown to be activated by Rac. Here, we explore the molecular basis of this interaction as well as the structural properties of PLCgamma(2) required for activation. Based on reconstitution experiments with isolated PLCgamma variants and Rac2, we show that an unusual pleckstrin homology (PH) domain, designated as the split PH domain (spPH), is both necessary and sufficient to effect activation of PLCgamma(2) by Rac2. We also demonstrate that Rac2 directly binds to PLCgamma(2) as well as to the isolated spPH of this isoform. Furthermore, through the use of NMR spectroscopy and mutational analysis, we determine the structure of spPH, define the structural features of spPH required for Rac interaction, and identify critical amino acid residues at the interaction interface. We further discuss parallels and differences between PLCgamma(1) and PLCgamma(2) and the implications of our findings for their respective signaling roles.

The Salmonella virulence protein SifA is a G protein antagonist

Salmonella's success at proliferating intracellularly and causing disease depends on the translocation of a major virulence protein, SifA, into the host cell. SifA recruits membranes enriched in lysosome associated membrane protein 1 (LAMP1) and is needed for growth of Salmonella induced filaments (Sifs) and the Salmonella containing vacuole (SCV). It directly binds a host protein called SKIP (SifA and kinesin interacting protein) which is critical for membrane stability and motor dynamics at the SCV. SifA also contains a WxxxE motif, predictive of G protein mimicry in bacterial effectors, but whether and how it mimics the action of a host G protein is not known. We show that SKIP's pleckstrin homology domain, which directly binds SifA, also binds to the late endosomal GTPase Rab9. Knockdown studies suggest that both SKIP and Rab9 function to maintain peripheral LAMP1 distribution in cells. The Rab9:SKIP interaction is GTP-dependent and is inhibited by SifA binding to the SKIP pleckstrin homology domain, suggesting that SifA may be a Rab9 antagonist. SifA:SKIP binding is significantly tighter than Rab9:SKIP binding and may thus allow SifA to bring SKIP to the SCV via SKIP's Rab9-binding site. Rab9 can measurably reverse SifA-dependent LAMP1 recruitment and the perinuclear location of the SCV in cells. Importantly, binding to SKIP requires SifA residues W197 and E201 of the conserved WxxxE signature sequence, leading to the speculation that bacterial G protein mimicry may result in G protein antagonism.

General and versatile autoinhibition of PLC isozymes

Phospholipase C (PLC) isozymes are directly activated by heterotrimeric G proteins and Ras-like GTPases to hydrolyze phosphatidylinositol 4,5-bisphosphate into the second messengers diacylglycerol and inositol 1,4,5-trisphosphate. Although PLCs play central roles in myriad signaling cascades, the molecular details of their activation remain poorly understood. As described here, the crystal structure of PLC-beta2 illustrates occlusion of the active site by a loop separating the two halves of the catalytic TIM barrel. Removal of this insertion constitutively activates PLC-beta2 without ablating its capacity to be further stimulated by classical G protein modulators. Similar regulation occurs in other PLC members, and a general mechanism of interfacial activation at membranes is presented that provides a unifying framework for PLC activation by diverse stimuli.

Dual activation of phospholipase C-epsilon by Rho and Ras GTPases

Phospholipase C-epsilon (PLC-epsilon) is a highly elaborated PLC required for a diverse set of signaling pathways. Here we use a combination of cellular assays and studies with purified proteins to show that activated RhoA and Ras isoforms directly engage distinct regions of PLC-epsilon to stimulate its phospholipase activity. Purified PLC-epsilon was activated in a guanine nucleotide- and concentration-dependent fashion by purified lipidated K-Ras reconstituted in PtdIns(4,5)P(2)-containing phospholipid vesicles. Whereas mutation of two critical lysine residues within the second Ras-association domain of PLC-epsilon prevented K-Ras-dependent activation of the purified enzyme, guanine nucleotide-dependent activation by RhoA was retained. Deletion of a loop unique to PLC-epsilon eliminated its activation by RhoA but not H-Ras. In contrast, removal of the autoinhibitory X/Y-linker region of the catalytic core of PLC-epsilon markedly activates the enzyme (Hicks, S. N., Jezyk, M. R., Gershburg, S., Seifert, J. P., Harden, T. K., and Sondek, J. (2008) Mol. Cell, 31, 383-394), but PLC-epsilon lacking this regulatory region retained activation by both Rho and Ras GTPases. Additive activation of PLC-epsilon by RhoA and K- or H-Ras was observed in intact cell studies, and this additivity was recapitulated in experiments in which activation of purified PLC-epsilon was quantified with PtdIns(4,5)P(2)-containing phospholipid vesicles reconstituted with purified, isoprenylated GTPases. A maximally effective concentration of activated RhoA also increased the sensitivity of purified PLC-epsilon to activation by K-Ras. These results indicate that PLC-epsilon can be directly and concomitantly activated by both RhoA and individual Ras GTPases resulting in diverse upstream control of signaling cascades downstream of PLC-epsilon.

Dynamic interaction between Arf GAP and PH domains of ASAP1 in the regulation of GAP activity

ASAP family Arf GAPs induce the hydrolysis of GTP bound to the Ras superfamily protein Arf1, regulate cell adhesion and migration and have been implicated in carcinogenesis. The ASAP proteins have a core catalytic domain of PH, Arf GAP and Ank repeat domains. The PH domain is necessary for both biological and catalytic functions of ASAP1 and has been proposed to be integrally folded with the Arf GAP domain. Protection studies and analytical ultracentrifugation studies previously reported indicated that the domains are, at least partly, folded together. Here, using NMR spectroscopy and biochemical analysis, we have further tested this hypothesis and characterized the interdomain interaction. A comparison of NMR spectra of three recombinant proteins comprised of either the isolated PH domain of ASAP1, the Arf GAP and ankyrin repeat domain or all three domains indicated that the PH domain did interact with the Arf GAP and Ank repeat domains; however, we found a significant amount of dynamic independence between the PH and Arf GAP domains, consistent with the interactions being transient. In contrast, the Arf GAP and Ank repeat domains form a relatively rigid structure. The PH-Arf GAP domain interaction partially occluded the phosphoinositide binding site in the soluble protein, but binding studies indicated the PIP2 binding site was accessible in ASAP1 bound to a lipid bilayer surface. Phosphoinositide binding altered the conformation of the PH domain, but had little effect on the structure of the Arf GAP domain. Mutations in a loop of the PH domain that contacts the Arf GAP domain affected PIP2 binding and the K(m) and k(cat) for converting Arf1 GTP to Arf1 GDP. Based on these results, we generated a homology model of a composite PH/Arf GAP/Ank repeat domain structure. We propose that the PH domain contributes to Arf GAP activity by either binding to or positioning Arf1 GTP that is simultaneously bound to the Arf GAP domain.

Structural determinants underlying the temperature-sensitive nature of a Galpha mutant in asymmetric cell division of Caenorhabditis elegans

Heterotrimeric G-proteins are integral to a conserved regulatory module that influences metazoan asymmetric cell division (ACD). In the Caenorhabditis elegans zygote, GOA-1 (Galpha(o)) and GPA-16 (Galpha(i)) are involved in generating forces that pull on astral microtubules and position the spindle asymmetrically. GPA-16 function has been analyzed in vivo owing notably to a temperature-sensitive allele gpa-16(it143), which, at the restrictive temperature, results in spindle orientation defects in early embryos. Here we identify the structural basis of gpa-16(it143), which encodes a point mutation (G202D) in the switch II region of GPA-16. Using Galpha(i1)(G202D) as a model in biochemical analyses, we demonstrate that high temperature induces instability of the mutant Galpha. At the permissive temperature, the mutant Galpha was stable upon GTP binding, but switch II rearrangement was compromised, as were activation state-selective interactions with regulators involved in ACD, including GoLoco motifs, RGS proteins, and RIC-8. We solved the crystal structure of the mutant Galpha bound to GDP, which indicates a unique switch II conformation as well as steric constraints that suggest activated GPA-16(it143) is destabilized relative to wild type. Spindle severing in gpa-16(it143) embryos revealed that pulling forces are symmetric and markedly diminished at the restrictive temperature. Interestingly, pulling forces are asymmetric and generally similar in magnitude to wild type at the permissive temperature despite defects in the structure of GPA-16(it143). These normal pulling forces in gpa-16(it143) embryos at the permissive temperature were attributable to GOA-1 function, underscoring a complex interplay of Galpha subunit function in ACD.