Specificity and structural requirements of phospholipase C-beta stimulation by Rho GTPases versus G protein beta gamma dimers

An integrated mechanism of Gq regulation of PLCβ enzymes

Phospholipase Cβ (PLCβ) enzymes are the principal effectors activated by Gq heterotrimers. Both Gαq and Gβγ subunits can activate PLCβ, which requires precise positioning of PLCβ at the plasma membrane to relieve structural autoinhibition and give the active site access to the phosphatidylinositol 4,5-bisphosphate (PIP2) substrate. PLCβ enzymes possess a unique distal C-terminal domain (dCTD) that is critical for activation by Gαq, but the reason for this is unclear. It is also not known how G protein activation affects the subcellular localization of PLCβ enzymes, some of which are found primarily in the cytosol despite needing to act at the plasma membrane. Here, we use bioluminescence spectroscopy, imaging, and gene editing to study the membrane disposition of PLCβ enzymes in living cells and to define the functional roles of the dCTD. We find that PLCβ translocates to the plasma membrane upon Gq activation, primarily by binding to Gαq subunits. This is rapidly counteracted by PIP2 hydrolysis, which promotes PLCβ translocation back into the cytosol. PLCβ translocation and activation require binding of Gαq to the catalytic domain and the dCTD at two distinct interfaces. Gαq binding to the dCTD is required for activation even when PLCβ is artificially tethered to the plasma membrane, suggesting that this domain has functions beyond simply recruiting the enzyme to the PIP2 substrate. We propose that in addition to associating PLCβ with the plasma membrane, the dCTD reorders the αN helix of active Gαq and thus participates directly in the precise positioning of the catalytic domain.

Activation Mechanisms and Diverse Functions of Mammalian Phospholipase C

Phospholipase C (PLC) plays pivotal roles in regulating various cellular functions by metabolizing phosphatidylinositol 4,5-bisphosphate in the plasma membrane. This process generates two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol, which respectively regulate the intracellular Ca²⁺ levels and protein kinase C activation. In mammals, six classes of typical PLC have been identified and classified based on their structure and activation mechanisms. They all share X and Y domains, which are responsible for enzymatic activity, as well as subtype-specific domains. Furthermore, in addition to typical PLC, atypical PLC with unique structures solely harboring an X domain has been recently discovered. Collectively, seven classes and 16 isozymes of mammalian PLC are known to date. Dysregulation of PLC activity has been implicated in several pathophysiological conditions, including cancer, cardiovascular diseases, and neurological disorders. Therefore, identification of new drug targets that can selectively modulate PLC activity is important. The present review focuses on the structures, activation mechanisms, and physiological functions of mammalian PLC.

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.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Phosphoinositide-Dependent Signaling in Cancer: A Focus on Phospholipase C Isozymes

Phosphoinositides (PI) form just a minor portion of the total phospholipid content in cells but are significantly involved in cancer development and progression. In several cancer types, phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P₃] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] play significant roles in regulating survival, proliferation, invasion, and growth of cancer cells. Phosphoinositide-specific phospholipase C (PLC) catalyze the generation of the essential second messengers diacylglycerol (DAG) and inositol 1,4,5 trisphosphate (InsP₃) by hydrolyzing PtdIns(4,5)P₂. DAG and InsP₃ regulate Protein Kinase C (PKC) activation and the release of calcium ions (Ca²⁺) into the cytosol, respectively. This event leads to the control of several important biological processes implicated in cancer. PLCs have been extensively studied in cancer but their regulatory roles in the oncogenic process are not fully understood. This review aims to provide up-to-date knowledge on the involvement of PLCs in cancer. We focus specifically on PLCβ, PLCγ, PLCδ, and PLCε isoforms due to the numerous evidence of their involvement in various cancer types.

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.

Gαq and the Phospholipase Cβ3 X-Y Linker Regulate Adsorption and Activity on Compressed Lipid Monolayers

Phospholipase Cβ (PLCβ) enzymes are peripheral membrane proteins required for normal cardiovascular function. PLCβ hydrolyzes phosphatidylinositol 4,5-bisphosphate, producing second messengers that increase intracellular Ca²⁺ level and activate protein kinase C. Under basal conditions, PLCβ is autoinhibited by its C-terminal domains and by the X-Y linker, which contains a stretch of conserved acidic residues required for interfacial activation. Following stimulation of G protein-coupled receptors, the heterotrimeric G protein subunit Gα_q allosterically activates PLCβ and helps orient the activated complex at the membrane for efficient lipid hydrolysis. However, the molecular basis for how the PLCβ X-Y linker, its C-terminal domains, Gα_q, and the membrane coordinately regulate activity is not well understood. Using compressed lipid monolayers and atomic force microscopy, we found that a highly conserved acidic region of the X-Y linker is sufficient to regulate adsorption. Regulation of adsorption and activity by the X-Y linker also occurs independently of the C-terminal domains. We next investigated whether Gα_q-dependent activation of PLCβ altered interactions with the model membrane. Gα_q increased PLCβ adsorption in a manner that was independent of the PLCβ regulatory elements and targeted adsorption to specific regions of the monolayer in the absence of the C-terminal domains. Thus, the mechanism of Gα_q-dependent activation likely includes a spatial component.

Activation of the Endoplasmic Reticulum Stress Response Impacts the NOD1 Signaling Pathway

Nucleotide-binding oligomerization domain 1 (NOD1) is an intracellular pattern recognition receptor (PRR) responsible for sensing bacterial peptidoglycan fragments. Stimulation of NOD1 leads to a robust innate immune response via activation of the major transcription factor NF-κB. In addition to peptidoglycan sensing, NOD1 and the closely related PRR NOD2 have been linked to inflammation by responding to the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR).

Nucleotide-binding oligomerization domain 1 (NOD1) is an intracellular pattern recognition receptor (PRR) responsible for sensing bacterial peptidoglycan fragments. Stimulation of NOD1 leads to a robust innate immune response via activation of the major transcription factor NF-κB. In addition to peptidoglycan sensing, NOD1 and the closely related PRR NOD2 have been linked to inflammation by responding to the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR). Here we show that differential ER stress induction renders cells more susceptible to Salmonella enterica serovar Typhimurium infection in a NOD1-dependent manner, measured by increased NF-κB activation and cytokine expression. In HeLa57A cells stably transfected with an NF-κB::luciferase reporter, we show that cells undergoing ER stress induced by thapsigargin display a significant increase in NF-κB activation in response to NOD1 stimulation by C12-iE-DAP (acylated derivative of the iE-DAP dipeptide [gamma-d-glutamyl-meso-diaminopimelic acid]) and the S. Typhimurium effector protein SopE. Tunicamycin-induced ER stress had no effect on NOD1-stimulated NF-κB activation. We further show that the mouse intestinal epithelial cell line MODE-K and RAW264.7 macrophages are more responsive to Salmonella infection when treated with thapsigargin but not with tunicamycin. These profound differences between thapsigargin- and tunicamycin-treated cells upon inflammation suggest that different components downstream of the UPR contribute to NOD1 activation. We found that the NOD1-induced inflammatory response is dependent on protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) activation in conjunction with stimulation of the inositol triphosphate receptor (IP3R). Together, these results suggest that differential UPR activation makes cells more responsive to bacterial infections in a NOD1-dependent manner.

Phospholipase Cβ3 Membrane Adsorption and Activation Are Regulated by Its C-Terminal Domains and Phosphatidylinositol 4,5-Bisphosphate

Phospholipase Cβ (PLCβ) enzymes hydrolyze phosphatidylinositol 4,5-bisphosphate to produce second messengers that regulate intracellular Ca²⁺, cell proliferation, and survival. Their activity is dependent upon interfacial activation that occurs upon localization to cell membranes. However, the molecular basis for how these enzymes productively interact with the membrane is poorly understood. Herein, atomic force microscopy demonstrates that the ∼300-residue C-terminal domain promotes adsorption to monolayers and is required for spatial organization of the protein on the monolayer surface. PLCβ variants lacking this C-terminal domain display differences in their distribution on the surface. In addition, a previously identified autoinhibitory helix that binds to the PLCβ catalytic core negatively impacts membrane binding, providing an additional level of regulation for membrane adsorption. Lastly, defects in phosphatidylinositol 4,5-bisphosphate hydrolysis also alter monolayer adsorption, reflecting a role for the active site in this process. Together, these findings support a model in which multiple elements of PLCβ modulate adsorption, distribution, and catalysis at the cell membrane.

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.

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.

Regulation of phosphatidylinositol-specific phospholipase C at the nuclear envelope in cardiac myocytes

Phosphatidylinositol 4,5-bisphosphate hydrolysis at the plasma membrane by phospholipase C is one of the major hormone regulated intracellular signaling systems. The system generates the diffusible second messenger IP3 and the membrane bound messenger diacylglycerol. Spatial regulation of this system has been thought to be through specific subcellular distributions of the IP3 receptor or PKC. As is becoming increasingly apparent, receptor-stimulated signaling systems are also found at intracellular membranes. As discussed in this issue, G protein-coupled receptors have been identified at the nuclear envelope implying intracellular localization of the signaling systems that respond to G protein-coupled receptors. Here, we discuss the evidence for the existence of PLC signals that regulate nuclear processes, as well as the evidence for nuclear and nuclear envelope localization of PLC signaling components, and their implications for cardiac physiology and disease.

The Novel Functions of the PLC/PKC/PKD Signaling Axis in G Protein-Coupled Receptor-Mediated Chemotaxis of Neutrophils

Chemotaxis, a directional cell migration guided by extracellular chemoattractant gradients, plays an essential role in the recruitment of neutrophils to sites of inflammation. Chemotaxis is mediated by the G protein-coupled receptor (GPCR) signaling pathway. Extracellular stimuli trigger activation of the PLC/PKC/PKD signaling axis, which controls several signaling pathways. Here, we concentrate on the novel functions of PLC/PKC/PKD signaling in GPCR-mediated chemotaxis of neutrophils.

Molecular mechanisms of phospholipase C β3 autoinhibition

Phospholipase C β (PLCβ) enzymes are dramatically activated by heterotrimeric G proteins. Central to this response is the robust autoinhibition of PLCβ by the X-Y linker region within its catalytic core and by the Hα2' helix in the C-terminal extension of the enzyme. The molecular mechanism of each and their mutual dependence are poorly understood. Herein, it is shown that distinct regions within the X-Y linker have specific roles in regulating activity. Most important,an acidic stretch within the linker stabilizes a lid that occludes the active site, consistent with crystal structures of variants lacking this region. Inhibition by the Hα2' helix is independent of the X-Y linker and likely regulates activity by limiting membrane interaction of the catalytic core. Full activation of PLCβ thus requires multiple independent molecular events induced by membrane association of the catalytic core and by the binding of regulatory proteins.

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.

Emerging non-canonical functions for heterotrimeric G proteins in cellular signaling

Classically heterotrimeric G proteins have been described as the principal signal transducing machinery for G-protein-coupled receptors. Receptor activation catalyzes nucleotide exchange on the Gα protein, enabling Gα-GTP and Gβγ-subunits to engage intracellular effectors to generate various cellular effects such as second messenger production or regulation of ion channel conductivity. Recent genetic and proteomic screens have identified novel heterotrimeric G-protein-interacting proteins and expanded their functional roles. This review highlights some examples of recently identified interacting proteins and summarizes how they functionally connect heterotrimeric G proteins to previously underappreciated cellular roles.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

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.

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.

Chemokines and the signaling modules regulating integrin affinity

Integrin-mediated adhesion is a general concept referring to a series of adhesive phenomena including tethering–rolling, affinity, valency, and binding stabilization altogether controlling cell avidity (adhesiveness) for the substrate. Arrest chemokines modulate each aspect of integrin activation, although integrin affinity regulation has been recognized as the prominent event in rapid leukocyte arrest induced by chemokines. A variety of inside-out and outside-in signaling mechanisms have been related to the process of integrin-mediated adhesion in different cellular models, but only few of them have been clearly contextualized to rapid integrin affinity modulation by arrest chemokines in primary leukocytes. Complex signaling processes triggered by arrest chemokines and controlling leukocyte integrin activation have been described for ras-related rap and for rho-related small GTPases. We summarize the role of rap and rho small GTPases in the regulation of rapid integrin affinity in primary leukocytes and provide a modular view of these pro-adhesive signaling events. A potential, albeit still speculative, mechanism of rho-mediated regulation of cytoskeletal proteins controlling the last step of integrin activation is also discussed. We also discuss data suggesting a functional integration between the rho- and rap-modules of integrin activation. Finally we examine the universality of signaling mechanisms regulating integrin triggering by arrest chemokines.

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.

LARG links histamine-H1-receptor-activated Gq to Rho-GTPase-dependent signaling pathways

Activation of heterotrimeric G proteins, such as G(12/13) and G(q), by cell surface receptors is coupled to the regulation of numerous cellular functions controlled by activated Rho GTPases. Previous studies have implicated the Rho guanine nucleotide exchange factor (RhoGEF) leukemia-associated RhoGEF (LARG) as a regulatory protein receiving stimulatory inputs from activated Gα(12/13) and Gα(q). However, the molecular mechanisms of the Gα(q)-mediated LARG activation are not fully understood and the structural elements of LARG involved in this process have remained unclear. In the present work, the specific coupling of the histamine H1 receptor (HRH1) exogenously expressed in COS-7 cells to G(q), but not to G(12/13), was used to conduct a detailed analysis of receptor- and Gα(q)-mediated LARG activation and to define its structural requirements. The results show that HRH1-mediated activation of the strictly Rho-dependent transcriptional activity of serum response factor requires the PDZ domain of LARG and can be mimicked by activated Gα(q)(Q209L). The functional interaction between activated Gα(q) and LARG requires no more than the catalytic DH-PH tandem of LARG, and is independent of PLCβ activation and distinct from the mechanisms of Gα(q)-mediated p63RhoGEF and PLCβ(3) activation. Activated Gα(q) physically interacts with the relevant portions of LARG in COS-7 cells and histamine causes activation of LARG in native HeLa cells endogenously expressing HRH1, G(q), and LARG. This work is the first positive demonstration of a stimulatory effect of LARG on the ability of a strictly G(q)-coupled receptor to cause activation of a Rho-GTPase-dependent signaling pathway.

One lipid, multiple functions: how various pools of PI(4,5)P(2) are created in the plasma membrane

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] is a minor lipid of the inner leaflet of the plasma membrane that controls the activity of numerous proteins and serves as a source of second messengers. This multifunctionality of PI(4,5)P(2) relies on mechanisms ensuring transient appearance of PI(4,5)P(2) clusters in the plasma membrane. One such mechanism involves phosphorylation of PI(4)P to PI(4,5)P(2) by the type I phosphatidylinositol-4-phosphate 5-kinases (PIP5KI) at discrete membrane locations coupled with PI(4)P delivery/synthesis at the plasma membrane. Simultaneously, both PI(4)P and PI(4,5)P(2) participate in anchoring PIP5KI at the plasma membrane via electrostatic bonds. PIP5KI isoforms are also selectively recruited and activated at the plasma membrane by Rac1, talin, or AP-2 to generate PI(4,5)P(2) in ruffles and lamellipodia, focal contacts, and clathrin-coated pits. In addition, PI(4,5)P(2) can accumulate at sphingolipid/cholesterol-based rafts following activation of distinct membrane receptors or be sequestered in a reversible manner due to electrostatic constrains posed by proteins like MARCKS.

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.

Structural insights into formation of an active signaling complex between Rac and phospholipase C gamma 2

Rho family GTPases are important cellular switches and control a number of physiological functions. Understanding the molecular basis of interaction of these GTPases with their effectors is crucial in understanding their functions in the cell. Here we present the crystal structure of the complex of Rac2 bound to the split pleckstrin homology (spPH) domain of phospholipase C-gamma(2) (PLCgamma(2)). Based on this structure, we illustrate distinct requirements for PLCgamma(2) activation by Rac and EGF and generate Rac effector mutants that specifically block activation of PLCgamma(2), but not the related PLCbeta(2) isoform. Furthermore, in addition to the complex, we report the crystal structures of free spPH and Rac2 bound to GDP and GTPgammaS. These structures illustrate a mechanism of conformational switches that accompany formation of signaling active complexes and highlight the role of effector binding as a common feature of Rac and Cdc42 interactions with a variety of effectors.

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.

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.

Rac1 is essential for phospholipase C-gamma2 activation in platelets

Platelet activation at sites of vascular injury is triggered through different signaling pathways leading to activation of phospholipase (PL) Cbeta or PLCgamma2. Active PLCs trigger Ca(2+) mobilization and entry, which is a prerequisite for adhesion, secretion, and thrombus formation. PLCbeta isoenzymes are activated downstream of G protein-coupled receptors (GPCRs), whereas PLCgamma2 is activated downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors, such as the major platelet collagen receptor glycoprotein (GP) VI or CLEC-2. The mechanisms underlying PLC regulation are not fully understood. An involvement of small GTPases of the Rho family (Rho, Rac, Cdc42) in PLC activation has been proposed but this has not been investigated in platelets. We here show that murine platelets lacking Rac1 display severely impaired GPVI- or CLEC-2-dependent activation and aggregation. This defect was associated with impaired production of inositol 1,4,5-trisphosphate (IP(3)) and intracellular calcium mobilization suggesting inappropriate activation of PLCgamma2 despite normal tyrosine phosphorylation of the enzyme. Rac1 ( -/- ) platelets displayed defective thrombus formation on collagen under flow conditions which could be fully restored by co-infusion of ADP and the TxA(2) analog U46619, indicating that impaired GPVI-, but not G-protein signaling, was responsible for the observed defect. In line with this, Rac1 ( -/- ) mice were protected in two collagen-dependent arterial thrombosis models. Together, these results demonstrate that Rac1 is essential for ITAM-dependent PLCgamma2 activation in platelets and that this is critical for thrombus formation in vivo.

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.

G protein βγ subunits: central mediators of G protein-coupled receptor signaling

G protein betagamma subunits are central participants in G protein-coupled receptor signaling pathways. They interact with receptors, G protein alpha subunits and downstream targets to coordinate multiple, different GPCR functions. Much is known about the biology of Gbetagamma subunits but mysteries remain. Here, we will review what is known about general aspects of structure and function of Gbetagamma as well as discuss emerging mechanisms for regulation of Gbetagamma signaling. Recent data suggest that Gbetagamma is a potential therapeutic drug target. Thus, a thorough understanding of the molecular and physiological functions of Gbetagamma has significant implications.

Constitutive serum response factor activation by the viral chemokine receptor homologue pUS28 is differentially regulated by Galpha(q/11) and Galpha(16)

Expression of the human cytomegalovirus (HCMV)-encoded chemokine receptor homologue pUS28 in mammalian cells results in ligand-dependent and -independent changes in the activity of multiple cellular signal transduction pathways. The ligand-dependent signalling activity of pUS28 has been shown to be predominantly mediated by heterotrimeric G proteins of the G(i/o) and G(12/13) subfamilies. Ligand-independent constitutive activity of pUS28 causing stimulation of inositol phosphate formation has been correlated with the coupling of pUS28 to G proteins of the G(q) family. It is well known that activation of G(q) proteins by cell surface receptors is coupled to activation of the Rho GTPase RhoA. Activated RhoA regulates numerous cellular functions, including the activity of the transcription factor serum response factor (SRF). The marked activation of G(q) proteins by pUS28 in transfected and HCMV-infected cells prompted us to investigate its effect on SRF activity. The results presented herein demonstrate that expression of pUS28 in COS-7 cells caused a vigorous induction of SRF activity. This effect was observed in the absence of chemokines known to interact with pUS28, and was specifically mediated by endogenous G(q) and/or G(11) as well as RhoA and/or a closely related Rho GTPase. The stimulatory effect of pUS28 and Galpha(q/11) was independent of phospholipase C-beta (PLCbeta) activation and was markedly sensitive to inhibition by wild-type, but not by constitutively active Galpha(16), thus identifying Galpha(16) as a modulator of Galpha(q/11) function likely to act by competing with Galpha(q/11) for and thus uncoupling Galpha(q/11) from activation by pUS28.

Activation of human phospholipase C-eta2 by Gbetagamma

Phospholipase C-eta2 (PLC-eta2) was recently identified as a novel broadly expressed phosphoinositide-hydrolyzing isozyme [Zhou, Y., et al. (2005) Biochem. J. 391, 667-676; Nakahara, M., et al. (2005) J. Biol. Chem. 280, 29128-29134]. In this study, we investigated the direct regulation of PLC-eta2 by Gbetagamma subunits of heterotrimeric G proteins. Coexpression of PLC-eta2 with Gbeta 1gamma 2, as well as with certain other Gbetagamma dimers, in COS-7 cells resulted in increases in inositol phosphate accumulation. Gbeta 1gamma 2-dependent increases in phosphoinositide hydrolysis also were observed with a truncation mutant of PLC-eta2 that lacks the long alternatively spliced carboxy-terminal domain of the isozyme. To begin to define the enzymatic properties of PLC-eta2 and its potential direct activation by Gbetagamma, a construct of PLC-eta2 encompassing the canonical domains conserved in all PLCs (PH domain through C2 domain) was purified to homogeneity after expression from a baculovirus in insect cells. Enzyme activity of purified PLC-eta2 was quantified after reconstitution with PtdIns(4,5)P 2-containing phospholipid vesicles, and values for K m (14.4 microM) and V max [12.6 micromol min (-1) (mg of protein) (-1)] were similar to activities previously observed with purified PLC-beta or PLC-epsilon isozymes. Moreover, purified Gbeta 1gamma 2 stimulated the activity of purified PLC-eta2 in a concentration-dependent manner similar to that observed with purified PLC-beta2. Activation was dependent on the presence of free Gbeta 1gamma 2 since its sequestration in the presence of Galpha i1 or GRK2-ct reversed Gbeta 1gamma 2-promoted activation. The PH domain of PLC-eta2 is not required for Gbeta 1gamma 2-mediated regulation since a purified fragment encompassing the EF-hand through C2 domains but lacking the PH domain nonetheless was activated by Gbeta 1gamma 2. Taken together, these studies illustrate that PLC-eta2 is a direct downstream effector of Gbetagamma and, therefore, of receptor-activated heterotrimeric G proteins.

Lipidomics: practical aspects and applications

Lipidomics is the characterization of the molecular species of lipids in biological samples. The polar lipids that comprise the bilayer matrix of the constituent cell membranes of living tissues are highly complex and number many hundreds of distinct lipid species. These differ in the nature of the polar group representing the different classes of lipid. Each class consists of a range of molecular species depending on the length, position of attachment and number of unsaturated double bonds in the associated fatty acids. The origin of this complexity is described and the biochemical processes responsible for homeostasis of the lipid composition of each morphologically-distinct membrane is considered. The practical steps that have been developed for the isolation of membranes and the lipids there from, their storage, separation, detection and identification by liquid chromatography coupled to mass spectrometry are described. Application of lipidomic analyses and examples where clinical screening for lipidoses in collaboration with mass spectrometry facilities are considered from the user point of view.

Stimulation of phospholipase Cbeta by membrane interactions, interdomain movement, and G protein binding--how many ways can you activate an enzyme?

Signaling proteins are usually composed of one or more conserved structural domains. These domains are usually regulatory in nature by binding to specific activators or effectors, or species that regulate cellular location, etc. Inositol-specific mammalian phospholipase C (PLC) enzymes are multidomain proteins whose activities are controlled by regulators, such as G proteins, as well as membrane interactions. One of these domains has been found to bind membranes, regulators, and activate the catalytic region. The recently solved structure of a major region of PLC-beta2 together with the structure of PLC-delta1 and a wealth of biochemical studies poises the system towards an understanding of the mechanism through which their regulations occurs.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

Crystal structure of Rac1 bound to its effector phospholipase C-β2

Although diverse signaling cascades require the coordinated regulation of heterotrimeric G proteins and small GTPases, these connections remain poorly understood. We present the crystal structure of the GTPase Rac1 bound to phospholipase C-beta2 (PLC-beta2), a classic effector of heterotrimeric G proteins. Rac1 engages the pleckstrin-homology (PH) domain of PLC-beta2 to optimize its orientation for substrate membranes. Gbetagamma also engages the PH domain to activate PLC-beta2, and these two activation events are compatible, leading to additive stimulation of phospholipase activity. In contrast to PLC-delta, the PH domain of PLC-beta2 cannot bind phosphoinositides, eliminating this mode of regulation. The structure of the Rac1-PLC-beta2 complex reveals determinants that dictate selectivity of PLC-beta isozymes for Rac GTPases over other Rho-family GTPases, and substitutions within PLC-beta2 abrogate its stimulation by Rac1 but not by Gbetagamma, allowing for functional dissection of this integral signaling node.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

Differences in CXCR4-mediated signaling in B cells

Among all chemokine receptors CXCR4 possesses a unique response profile and distinguishes itself through a prolonged signaling capacity. Here, we investigated the signaling capacity of CXCR4 to its so far known unique ligand CXCL12 in B cell lines and primary CD19(+) B lymphocytes. During lymphopoiesis, CXCR4 is continuously expressed on the surface of B cells. However, its signaling profile changes inasmuch preB and proB cells migrate towards CXCL12, mobilize intracellular calcium and activate the small GTPases Rac1 and Cdc42, whereas mature B cells do not show these responses, albeit the cells retain the capability to migrate in response to CXCL13 and CCL21. By contrast, stimulation of B cells with CXCL12 at all stages of development results in the activation of the MAP-kinase cascade and in rapid CXCR4 internalization. The pathways leading to ERK1/2 activation are different in preB and mature B cell lines. In either case, ERK1/2 activation is pertussis toxin sensitive, but only in mature B-cells inhibition of PI3-kinase causes an almost complete block of ERK1/2 activation. Taken together, the results show that CXCR4 changes its coupling to downstream signal-transduction pathways in B cells, suggesting that receptor activity may depend on accessory proteins.