EGF-and NGF-stimulated translocation of cytohesin-1 to the plasma membrane of PC12 cells requires PI 3-kinase activation and a functional cytohesin-1 PH domain

HGF-induced migration depends on the PI(3,4,5)P3-binding microexon-spliced variant of the Arf6 exchange factor cytohesin-1

Splice variants of the Arf6 guanine exchange factor cytohesin-1 display differential affinity for PI(4,5)P₂ and PI(3,4,5)P₃. Ratcliffe et al. show that the specific lipid binding of the diglycine variant of cytohesin-1 is needed for HGF-dependent cell migration and establishment of the leading edge, thereby regulating cancer cell migration following activation of the proto-oncogenic receptor tyrosine kinase Met.

Differential inclusion or skipping of microexons is an increasingly recognized class of alternative splicing events. However, the functional significance of microexons and their contribution to signaling diversity is poorly understood. The Met receptor tyrosine kinase (RTK) modulates invasive growth and migration in development and cancer. Here, we show that microexon switching in the Arf6 guanine nucleotide exchange factor cytohesin-1 controls Met-dependent cell migration. Cytohesin-1 isoforms, differing by the inclusion of an evolutionarily conserved three-nucleotide microexon in the pleckstrin homology domain, display differential affinity for PI(4,5)P₂ (triglycine) and PI(3,4,5)P₃ (diglycine). We show that selective phosphoinositide recognition by cytohesin-1 isoforms promotes distinct subcellular localizations, whereby the triglycine isoform localizes to the plasma membrane and the diglycine to the leading edge. These data highlight microexon skipping as a mechanism to spatially restrict signaling and provide a mechanistic link between RTK-initiated phosphoinositide microdomains and Arf6 during signal transduction and cancer cell migration.

Quantifying lipid changes in various membrane compartments using lipid binding protein domains

One of the largest challenges in cell biology is to map the lipid composition of the membranes of various organelles and define the exact location of processes that control the synthesis and distribution of lipids between cellular compartments. The critical role of phosphoinositides, low-abundant lipids with rapid metabolism and exceptional regulatory importance in the control of almost all aspects of cellular functions created the need for tools to visualize their localizations and dynamics at the single cell level. However, there is also an increasing need for methods to determine the cellular distribution of other lipids regulatory or structural, such as diacylglycerol, phosphatidic acid, or other phospholipids and cholesterol. This review will summarize recent advances in this research field focusing on the means by which changes can be described in more quantitative terms.

Willing to Be Involved in Cancer

Genome sequencing is now a common procedure, but prior to this, screening experiments using protein baits was one of the routinely used methods that, occasionally, allowed the identification of new gene products. One such experiment uncovered the gene product called willin/human Expanded/FRMD6. Initial characterization studies found that willin bound phospholipids and was strongly co-localised with actin. However, subsequently, willin was found to be the closest human sequence homologue of the Drosophila protein Expanded (Ex), sharing 60% homology with the Ex FERM domain. This in turn suggested, and then was proven that willin could activate the Hippo signalling pathway. This review describes the increasing body of knowledge about the actions of willin in a number of cellular functions related to cancer. However, like many gene products involved in aspects of cell signalling, a convincing direct role for willin in cancer remains tantalisingly elusive, at present.

Neuron membrane trafficking and protein kinases involved in autism and ADHD

A brain-enriched multi-domain scaffolding protein, neurobeachin has been identified as a candidate gene for autism patients. Mutations in the synaptic adhesion protein cell adhesion molecule 1 (CADM1) are also associated with autism spectrum disorder, a neurodevelopmental disorder of uncertain molecular origin. Potential roles of neurobeachin and CADM1 have been suggested to a function of vesicle transport in endosomal trafficking. It seems that protein kinase B (AKT) and cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) have key roles in the neuron membrane trafficking involved in the pathogenesis of autism. Attention deficit hyperactivity disorder (ADHD) is documented to dopaminergic insufficiencies, which is attributed to synaptic dysfunction of dopamine transporter (DAT). AKT is also essential for the DAT cell-surface redistribution. In the present paper, we summarize and discuss the importance of several protein kinases that regulate the membrane trafficking involved in autism and ADHD, suggesting new targets for therapeutic intervention.

Cytohesin 2/ARF6 regulates preadipocyte migration through the activation of ERK1/2

Preadipocyte migration is vital for the development of adipose tissue, which plays a crucial role in lipid metabolism. ADP-ribosylation factor 6 (ARF6) small GTPase, which regulates membrane trafficking, is activated by GTP-exchange factors (GEFs) such as cytohesin 2. Cytohesin 2 and ARF6 have previously been implicated in the regulation of 3T3-L1 preadipocyte migration. We investigated here the molecular mechanism underlying the cytohesin 2 and ARF6 mediated regulation of preadipocyte migration. Preadipocyte migration and the activation of ARF6 and ERK1/2 were studied by using a number of approaches, including pharmacological inhibitors, siRNA and the inhibitory peptides. The siRNA mediated down regulation of ARF6 and cytohesin 2 expression confirmed the requirement of both for migration of preadipocytes. Phosphatidylinositol 3-kinase (PI3K) and PI 4,5-bisphosphate (PIP2) have also found to be essential for the cytohesin 2/ARF6 induced preadipocyte migration. Pharmacological inhibition of the activation of ARF6, ERK1/2 or dynamin led to significant reduction in migration of 3T3-L1 preadipocytes. Furthermore, our study revealed the activation of ARF6 and ERK1/2 during migration of preadipocytes. In the migrating preadipocytes, ARF6 activation was inhibited with SecinH3 (cytohesin inhibitor) and LY294002 (PI3K inhibitor) whereas the ERK1/2 phosphorylation was inhibited with SecinH3, LY294002, PBP10 (a PIP2 sequester peptide) and PD98059 (MAPKK inhibitor). However, dynosore (dynamin inhibitor) had inhibited neither ARF6 activation nor ERK1/2 phosphorylation during preadipocyte migration. These results together suggest that cytohesin 2 activates ARF6 in a PI3K dependent manner and then the active ARF6 causes phosphorylation of ERK1/2 during preadipocyte migration.

Structural basis for membrane recruitment and allosteric activation of cytohesin family Arf GTPase exchange factors

Membrane recruitment of cytohesin family Arf guanine nucleotide exchange factors depends on interactions with phosphoinositides and active Arf GTPases that, in turn, relieve autoinhibition of the catalytic Sec7 domain through an unknown structural mechanism. Here, we show that Arf6-GTP relieves autoinhibition by binding to an allosteric site that includes the autoinhibitory elements in addition to the PH domain. The crystal structure of a cytohesin-3 construct encompassing the allosteric site in complex with the head group of phosphatidyl inositol 3,4,5-trisphosphate and N-terminally truncated Arf6-GTP reveals a large conformational rearrangement, whereby autoinhibition can be relieved by competitive sequestration of the autoinhibitory elements in grooves at the Arf6/PH domain interface. Disposition of the known membrane targeting determinants on a common surface is compatible with multivalent membrane docking and subsequent activation of Arf substrates, suggesting a plausible model through which membrane recruitment and allosteric activation could be structurally integrated.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

ARF6 activated by the LHCG receptor through the cytohesin family of guanine nucleotide exchange factors mediates the receptor internalization and signaling

The luteinizing hormone chorionic gonadotropin receptor (LHCGR) is a G(s)-coupled GPCR that is essential for the maturation and function of the ovary and testis. LHCGR is internalized following its activation, which regulates the biological responsiveness of the receptor. Previous studies indicated that ADP-ribosylation factor (ARF)6 and its GTP-exchange factor (GEF) cytohesin 2 regulate LHCGR internalization in follicular membranes. However, the mechanisms by which ARF6 and cytohesin 2 regulate LHCGR internalization remain incompletely understood. Here we investigated the role of the ARF6 signaling pathway in the internalization of heterologously expressed human LHCGR (HLHCGR) in intact cells using a combination of pharmacological inhibitors, siRNA and the expression of mutant proteins. We found that human CG (HCG)-induced HLHCGR internalization, cAMP accumulation and ARF6 activation were inhibited by Gallein (βγ inhibitor), Wortmannin (PI 3-kinase inhibitor), SecinH3 (cytohesin ARF GEF inhibitor), QS11 (an ARF GAP inhibitor), an ARF6 inhibitory peptide and ARF6 siRNA. However, Dynasore (dynamin inhibitor), the dominant negative mutants of NM23-H1 (dynamin activator) and clathrin, and PBP10 (PtdIns 4,5-P2-binding peptide) inhibited agonist-induced HLHCGR and cAMP accumulation but not ARF6 activation. These results indicate that heterotrimeric G-protein, phosphatidylinositol (PI) 3-kinase (PI3K), cytohesin ARF GEF and ARF GAP function upstream of ARF6 whereas dynamin and clathrin act downstream of ARF6 in the regulation of HCG-induced HLHCGR internalization and signaling. In conclusion, we have identified the components and molecular details of the ARF6 signaling pathway required for agonist-induced HLHCGR internalization.

Imaging phosphoinositide dynamics using GFP-tagged protein domains

Phosphoinositides are important regulators of cellular homoeostasis and numerous signal-transduction pathways. One of their major features is their ability to recruit signalling proteins to membranes by direct interaction with phosphoinositide-binding modules. The distribution and dynamics of membrane phosphoinositides are therefore major determinants in the spatiotemporal control of cell signalling and membrane trafficking. However, standard biochemical approaches cannot reveal the dynamics of phosphoinositides at the single-cell level. A major technical advance has been the development of genetically encoded fluorescent phosphoinositide probes on the basis of the phosphoinositide-binding domains found in signalling proteins, such as the PH (pleckstrin homology) domain. This review describes the diverse fluorescent phosphoinositide probes available for imaging specific phosphoinositide species and how their use has improved the understanding of phosphoinositide signalling at the single-cell level.

Regulation of growth factor receptor degradation by ADP-ribosylation factor domain protein (ARD) 1

ADP-ribosylation factor domain protein 1 (ARD1) is a 64-kDa protein containing a functional ADP-ribosylation factor (GTP hydrolase, GTPase), GTPase-activating protein, and E3 ubiquitin ligase domains. ARD1 activation by the guanine nucleotide-exchange factor cytohesin-1 was known. GTPase and E3 ligase activities of ARD1 suggest roles in protein transport and turnover. To explore this hypothesis, we used mouse embryo fibroblasts (MEFs) from ARD1-/- mice stably transfected with plasmids for inducible expression of wild-type ARD1 protein (KO-WT), or ARD1 protein with inactivating mutations in E3 ligase domain (KO-E3), or containing persistently active GTP-bound (KO-GTP), or inactive GDP-bound (KO-GDP) GTPase domains. Inhibition of proteasomal proteases in mifepristone-induced KO-WT, KO-GDP, or KO-GTP MEFs resulted in accumulation of these ARD1 proteins, whereas KO-E3 accumulated without inhibitors. All data were consistent with the conclusion that ARD1 regulates its own steady-state levels in cells by autoubiquitination. Based on reported growth factor receptor-cytohesin interactions, EGF receptor (EGFR) was investigated in induced MEFs. Amounts of cell-surface and total EGFR were higher in KO-GDP and lower in KO-GTP than in KO-WT MEFs, with levels in both mutants greater (p = 0.001) after proteasomal inhibition. Significant differences among MEF lines in content of TGF-β receptor III were similar to those in EGFR, albeit not as large. Differences in amounts of insulin receptor mirrored those in EGFR, but did not reach statistical significance. Overall, the capacity of ARD1 GTPase to cycle between active and inactive forms and its autoubiquitination both appear to be necessary for the appropriate turnover of EGFR and perhaps additional growth factor receptors.

Imaging the dynamics of intracellular protein translocation by photoconversion of phamret-cybr/ROM

Cybr/Reduced On-random Motile (ROM) is a scaffold protein, containing a postsynaptic density protein-95/discs-large/ZO-1 (PDZ) domain, a LEU region and a PDZ domain binding region at the C-terminus. In the immune system, Cybr/ROM was found to localize in vesicles and at the plasma membrane, through interactions with cytohesin-1. In this investigation, we reported Cybr/ROM as occurring in vesicles, the cytoplasm and at membrane ruffles of H1299 lung cancer cells. Its localization at the ruffles was dependent on intact actin structures as indicated by latrunculin A treatment, which abrogated ruffle formation and staining of Cybr/ROM at the cells' periphery. Transfection of truncation mutants consisting of either the PDZ or LEU domain showed that the LEU domain of ROM was localized to membrane ruffles, vesicles and the cytoplasm, whereas, the PDZ domain localized to the membrane ruffles and cytoplasm only. There was therefore, domain/molecular segregation of Cybr/ROM in different cellular compartments. Cybr/ROM was subcloned into a plasmid carrying the photoactivation-mediated resonance energy transfer (Phamret) protein. The photoconversion experiments demonstrated the diffusion of ROM from the cytoplasm to the membrane ruffling sites and conversely from membrane ruffles to the cytoplasm. Large variances in the transport velocity of Cybr/ROM in the cytoplasm suggested that its movements were facilitated by other mechanisms in addition to diffusion.

The SNX-PX-BAR family in macropinocytosis: the regulation of macropinosome formation by SNX-PX-BAR proteins

BACKGROUND

Macropinocytosis is an actin-driven endocytic process, whereby membrane ruffles fold back onto the plasma membrane to form large (>0.2 µm in diameter) endocytic organelles called macropinosomes. Relative to other endocytic pathways, little is known about the molecular mechanisms involved in macropinocytosis. Recently, members of the Sorting Nexin (SNX) family have been localized to the cell surface and early macropinosomes, and implicated in macropinosome formation. SNX-PX-BAR proteins form a subset of the SNX family and their lipid-binding (PX) and membrane-curvature sensing (BAR) domain architecture further implicates their functional involvement in macropinosome formation.

METHODOLOGY/PRINCIPAL FINDINGS

We exploited the tractability of macropinosomes through image-based screening and systematic overexpression of SNX-PX-BAR proteins to quantitate their effect on macropinosome formation. SNX1 (40.9+/-3.19 macropinosomes), SNX5 (36.99+/-4.48 macropinosomes), SNX9 (37.55+/-2.4 macropinosomes), SNX18 (88.2+/-8 macropinosomes), SNX33 (65.25+/-6.95 macropinosomes) all exhibited statistically significant (p<0.05) increases in average macropinosome numbers per 100 transfected cells as compared to control cells (24.44+/-1.81 macropinosomes). SNX1, SNX5, SNX9, and SNX18 were also found to associate with early-stage macropinosomes within 5 minutes following organelle formation. The modulation of intracellular PI(3,4,5)P(3) levels through overexpression of PTEN or a lipid phosphatase-deficient mutant PTEN(G129E) was also observed to significantly reduce or elevate macropinosome formation respectively; coexpression of PTEN(G129E) with SNX9 or SNX18 synergistically elevated macropinosome formation to 119.4+/-7.13 and 91.4+/-6.37 macropinosomes respectively (p<0.05).

CONCLUSIONS/SIGNIFICANCE

SNX1, SNX5, SNX9, SNX18, and SNX33 were all found to elevate macropinosome formation and (with the exception of SNX33) associate with early-stage macropinosomes. Moreover the effects of SNX9 and SNX18 overexpression in elevating macropinocytosis is likely to be synergistic with the increase in PI(3,4,5)P(3) levels, which is known to accumulate on the cell surface and early-stage macropinocytic cups. Together these findings represent the first systematic functional study into the impact of the SNX-PX-BAR family on macropinocytosis.

Visualization of cellular phosphoinositide pools with GFP-fused protein-domains

This unit describes the method of following phosphoinositide dynamics in live cells. Inositol phospholipids have emerged as universal signaling molecules present in virtually every membrane of eukaryotic cells. Phosphoinositides are present in only tiny amounts as compared to structural lipids, but they are metabolically very active as they are produced and degraded by the numerous inositide kinase and phosphatase enzymes. Phosphoinositides control the membrane recruitment and activity of many membrane protein signaling complexes in specific membrane compartments, and they have been implicated in the regulation of a variety of signaling and trafficking pathways. It has been a challenge to develop methods that allow detection of phosphoinositides at the single-cell level. The only available technique in live cell applications is based on the use of the same protein domains selected by evolution to recognize cellular phosphoinositides. Some of these isolated protein modules, when fused to fluorescent proteins, can follow dynamic changes in phosphoinositides. While this technique can provide information on phosphoinositide dynamics in live cells with subcellular localization, and it has rapidly gained popularity, it also has several limitations that must be taken into account when interpreting the data. This unit summarizes the design and practical use of these constructs and also reviews important considerations for interpretation of the data obtained by this technique.

Live cell imaging of phosphoinositides with expressed inositide binding protein domains

Inositol lipids and calcium signaling has been inseparable twins during the 1980s when the molecular details of phospholipase C-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and its Ca2+ mobilizing action were discovered. Since then, both the Ca2+ and inositol lipid signaling fields have hugely expanded and the tools allowing dissection of the finest details of their molecular organization also followed closely. Although phosphoinositides regulate many cell functions unrelated to Ca2+ signaling there are still many open questions even in the Ca2+ field that would benefit from single cell monitoring of PtdIns(4,5)P2 or InsP3 changes during agonist stimulation. This chapter is designed to provide practical guidance as well as some theoretical background on measurements of phosphoinositides in live cells using protein domain-GFP chimeras that could be also useful for people working on calcium signaling.

Function and dysfunction of the PI system in membrane trafficking

The phosphoinositides (PIs) function as efficient and finely tuned switches that control the assembly-disassembly cycles of complex molecular machineries with key roles in membrane trafficking. This important role of the PIs is mainly due to their versatile nature, which is in turn determined by their fast metabolic interconversions. PIs can be tightly regulated both spatially and temporally through the many PI kinases (PIKs) and phosphatases that are distributed throughout the different intracellular compartments. In spite of the enormous progress made in the past 20 years towards the definition of the molecular details of PI-protein interactions and of the regulatory mechanisms of the individual PIKs and phosphatases, important issues concerning the general principles of the organisation of the PI system and the coordination of the different PI-metabolising enzymes remain to be addressed. The answers should come from applying a systems biology approach to the study of the PI system, through the integration of analyses of the protein interaction data of the PI enzymes and the PI targets with those of the 'phenomes' of the genetic diseases that involve these PI-metabolising enzymes.

Analysis of phosphoinositide dynamics during phagocytosis using genetically encoded fluorescent biosensors

Phosphoinositide signaling is essential for successful phagocytosis. Phosphoinositides regulate processes such as actin assembly and the recruitment of molecular motors required for ingestion, as well as fusion events required for the maturation of the phagosome. Phosphoinositides not only serve as substrates for the generation of second messengers, but also function to anchor to the membrane cytosolic proteins that contain phosphoinositide-binding motifs. Conventional methods for the detection of phosphoinositides involve their extraction from the cells and separation by chromatographic procedures. These approaches are laborious and expensive and fail to provide spatio-temporal information, which is critical when analyzing localized and transient phenomena like phagocytosis. In this chapter we describe a method to monitor phosphoinositides dynamically by transfection of fluorescently tagged probes (biosensors) into cultured macrophages. These biosensors are based on the fusion of phosphoinositide-binding protein domains with fluorescent proteins. Some specifications for live cell imaging of such phosphoinositide-specific probes are also provided.

ARL4D recruits cytohesin-2/ARNO to modulate actin remodeling

ARL4D is a developmentally regulated member of the ADP-ribosylation factor/ARF-like protein (ARF/ARL) family of Ras-related GTPases. Although the primary structure of ARL4D is very similar to that of other ARF/ARL molecules, its function remains unclear. Cytohesin-2/ARF nucleotide-binding-site opener (ARNO) is a guanine nucleotide-exchange factor (GEF) for ARF, and, at the plasma membrane, it can activate ARF6 to regulate actin reorganization and membrane ruffling. We show here that ARL4D interacts with the C-terminal pleckstrin homology (PH) and polybasic c domains of cytohesin-2/ARNO in a GTP-dependent manner. Localization of ARL4D at the plasma membrane is GTP- and N-terminal myristoylation-dependent. ARL4D(Q80L), a putative active form of ARL4D, induced accumulation of cytohesin-2/ARNO at the plasma membrane. Consistent with a known action of cytohesin-2/ARNO, ARL4D(Q80L) increased GTP-bound ARF6 and induced disassembly of actin stress fibers. Expression of inactive cytohesin-2/ARNO(E156K) or small interfering RNA knockdown of cytohesin-2/ARNO blocked ARL4D-mediated disassembly of actin stress fibers. Similar to the results with cytohesin-2/ARNO or ARF6, reduction of ARL4D suppressed cell migration activity. Furthermore, ARL4D-induced translocation of cytohesin-2/ARNO did not require phosphoinositide 3-kinase activation. Together, these data demonstrate that ARL4D acts as a novel upstream regulator of cytohesin-2/ARNO to promote ARF6 activation and modulate actin remodeling.

Visualization and manipulation of phosphoinositide dynamics in live cells using engineered protein domains

There is hardly a membrane-associated molecular event that is not regulated by phosphoinositides, a minor but critically important class of phospholipids of cellular membranes. The rapid formation, elimination, and conversion of these lipids in specific membrane compartments are ensured by a wealthy number of inositol lipid kinases and phosphatases with unique localization and regulatory properties. The existence of multiple inositol lipid pools have been indicated by metabolic labeling studies, but the level of functional compartmentalization revealed by the identification of numerous protein effectors acted upon by phosphoinositides could not have been foreseen. The changing perception of inositides from just serving as lipid precursors of second messengers to becoming highly dynamic local membrane-bound regulators poses new challenges concerning the detection of their rapid localized changes. Moreover, it is increasingly evident that manipulation of lipids in highly defined compartments would be a highly superior approach to soaking the cells with a particular phosphoinositide when studying the local regulation of the lipid on any effectors. In this review, we will summarize our efforts to improve our tools in studying phosphoinositide dynamics and discuss our views on the values of these methods compared to other options currently used or being explored.

The Arl4 family of small G proteins can recruit the cytohesin Arf6 exchange factors to the plasma membrane

The small GTPase Arf6 regulates endocytosis, actin dynamics, and cell adhesion, and one of its major activators is the exchange factor Arf nucleotide-binding site opener (ARNO), also called cytohesin-2 [1, 2]. ARNO must be recruited from the cytosol to the plasma membrane in order to activate Arf6, and in addition to a Sec7 nucleotide-exchange domain it contains a C-terminal pleckstrin homology (PH) domain that binds phosphoinositides [3, 4]. ARNO and its three relatives, cytohesin-1, Grp1/cytohesin-3, and cytohesin-4, are expressed as two splice variants, with either two or three glycines in a loop in the phosphoinositide-binding pocket of the PH domain [5, 6]. The diglycine form binds PtdIns(3,4,5)P(3) with high affinity and mediates recruitment of cytohesins to the plasma membrane in response to insulin and growth factors [7, 8]. However, the triglycine form has only micromolar affinity for both PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2), affinities that are insufficient to confer membrane recruitment, raising the question of how the triglycine forms of cytohesins are regulated [5, 9]. Here we show that three related Arf-like GTPases of unknown function, Arl4a, Arl4c, and Arl4d, are able to recruit ARNO and other cytohesins to the plasma membrane by binding to their PH domains irrespective of whether they are in the diglycine or triglycine form. The Arl4 family thus defines a signal-transduction pathway that can mediate the plasma-membrane recruitment of cytohesins independently of a requirement for the generation of PtdIns(3,4,5)P(3).

Active Arf6 recruits ARNO/cytohesin GEFs to the PM by binding their PH domains

ARNO is a soluble guanine nucleotide exchange factor (GEF) for the Arf family of GTPases. Although in biochemical assays ARNO prefers Arf1 over Arf6 as a substrate, its localization in cells at the plasma membrane (PM) suggests an interaction with Arf6. In this study, we found that ARNO activated Arf1 in HeLa and COS-7 cells resulting in the recruitment of Arf1 on to dynamic PM ruffles. By contrast, Arf6 was activated less by ARNO than EFA6, a canonical Arf6 GEF. Remarkably, Arf6 in its GTP-bound form recruited ARNO to the PM and the two proteins could be immunoprecipitated. ARNO binding to Arf6 was not mediated through the catalytic Sec7 domain, but via the pleckstrin homology (PH) domain. Active Arf6 also bound the PH domain of Grp1, another ARNO family member. This interaction was direct and required both inositol phospholipids and GTP. We propose a model of sequential Arf activation at the PM whereby Arf6-GTP recruits ARNO family GEFs for further activation of other Arf isoforms.

PI-3-kinase-dependent membrane recruitment of centaurin-alpha2 is essential for its effect on ARF6-mediated actin cytoskeleton reorganisation

GTPase activating proteins (GAPs) of the centaurin family regulate the actin cytoskeleton and vesicle trafficking through inactivation of the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. We report the functional characterisation of centaurin-alpha(2), which is structurally related to the centaurin-alpha(1) ARF6 GAP. centaurin-alpha(2) contains an N-terminal GAP domain followed by two pleckstrin homology (PH) domains (N-PH and C-PH). In vitro, GFP-centaurin-alpha(2) specifically binds the phosphatidylinositol (PI) 3-kinase lipid products, PI 3,4-P(2) and PI 3,4,5-P(3) (PIP(3)), through its C-terminal PH domain. In agreement with this observation, GFP-centaurin-alpha(2) was recruited to the plasma membrane from the cytosol in EGF-stimulated cells in a PI-3-kinase-dependent manner. Moreover, the C-PH domain is sufficient and necessary for membrane recruitment of centaurin-alpha(2). centaurin-alpha(2) shows sustained kinetics of PI-3-kinase-mediated membrane recruitment in EGF-stimulated cells, owing to its binding to PI 3,4-P(2). centaurin-alpha(2) prevents ARF6 translocation to, and cortical actin formation at, the plasma membrane, which are phenotypic indications for ARF6 activation in EGF-stimulated cells. Moreover, the constitutively active mutant of ARF6 reverses the effect of centaurin-alpha(2) on cortical actin formation. The membrane targeted centaurin-alpha(2) is constitutively active. Together, these studies indicate that centaurin-alpha(2) is recruited in a sustained manner to the plasma membrane through binding to PI 3,4-P(2) and thereby regulates actin reorganisation via ARF6.

A kinase-deficient TrkC receptor isoform activates Arf6-Rac1 signaling through the scaffold protein tamalin

Neurotrophins play an essential role in mammalian development. Most of their functions have been attributed to activation of the kinase-active Trk receptors and the p75 neurotrophin receptor. Truncated Trk receptor isoforms lacking the kinase domain are abundantly expressed during development and in the adult; however, their function and signaling capacity is largely unknown. We show that the neurotrophin-3 (NT3) TrkCT1-truncated receptor binds to the scaffold protein tamalin in a ligand-dependent manner. Moreover, NT3 initiation of this complex leads to activation of the Rac1 GTPase through adenosine diphosphate-ribosylation factor 6 (Arf6). At the cellular level, NT3 binding to TrkCT1–tamalin induces Arf6 translocation to the membrane, which in turn causes membrane ruffling and the formation of cellular protrusions. Thus, our data identify a new signaling pathway elicited by the kinase-deficient TrkCT1 receptor. Moreover, we establish NT3 as an upstream regulator of Arf6.

A novel 4.1 ezrin radixin moesin (FERM)-containing protein, 'Willin'

The 4.1 superfamily of proteins contain a 4.1 Ezrin Radixin Moesin (FERM) domain and are described as linking the cytoskeleton with the plasma membrane. Here, we describe a new FERM domain-containing protein called Willin. Willin has a recognizable FERM domain within its N-terminus and is capable of binding phospholipids. Its intra-cellular distribution can be cytoplasmic or at the plasma membrane where it can co-localize with actin. However, the plasma membrane location of Willin is not influenced by cytochalasin D induced actin disruption but it is induced by the addition of epidermal growth factor.

Leukocytes on the move with phosphoinositide 3-kinase and its downstream effectors

Cell signalling mediators derived from membrane phospholipids are frequent participants in biological processes. The family of phosphoinositide 3-kinases (PI3Ks) phosphorylate the membrane lipid phosphatidylinositol, generating second messengers that direct diverse responses. These PI3K products are fundamental for leukocyte migration or chemotaxis, a pivotal event during the immune response. This system is therefore of significant biomedical interest. This review focuses on the biochemistry and signalling pathways of PI3K, with particular emphasis on chemokine (chemotactic cytokine)-directed responses. The key objectives of chemotaxis are motility and direction. The latter--direction--requires distinct events at the front and back of a cell. In light of this, the coordinated localisation of signalling factors, an event choreographed by a sharp intracellular gradient of PI3K-derived products, is a common theme.

Cytohesin-1: structure, function, and ARF activation

Mammalian cytohesins are a family of very similar guanine nucleotide-exchange proteins (GEPs) that activate ADP-ribosylation factors (ARFs). Cytohesins are multifunctional molecules comprising a Sec7 domain that is responsible for the GEP activity, a PH domain that binds specific phosphatidylinositol phosphates, and a coiled-coil domain responsible for homodimerization and interaction with other proteins. Cytohesin proteins are ubiquitous and have been implicated in several functions including cell spreading and adhesion, chemotaxis, protein trafficking, and cytoskeletal rearrangements, only some of which appear to depend on their ability to activate ARFs. Unlike the GEP activity of BIG1 and BIG2, the acceleration by cytohesins of guanine nucleotide exchange to generate active ARF-GTP is not inhibited by the fungal metabolite brefeldin, A (BFA). This chapter is concerned for the most part with cytohesin-1 and the assay of its GEP activity.

Structural determinants of phosphoinositide selectivity in splice variants of Grp1 family PH domains

The pleckstrin homology (PH) domains of the homologous proteins Grp1 (general receptor for phosphoinositides), ARNO (Arf nucleotide binding site opener), and Cytohesin-1 bind phosphatidylinositol (PtdIns) 3,4,5-trisphosphate with unusually high selectivity. Remarkably, splice variants that differ only by the insertion of a single glycine residue in the beta1/beta2 loop exhibit dual specificity for PtdIns(3,4,5)P(3) and PtdIns(4,5)P(2). The structural basis for this dramatic specificity switch is not apparent from the known modes of phosphoinositide recognition. Here, we report crystal structures for dual specificity variants of the Grp1 and ARNO PH domains in either the unliganded form or in complex with the head groups of PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3). Loss of contacts with the beta1/beta2 loop with no significant change in head group orientation accounts for the significant decrease in PtdIns(3,4,5)P(3) affinity observed for the dual specificity variants. Conversely, a small increase rather than decrease in affinity for PtdIns(4,5)P(2) is explained by a novel binding mode, in which the glycine insertion alleviates unfavorable interactions with the beta1/beta2 loop. These observations are supported by a systematic mutational analysis of the determinants of phosphoinositide recognition.

Membrane targeting by pleckstrin homology domains

Pleckstrin homology (PH) domains are small modular domains that occur once, or occasionally several times, in a large variety of signalling proteins. In a number of instances, PH domains act to target their host protein to the cytosolic face of cellular membranes through an ability to associate with phosphoinositides. In this review, we discuss recent advances in our understanding of PH domain function. In particular we describe the structural aspects of how PH domains have evolved to bind various phosphoinositides, how PH domains regulate phosphoinositide-mediated association to plasma and internals membranes, and finally raise the issue of PH domains in protein:protein interactions and the allosteric regulation of their host protein.

Interaction protein for cytohesin exchange factors 1 (IPCEF1) binds cytohesin 2 and modifies its activity

The ADP-ribosylation factor 6 (ARF6) small GTPase functions as a GDP/GTP-regulated switch in the pathways that stimulate actin reorganization and membrane ruffling. The formation of active ARF6GTP is stimulated by guanine nucleotide exchange factors (GEFs) such as cytohesins, which translocate to the plasma membrane in agonist-stimulated cells by binding the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate through the pleckstrin homology domain with subsequent ARF6 activation. Using cytohesin 2 as bait in yeast two-hybrid screening, we have isolated a cDNA encoding a protein termed interaction protein for cytohesin exchange factors 1 (IPCEF1). Using yeast two-hybrid and glutathione S-transferase pull-down assays coupled with deletion mutational analysis, the specific domains required for the cytohesin 2-IPCEF1 interaction were mapped to the coiled-coil domain of cytohesin 2 and the C-terminal 121 amino acids of IPCEF1. IPCEF1 also interacts with the other members of the cytohesin family of ARF GEFs, suggesting that the interaction with IPCEF1 is highly conserved among the cytohesin family of ARF GEFs. The interaction of cytohesin 2 and IPCEF1 in mammalian cells was demonstrated by immunoprecipitation. Immunofluorescence analysis revealed that IPCEF1 co-localizes with cytohesin 2 to the cytosol in unstimulated cells and translocates to the plasma membrane via binding to cytohesin 2 in epidermal growth factor-stimulated cells. However, a deletion mutant of IPCEF1 that lacks the cytohesin 2 binding site failed to co-migrate with cytohesin 2 to the membrane in stimulated cells. The functional significance of the IPCEF1-cytohesin 2 interaction is demonstrated by showing that IPCEF1 increases the in vitro and in vivo stimulation of ARFGTP formation by cytohesin 2.

Production of PtdInsP3 at endomembranes is triggered by receptor endocytosis

Phosphatidylinositol-3,4,5-trisphosphate (PtdInsP(3)) regulates diverse cellular functions, including cell proliferation and apoptosis, and has roles in the progression of diabetes and cancer. However, little is known about its production. Here, we describe fluorescent indicators for PtdInsP(3) that allow a spatio-temporal examination of PtdInsP(3) production in single living cells. After ligand stimulation, PtdInsP(3) levels increased to a larger extent at the endomembranes (that is, the endoplasmic reticulum and the Golgi) than at the plasma membrane. This increase was found to originate from in situ production at the endomembranes, a process stimulated directly by receptor tyrosine kinases endocytosed from the plasma membrane to the endomembranes. The demonstration of PtdInsP(3) production through receptor endocytosis addresses a long-standing question about how signalling pathways downstream of PtdInsP(3) are activated at intracellular compartments remote from the plasma membrane.

Phosphoinositide binding by the pleckstrin homology domains of Ipl and Tih1

The Ipl protein consists of a single pleckstrin homology (PH) domain with short N- and C-terminal extensions. This protein is highly conserved among vertebrates, and it acts to limit placental growth in mice. However, its biochemical function is unknown. The closest paralogue of Ipl is Tih1, another small PH domain protein. By sequence comparisons, Ipl and Tih1 define an outlying branch of the PH domain superfamily. Here we describe phosphatidylinositol phosphate (PIP) binding by these proteins. Ipl and Tih1 bind to immobilized PIPs with moderate affinity, but this binding is weaker and more promiscuous than that of prototypical PH domains from the general receptor for phosphoinositides (GRP1), phospholipase C delta1, and dual adaptor for phosphoinositides and phosphotyrosine 1. In COS7 cells exposed to epidermal growth factor, green fluorescent protein (GFP)-Ipl and GFP-Tih1 accumulate at membrane ruffles without clearing from the cytoplasm, whereas control GFP-GRP1 translocates rapidly to the plasma membrane and clears from the cytoplasm. Ras*-Ipl and Ras*-Tih1 fusion proteins both rescue cdc25ts Saccharomyces cerevisiae, but Ras*-Ipl rescues more efficiently in the presence of phosphatidylinositol 3-kinase (PI3K), whereas PI3K-independent rescue is more efficient with Ras*-Tih1. Site-directed mutagenesis defines amino acids in the beta1-loop1-beta2 regions of Ipl and Tih1 as essential for growth rescue in this assay. Thus, Ipl and Tih1 are bona fide PH domain proteins, with broad specificity and moderate affinity for PIPs.

Isolation of differentially expressed genes in NPM-ALK-positive anaplastic large cell lymphoma

In this study, we used subtractive suppression hybridization to compare gene expression between an ALK-positive anaplastic large cell lymphoma (ALCL)-derived cell line and a clinical case of ALK-negative ALCL. Construction and screening of a subtracted library resulted in the cloning of 29 cDNAs which were differentially expressed. Most of these clones corresponded to novel genes with unknown function (EST) or to genes implicated in the differentiation, activation or signalling of T cells such as Ran/TC4, interleukin 1-receptor, thymosin beta4, thymosin beta10, moesin and cytohesin-1. Other genes involved in the regulation of apoptosis, such as human inhibitor of apoptosis-1 (HIAP-1), Bax inhibitor-1 and MCL-1, or DNA repair, such as poly (ADP-ribose) polymerase 1 (PARP-1), X-associated protein-1 (XAP-1), SUMO-1 (sentrin-1) and RanGTPase-activating protein 1 (RanGAP-1), were isolated. Interestingly, we found that both RNA and protein levels of human sterol isomerase (hSI), also referred to as emopamil binding protein (EBP), were overexpressed in ALK+ tumours. This protein is involved in the biosynthesis of cholesterol and may be activated by NPM-ALK. Overall, our results suggest that all the genes described above are upregulated in the NPM-ALK-driven transformation process, and that moesin and cytohesin-1 may be more specifically implicated in a signalling pathway involving PLCgamma and PI3K.

The N-terminal coiled coil domain of the cytohesin/ARNO family of guanine nucleotide exchange factors interacts with the scaffolding protein CASP

Cytohesin is a guanine nucleotide exchange factor that regulates members of the ADP-ribosylation factor (ARF) family of small GTPases. All of the members of the cytohesin family (including ARNO, ARNO3, and the newly characterized cytohesin-4) have a similar domain distribution consisting of a Sec7 homology domain, a pleckstrin homology domain, and an N-terminal coiled coil. In this study, we attempt to identify proteins that interact specifically with the coiled coil motif of cytohesin. Yeast two-hybrid screening of a B cell library using the cytohesin N terminus as bait, identified CASP, a scaffolding protein of previously unknown function, as a binding partner. CASP contains an internal coiled coil motif that is required for cytohesin binding both in vitro and in COS-1 cells. The specificity of the coiled coil of CASP is not restricted to cytohesin, however, because it is also capable of interacting with other members of the cytohesin/ARNO family, ARNO and ARNO3. In immunofluorescence experiments, CASP localizes to perinuclear tubulovesicular structures that are in close proximity to the Golgi. These structures remain relatively undisturbed when the cells are treated with brefeldin A. In epidermal growth factor-stimulated COS-1 cells overexpressing cytohesin and CASP, cytohesin recruits CASP to membrane ruffles, revealing a functional interaction between the two proteins. These observations collectively suggest that CASP is a scaffolding protein that facilitates the function of at least one member of the cytohesin/ARNO family in response to specific cellular stimuli.

Visualizing cellular phosphoinositide pools with GFP-fused protein-modules

Inositol phospholipids are well known for their pivotal role in calcium signaling as precursors of important second messengers generated in response to various stimuli. However, over the last 10 years, inositides have also emerged as universal signaling components present in virtually every membrane of eukaryotic cells. These lipids are locally produced and degraded by the numerous inositide kinase and phosphatase enzymes, to control the recruitment and activity of protein signaling complexes in specific membrane compartments. The spatial and temporal constraints imposed on changes in cellular inositides pose new challenges in finding experimental techniques through which such changes can be examined. Taking advantage of the protein domains selected by evolution to recognize cellular phosphoinositides, we have created fluorescent molecules by fusing these domains to the improved version of green fluorescent protein (EGFP); the distribution of these fusion proteins can be followed within live cells, thereby reporting on changes in phosphoinositides. Although this technique is one of the few that provide information on phosphoinositide dynamics in live cells with subcellular resolution and has rapidly gained popularity, it also has limitations that need to be taken into account when interpreting the data. Here, we summarize our experience in designing and using these constructs and review our position concerning the interpretation of the data obtained by this technique.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Localization of mRNAs for subfamily of guanine nucleotide-exchange proteins (GEP) for ARFs (ADP-ribosylation factors) in the brain of developing and mature rats under normal and postaxotomy conditions

ADP-ribosylation factors (ARFs) play important roles in vesicular trafficking and cytoskeletal regulation and its activation depends on guanine nucleotide-exchange proteins (GEPs). By way of in situ hybridization histochemistry, the localization of mRNAs for subfamily members of low-molecular-weight ARF-GEPs in the rat brain was studied at embryonic and postnatal stages. In the embryonic brain, the gene expression for msec7-1 was distinct in the ventricular zone while that for msec7-1, -3 and EFA6 in the mantle zone. In early postnatal brain, the expression for msec7-1, -2, -3 and EFA6 was seen widely in various loci of the gray matter with different intensity, and the expression of msec7-1 and -2 mRNAs was evident in the cerebellar external granule cell layer. In the adult brain, the gene expression for the four ARF-GEPs decreased more or less in most gray matter and the distinct expression was maintained mainly in the hippocampal and dentate neuronal layers and cerebellar cortex. The expression of EFA6 mRNA was also evident in the molecular layer of the hippocampus and dentate gyrus. No obvious gene expression for cytohesin-4 and ARF-GEP100 was detected in the brain at any stages of development. The present findings suggest that ARF-GEPs are differentially involved in some processes essential to neuronal differentiation and maturation in association with ARFs.

Signaling complexes of the FERM domain-containing protein GRSP1 bound to ARF exchange factor GRP1

GRP1 is a member of a family of proteins that contain a coiled-coil region, a Sec7 homology domain with guanosine nucleotide exchange activity for the ARF GTP-binding proteins, and a pleckstrin homology domain at the C terminus. The pleckstrin homology domain of GRP1 binds phosphatidylinositol (3,4,5) trisphosphate and mediates the translocation of GRP1 to the plasma membrane upon agonist stimulation of PI 3-kinase activity. Using a (32)P-labeled GRP1 probe to screen a mouse brain cDNA expression library, we isolated a cDNA clone encoding a GRP1-binding partner (GRSP1) that exists as two different splice variants in brain and lung. The GRSP1 protein contains a FERM protein interaction domain as well as two coiled coil domains and may therefore function as a scaffolding protein. Mapping experiments revealed that the interaction of GRP1 and GRSP1 occurs through the coiled coil domains in the two proteins. Immunodepletion experiments indicate that virtually all of the endogenous GRSP1 protein exists as a complex with GRP1 in lung. When co-expressed in Chinese hamster ovary cells expressing the human insulin receptor, both proteins display a diffuse, cytoplasmic localization. Acute translocation and co-localization of GRSP1 and GRP1 to ruffles in the plasma membrane was evident after insulin stimulation. These results identify GRSP1 as a novel member of GRP1 signaling complexes that are acutely recruited to plasma membrane ruffles in response to insulin receptor signaling.

Actin cytoskeletal association of cytohesin-1 is regulated by specific phosphorylation of its carboxyl-terminal polybasic domain

Cell adhesion mediated by integrin receptors is controlled by intracellular signal transduction cascades. Cytohesin-1 is an integrin-binding protein and guanine nucleotide exchange factor that activates binding of the leukocyte integrin leukocyte function antigen-1 to its ligand, intercellular adhesion molecule 1. Cytohesin-1 bears a carboxyl-terminal pleckstrin homology domain that aids in reversible membrane recruitment and functional regulation of the protein. Although phosphoinositide-dependent membrane attachment of cytohesin-1 is mediated primarily by the pleckstrin homology domain, this function is further strengthened by a short carboxyl-terminal polybasic amino acid sequence. We show here that a serine/threonine motif within the short polybasic stretch of cytohesin-1 is phosphorylated by purified protein kinase C delta in vitro. Furthermore, the respective residues are also found to be phosphorylated after phorbol ester stimulation in vivo. Biochemical and functional analyses show that phosphorylated cytohesin-1 is able to tightly associate with the actin cytoskeleton, and we further demonstrate that phosphorylation of the protein is required for maximal leukocyte function antigen-1-mediated adhesion of Jurkat cells to intercellular adhesion molecule 1. These data suggest that both phosphatidylinositol 3-kinase and protein kinase C-dependent intracellular pathways that stimulate beta(2)-integrin-mediated adhesion of T lymphocytes converge on cytohesin-1 as functional integrator.

Phosphatidylinositol 3-kinase-dependent translocation of phospholipase Cgamma2 in mouse megakaryocytes is independent of Bruton tyrosine kinase translocation

Activation of the collagen receptor glycoprotein VI (GPVI) by a collagen-related peptide (CRP) induces stimulation of platelets and megakaryocytes through the phosphatidylinositol (PI) 3-kinase-dependent pathway leading to activation of Bruton tyrosine kinase (Btk) and phospholipase Cgamma2 (PLCgamma2). Here, we present evidence that both proteins undergo PI 3-kinase-dependent translocation to the plasma membrane on CRP stimulation that is markedly inhibited by wortmannin and LY294002. Translocation of PLCgamma2 but not Btk is also seen in megakaryocytes from X-linked immunodeficiency mice, which have a mutation that reduces the affinity of the pleckstrin homology (PH) domain of Btk for PI 3,4,5-trisphosphate (PI 3,4,5-P3). Activation of PC12 cells by epidermal growth factor (EGF) results in increased PI 3-kinase activity and high PI 3,4,5-P3 levels that trigger translocation of the green fluorescent protein (GFP)-labeled PH of Btk, but not the GFP-labeled PH and tandem Src homology 2 (SH2) domains of PLCgamma2. In contrast to the results with CRP, the G protein-coupled receptor agonist thrombin stimulates PI 3-kinase-independent translocation of Btk but not PLCgamma2. In conclusion, these results demonstrate that in mouse megakaryocytes, CRP leads to PI 3-kinase-dependent translocation of PLCgamma2 and Btk that are independent of one another, whereas thrombin only induces translocation of Btk through a pathway that is independent of PI 3-kinase activity.

Sprouty proteins are targeted to membrane ruffles upon growth factor receptor tyrosine kinase activation. Identification of a novel translocation domain

Sprouty (Spry) was first identified in a genetic screen in Drosophila to be an antagonist of fibroblast growth factor and epidermal growth factor (EGF) signaling, seemingly by inhibiting the Ras/MAP kinase pathway. Data base searches lead to the identification and cloning of, to date, four mammalian sprouty genes. The primary sequences of the mammalian sprouty gene products share a well conserved cysteine-rich C-terminal domain with the Drosophila protein. The N-terminal regions, however, do not exhibit significant homology. This study aimed at determining the disposition of Spry proteins in intact cells before and after stimulation of the EGF receptor tyrosine kinase. Full-length or deletion mutants of Spry, tagged at the N termini with the FLAG-epitope, were expressed in COS-1 cells by transient transfection and analyzed by immunofluorescence microscopy before and after EGF stimulation of the cells. In unstimulated cells, the Spry proteins were distributed throughout the cytosol except for human Sprouty2 (hSpry2), which, although generally located in the cytosol, co-localized with microtubules. In all cases, the Spry proteins underwent rapid translocation to membrane ruffles following EGF stimulation. The optimal translocation domain was identified by deletion and immunofluorescence analysis to be a highly conserved 105-amino acid domain in the C-terminal half of the hSpry2 protein. The translocation of this conserved domain, based on hSpry2 data, was independent of the activation of phosphatidylinositol-3 kinase.

Distinct polyphosphoinositide binding selectivities for pleckstrin homology domains of GRP1-like proteins based on diglycine versus triglycine motifs

GRP1 and the related proteins ARNO and cytohesin-1 are ARF exchange factors that contain a pleckstrin homology (PH) domain thought to target these proteins to cell membranes through binding polyphosphoinositides. Here we show the PH domains of all three proteins exhibit relatively high affinity for dioctanoyl phosphatidylinositol 3,4,5-triphosphate (PtdIns(3,4,5)P(3)), with K(D) values of 0.05, 1.6 and 1.0 micrometer for GRP1, ARNO, and cytohesin-1, respectively. However, the GRP1 PH domain was unique among these proteins in its striking selectivity for PtdIns(3,4, 5)P(3) versus phosphatidylinositol 4,5-diphosphate (PtdIns(4,5)P(2)), for which it exhibits about 650-fold lower apparent affinity. Addition of a glycine to the Gly(274)-Gly(275) motif in GRP1 greatly increased its binding affinity for PtdIns(4,5)P(2) with little effect on its binding to PtdIns(3,4,5)P(3), while deletion of a single glycine in the corresponding triglycine motif of the ARNO PH domain markedly reduced its binding affinity for PtdIns(4,5)P(2) but not for PtdIns(3,4,5)P(3). In intact cells, the hemagglutinin epitope-tagged PH domain of GRP1 was recruited to ruffles in the cell surface in response to insulin, as were full-length GRP1 and cytohesin-1, but the PH domain of cytohesin-1 was not. These data indicate that the unique diglycine motif in the GRP1 PH domain, as opposed to the triglycine in ARNO and cytohesin-1, directs its remarkable PtdIns(3,4,5)P(3) binding selectivity.

Integrin cytoplasmic domain-binding proteins

Integrins are a large family of cell surface receptors that mediate cell adhesion and influence migration, signal transduction, and gene expression. The cytoplasmic domains of integrins play a pivotal role in these integrin-mediated cellular functions. Through interaction with the cytoskeleton, signaling molecules, and other cellular proteins, integrin cytoplasmic domains transduce signals from both the outside and inside of the cell and regulate integrin-mediated biological functions. Identification and functional analyses of integrin cytoplasmic domain-binding proteins have been pursued intensively. In recent years, more cellular proteins have been reported to directly interact with integrin cytoplasmic domains and some of these interactions may play important roles in integrin-mediated biological responses. Integrin (β) chains, for example, interact with actin-binding proteins (e.g. talin and filamin), which form mechanical links to the cytoskeleton. These and other proteins (e.g. FAK, ILK and novel proteins such as TAP20) might also link integrins to signaling mechanisms and, in some cases (e.g. JAB1) mediate integrin-dependent gene regulation.

Cytohesins and centaurins: mediators of PI 3-kinase-regulated Arf signaling

Receptor-activated phosphoinositide (PI) 3-kinases produce PtdIns(3, 4,5)P(3) and its metabolite PtdIns(3,4)P(2) that function as second messengers in membrane recruitment and activation of target proteins. The cytohesin and centaurin protein families are potential targets for PtdIns(3,4,5)P(3) that also regulate and interact with Arf GTPases. Consequently, these families are poised to transduce PI 3-kinase activation into coordinated control of Arf-dependent pathways. Proposed downstream events in PI 3-kinase-regulated Arf cascades include modulation of vesicular trafficking and the actin cytoskeleton.

GAP1IP4BP contains a novel group I pleckstrin homology domain that directs constitutive plasma membrane association

The group I family of pleckstrin homology (PH) domains are characterized by their inherent ability to specifically bind phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) and its corresponding inositol head-group inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). In vivo this interaction results in the regulated plasma membrane recruitment of cytosolic group I PH domain-containing proteins following agonist-stimulated PtdIns(3,4,5)P(3) production. Among group I PH domain-containing proteins, the Ras GTPase-activating protein GAP1(IP4BP) is unique in being constitutively associated with the plasma membrane. Here we show that, although the GAP1(IP4BP) PH domain interacts with PtdIns(3,4, 5)P(3), it also binds, with a comparable affinity, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) (K(d) values of 0.5 +/- 0.2 and 0.8 +/- 0.5 microm, respectively). Intriguingly, whereas this binding site overlaps with that for Ins(1,3,4,5)P(4), consistent with the constitutive plasma membrane association of GAP1(IP4BP) resulting from its PH domain-binding PtdIns(4,5)P(2), we show that in vivo depletion of PtdIns(4,5)P(2), but not PtdIns(3,4,5)P(3), results in dissociation of GAP1(IP4BP) from this membrane. Thus, the Ins(1,3,4,5)P(4)-binding PH domain from GAP1(IP4BP) defines a novel class of group I PH domains that constitutively targets the protein to the plasma membrane and may allow GAP1(IP4BP) to be regulated in vivo by Ins(1,3,4,5)P(4) rather than PtdIns(3,4,5)P(3).

N-terminal targeting of guanine nucleotide exchange factors (GEF) for ADP ribosylation factors (ARF) to the Golgi

B2-1 (cytohesin-1) is a member of a group of proteins (including ARNO and ARNO3) that are all of similar size and domain composition. The three proteins contain an N-terminal coiled-coil domain, followed by a Sec7 and a pleckstrin homology (PH) domain. While it is well established that the Sec7 domain functions as a guanine nucleotide exchange factor (GEF) for ADP-ribosylation factors (ARFs) and the PH domain anchors the proteins to membrane phosphoinositols, the function of the N-terminal domain is unknown. Here we show that the N terminus of B2-1 (residues 1–54) is necessary and sufficient to target the protein to the Golgi. The Sec7+PH domains of B2-1 (residues 55–398) are not sufficient for Golgi localization. Further deletion analysis and point mutagenesis indicate that the coiled-coil domain within the N terminus is responsible for Golgi targeting. Furthermore, ARNO and ARNO3 N termini also have the same capability of targeting to the Golgi. We conclude that the N-terminal, (α)-helical, coiled-coil domain is used to target this family of proteins to the Golgi complex.

Phosphoinositides determine specificity of the guanine-nucleotide exchange activity of cytohesin-1 for ADP-ribosylation factors derived from a mammalian expression system

ADP-ribosylation factors (ARFs) are small Ras-like GTPases which play important roles in intracellular vesicle transport and in the remodeling of the actin cytoskeleton. Guanine nucleotide exchange factors (GEFs) for ARFs have recently been identified. One of them, cytohesin-1, a 47-kDa cytoplasmic protein acts as an inside-out signaling molecule and regulates binding of the beta2 integrin leukocyte function antigen 1 (LFA-1) to its ligand intercellular adhesion molecule 1 (ICAM-1). In this study, we address the regulation of the GEF activity of cytohesin-1 by phosphoinositides, using mammalian expression of functional ARF-Ig chimeras. The fusion proteins, which can be quantitatively immunoprecipitated on protein A-Sepharose, target to the expected intracellular compartments, and they are readily induced to bind GTP in vitro. We show that both ARF1-Ig and ARF6-Ig chimeras are activated in vitro by cytohesin-1. However, GEF activity towards ARF6 is strongly suppressed by phosphatidylinositol-(3,4,5)-trisphosphate (PtdInsP3). In contrast, cytohesin-1-dependent GTP binding of ARF1 is significantly enhanced by PtdInsP3. We conclude that the membrane phospholipid PtdInsP3 determines the specificity of the GEF activity of cytohesin-1.

Signaling by distinct classes of phosphoinositide 3-kinases

Many signaling pathways converge on and regulate phosphoinositide 3-kinase (PI3K) enzymes whose inositol lipid products are key mediators of intracellular signaling. Different PI3K isoforms generate specific lipids that bind to FYVE and pleckstrin homology (PH) domains in a variety of proteins, affecting their localization, conformation, and activities. Here we review the activation mechanisms of the different types of PI3Ks and their downstream actions, with focus on the PI3Ks that are acutely triggered by extracellular stimulation.