A comparative analysis of the phosphoinositide binding specificity of pleckstrin homology domains

Phosphoinositide 3-kinase signalling pathways

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the 3′-OH position of the inositol ring of inositol phospholipids, producing three lipid products: PtdIns(3)P, PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3). These lipids bind to the pleckstrin homology (PH) domains of proteins and control the activity and subcellular localisation of a diverse array of signal transduction molecules. Three major classes of signalling molecule are regulated by binding of D-3 phosphoinositides to PH domains: guanine-nucleotide-exchange proteins for Ρ family GTPases, the TEC family tyrosine kinases such as BTK and ITK in B and T lymphocytes, respectively, and the AGC superfamily of serine/threonine protein kinases. These molecules are activated by a variety of extracellular stimuli and have been implicated in a wide range of cellular processes, including cell cycle progression, cell growth, cell motility, cell adhesion and cell survival.

A critical role for phosphoinositide 3-kinase upstream of Gab1 and SHP2 in the activation of ras and mitogen-activated protein kinases by epidermal growth factor

Although the mechanisms involved in the activation of mitogen-activated protein kinases (MAPK) by receptor tyrosine kinases do not display an obvious role for phosphoinositide 3-kinases (PI3Ks), we have observed in the nontransformed cell line Vero stimulated with epidermal growth factor (EGF) that wortmannin and LY294002 nearly abolished MAPK activation. The effect was observed under strong stimulation and was independent of EGF concentration. In addition, three mutants of class Ia PI3Ks were found to inhibit MAPK activation to an extent similar to their effect on Akt/protein kinase B activation. To determine the importance of PI3K lipid kinase activity in MAPK activation, we have used the phosphatase PTEN and the pleckstrin homology domain of Tec kinase. Overexpression of these proteins, but not control mutants, was found to inhibit MAPK activation, suggesting that the lipid products of class Ia PI3K are necessary for MAPK signaling. We next investigated the location of PI3K in the MAPK cascade. Pharmacological inhibitors and dominant negative forms of PI3K were found to block the activation of Ras induced by EGF. Upstream from Ras, although association of Grb2 with its conventional effectors was independent of PI3K, we have observed that the recruitment of the tyrosine phosphatase SHP2 required PI3K. Because SHP2 was also essential for Ras activation, this suggested the existence of a PI3K/SHP2 pathway leading to the activation of Ras. In addition, we have observed that the docking protein Gab1, which is involved in PI3K activation during EGF stimulation, is also implicated in this pathway downstream of PI3K. Indeed, the association of Gab1 with SHP2 was blocked by PI3K inhibitors, and expression of Gab1 mutant deficient for binding to SHP2 was found to inhibit Ras stimulation without interfering with PI3K activation. These results show that, in addition to Shc and Grb2, a PI3K-dependent pathway involving Gab1 and SHP2 is essential for Ras activation under EGF stimulation.

Membrane recruitment of DOCK180 by binding to PtdIns(3,4,5)P3

DOCK180 was originally identified as one of two major proteins bound to the Crk oncogene product and became an archetype of the CDM family of proteins, including Ced-5 of Caenorhabditis elegans and Mbc of Drosophila melanogaster. Further study has suggested that DOCK180 is involved in the activation of Rac by the CrkII-p130(Cas) complex. With the use of deletion mutants of DOCK180, we found that the C-terminal region containing a cluster of basic amino acids was required for binding to and activation of Rac. This region showed high amino-acid sequence similarity to the consensus sequence of the phosphoinositide-binding site; this led us to examine whether this basic region binds to phosphoinositides. For this purpose we used PtdIns(3,4,5)P(3)-APB beads, as reported previously [Shirai, Tanaka, Terada, Sawada, Shirai, Hashimoto, Nagata, Iwamatsu, Okawa, Li et al. (1998) Biochim. Biophys. Acta 1402, 292-302]. By using various competitors, we demonstrated the specific binding of DOCK180 to PtdIns(3,4,5)P(3). The expression of active phosphoinositide 3-kinase (PI-3K) did not enhance a DOCK180-induced increase in GTP-Rac; however, the expression of PI-3K translocated DOCK180 to the plasma membrane. Thus DOCK180 contained a phosphoinositide-binding domain, as did the other guanine nucleotide exchange factors with a Dbl homology domain, and was translocated to the plasma membrane on the activation of PI-3K.

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.

Regulation of apoptosis by phosphatidylinositol 4,5-bisphosphate inhibition of caspases, and caspase inactivation of phosphatidylinositol phosphate 5-kinases

Phosphoinositides such as phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate promote cell survival and protect against apoptosis by activating Akt/PKB, which phosphorylates components of the apoptotic machinery. We now report that another phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP2) is a direct inhibitor of initiator caspases 8 and 9, and their common effector caspase 3. PIP2 inhibited procaspase 9 processing in cell extracts and in a reconstituted procaspase 9/Apaf1 apoptosome system. It inhibited purified caspase 3 and 8 activity, at physiologically attainable PIP2 levels in mixed lipid vesicles. Caspase 3 binding to PIP2 was confirmed by cosedimentation with mixed lipid vesicles. Overexpression of phosphatidylinositol phosphate 5-kinase alpha (PIP5KIalpha), which synthesizes PIP2, suppressed apoptosis, whereas a kinase-deficient mutant did not. Protection by the wild-type PIP5KIalpha was accompanied by decreases in the generation of activated caspases and of caspase 3-cleaved PARP. Protection was not mediated through PIP3 or Akt activation. An anti-apoptotic role for PIP(2) is further substantiated by our finding that PIP5KIalpha was cleaved by caspase 3 during apoptosis, and cleavage inactivated PIP5KIalpha in vitro. Mutation of the P(4) position (D279A) of the PIP5KIalpha caspase 3 cleavage consensus prevented cleavage in vitro, and during apoptosis in vivo. Significantly, the caspase 3-resistant PIP5KIalpha mutant was more effective in suppressing apoptosis than the wild-type kinase. These results show that PIP2 is a direct regulator of apical and effector caspases in the death receptor and mitochondrial pathways, and that PIP5KIalpha inactivation contributes to the progression of apoptosis. This novel feedforward amplification mechanism for maintaining the balance between life and death of a cell works through phosphoinositide regulation of caspases and caspase regulation of phosphoinositide synthesis.

Dynactin-dependent, dynein-driven vesicle transport in the absence of membrane proteins: a role for spectrin and acidic phospholipids

We reconstituted dynein-driven, dynactin-dependent vesicle transport using protein-free liposomes and soluble components from squid axoplasm. Dynein and dynactin, while necessary, are not the only essential cytosolic factors; axonal spectrin is also required. Spectrin is resident on axonal vesicles, and rebinds from cytosol to liposomes or proteolysed vesicles, concomitant with their dynein-dynactin-dependent motility. Binding of purified axonal spectrin to liposomes requires acidic phospholipids, as does motility. Using dominant negative spectrin polypeptides and a drug that releases PH domains from membranes, we show that spectrin is required for linking dynactin, and thereby dynein, to acidic phospholipids in the membrane. We verify this model in the context of liposomes, isolated axonal vesicles, and whole axoplasm. We conclude that spectrin has an essential role in retrograde axonal transport.

A phosphatidylinositol 3,4,5-trisphosphate analogue with low serum protein-binding affinity

Phosphatidylinositol 3,4,5-trisphosphate (PIP3) plays an important role in the regulation of diverse physiological functions. Recent evidence indicates that PIP3 is cell permeant, and can be added exogenously to modulate cellular responses. However, like many other phospholipids, PIP3 binds serum proteins with high affinity, resulting in rapid deactivation of this lipid second messenger. Our study indicates that bovine serum albumin (BSA) at concentrations as low as 10 microg/mL abrogated the biological activity of dipalmitoyl-PIP3. This nonspecific interaction with serum proteins hampers the use of PIP3 in biological studies where serum is needed. We report here an ether-linked PIP3 analogue, 1-O-(1-O-hexadecyl-2-O-methyl-sn-glycero-3-phosphoryl)-myo-inositol 3,4,5-trisphosphate (C16Me-PIP3). which displays low serum protein-binding affinity while retaining the biological function of PIP3. The affinity of C16Me-PIP3 with BSA was two orders of magnitude lower than that of its dipalmitoyl-counterpart. Biochemical data indicate that C16Me-PIP3 was able to stimulate Ca2+ influx in T cells in the presence of moderate levels (up to 1 mg/mL) of BSA. Thus. C16Me-PIP3 may provide a useful tool to study the physiological function of phosphoinositide (PI) 3-kinase in vivo.

Tec kinase signaling in T cells is regulated by phosphatidylinositol 3-kinase and the Tec pleckstrin homology domain

Tec, the prototypical member of the Tec family of tyrosine kinases, is abundantly expressed in T cells and other hemopoietic cell types. Although the functions of Itk and Txk have recently been investigated, little is known about the role of Tec in T cells. Using antisense oligonucleotide treatment to deplete Tec protein from primary T cells, we demonstrate that Tec plays a role in TCR signaling leading to IL-2 gene induction. Interestingly, Tec kinases are the only known family of tyrosine kinases containing a pleckstrin homology (PH) domain. Using several PH domain mutants overexpressed in Jurkat T cells, we show that the Tec PH domain is required for Tec-mediated IL-2 gene induction and TCR-mediated Tec tyrosine phosphorylation. Furthermore, we show that Tec colocalizes with the TCR after TCR cross-linking, and that both the Tec PH and Src homology (SH) 2 domains play a role in this association. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, abolishes Tec-mediated IL-2 gene induction and Tec tyrosine phosphorylation, and partially suppresses Tec colocalization with the activated TCR. Thus, our data implicate the Tec kinase PH domain and phosphatidylinositol 3-kinase in Tec signaling downstream of the TCR.

Protein-tyrosine phosphatase D1, a potential regulator and effector for Tec family kinases

Etk, also named Bmx, is a member of the Tec tyrosine kinase family, which is characterized by a multimodular structure including a pleckstrin homology (PH) domain, an SH3 domain, an SH2 domain, and a catalytic domain. The signaling mechanisms regulating Etk kinase activity remain largely unknown. To identify factor(s) regulating Etk activity, we used the PH domain and a linker region of Etk as a bait for a yeast two-hybrid screen. Three independent clones encoding protein-tyrosine phosphatase D1 (PTPD1) fragments were isolated. The binding of PTPD1 to Etk is specific since PTPD1 cannot associate with either the Akt PH domain or lamin. In vitro and in vivo binding studies demonstrated that PTPD1 can interact with Etk and that residues 726-848 of PTPD1 are essential for this interaction. Deletion analysis of Etk indicated that the PH domain is essential for PTPD1 interaction. Furthermore, the Etk-PTPD1 interaction stimulated the kinase activity of Etk, resulting in an increased phosphotyrosine content in both factors. The Etk-PTPD1 interaction also increased Stat3 activation. The effect of PTPD1 on Etk activation is specific since PTPD1 cannot potentiate Jak2 activity upon Stat3 activation. In addition, Tec (but not Btk) kinase can also be activated by PTPD1. Taken together, these findings indicate that PTPD1 can selectively associate with and stimulate Tec family kinases and modulate Stat3 activation.

The transcription factor Bright associates with Bruton's tyrosine kinase, the defective protein in immunodeficiency disease

Binding of the transcription factor Bright to Ig heavy chain loci after B cell activation is associated with increased heavy chain transcription. We now report that Bright coprecipitates with Bruton's tyrosine kinase (Btk), the defective enzyme in X-linked immunodeficiency disease (xid). Furthermore, we observed Btk in the nucleus of activated murine B cells, and mobility shift assays suggest that it is a component of the Bright DNA-binding complex. While BRIGHT protein was synthesized in activated spleen cells from xid mice, it did not bind DNA or associate stably with Btk. These data suggest that deficiencies in BRIGHT DNA-binding activity may contribute to the defects in Ig production seen in xid mice.

Signaling network of the Btk family kinases

The Btk family kinases represent new members of non-receptor tyrosine kinases, which include Btk/Atk, Itk/Emt/Tsk, Bmx/Etk, and Tec. They are characterized by having four structural modules: PH (pleckstrin homology) domain, SH3 (Src homology 3) domain, SH2 (Src homology 2) domain and kinase (Src homology 1) domain. Increasing evidence suggests that, like Src-family kinases, Btk family kinases play central but diverse modulatory roles in various cellular processes. They participate in signal transduction in response to virtually all types of extracellular stimuli which are transmitted by growth factor receptors, cytokine receptors, G-protein coupled receptors, antigen-receptors and integrins. They are regulated by many non-receptor tyrosine kinases such as Src, Jak, Syk and FAK family kinases. In turn, they regulate many of major signaling pathways including those of PI3K, PLCgamma and PKC. Both genetic and biochemical approaches have been used to dissect the signaling pathways and elucidate their roles in growth, differentiation and apoptosis. An emerging new role of this family of kinases is cytoskeletal reorganization and cell motility. The physiological importance of these kinases was amply demonstrated by their link to the development of immunodeficiency diseases, due to germ-line mutations. The present article attempts to review the structure and functions of Btk family kinases by summarizing our current knowledge on the interacting partners associated with the different modules of the kinases and the diverse signaling pathways in which they are involved.

The Ras/p120 GTPase-activating protein (GAP) interaction is regulated by the p120 GAP pleckstrin homology domain

Pleckstrin homology domains are structurally conserved functional domains that can undergo both protein/protein and protein/lipid interactions. Pleckstrin homology domains can mediate inter- and intra-molecular binding events to regulate enzyme activity. They occur in numerous proteins including many that interact with Ras superfamily members, such as p120 GAP. The pleckstrin homology domain of p120 GAP is located in the NH(2)-terminal, noncatalytic region of p120 GAP. Overexpression of the noncatalytic domains of p120 GAP may modulate Ras signal transduction pathways. Here, we demonstrate that expression of the isolated pleckstrin homology domain of p120 GAP specifically inhibits Ras-mediated signaling and transformation but not normal cellular growth. Furthermore, we show that the pleckstrin homology domain binds the catalytic domain of p120 GAP and interferes with the Ras/GAP interaction. Thus, we suggest that the pleckstrin homology domain of p120 GAP may specifically regulate the interaction of Ras with p120 GAP via competitive intra-molecular binding.

Tiam1 mutations in human renal-cell carcinomas

Tiam1 activates the Rho-like GTPase Rac1, and studies indicate that Tiam1-Rac1 signaling affects invasion in different ways depending on the cell type studied. However, no investigations on Tiam1 in human tumors have been reported. Here, we show that for 4 of 5 human renal-cell carcinoma (RCC) cell lines the expression levels of Tiam1 tended to be inversely correlated with in vitro invasiveness, whereas no obvious correlation could be found between the expression levels of Rac1 and invasion. Subsequent mutation analysis of these cell lines revealed no mutations in Rac1 but up to 5 different point mutations in the Tiam1 gene. Of these, 1 mutation (A441G) was located in the NH2-terminal pleckstrin homology domain, which is essential for membrane localization and functional activity of Tiam1. By analysis of an additional 30 primary human RCCs, mutation A441G was found in 4 of 35 tumors and tumor cell lines (11.5%) but not in the respective normal kidney tissues. By enzymatic digestion, mutation A441G proved to be heterozygous, suggesting a dominant active function. This was supported by showing that stable over-expression of mutated A441G-Tiam1 induced transformation of NIH3T3 cells, as determined in a colony formation assay, whereas empty vector and wild-type Tiam1 failed to do so. In conclusion, a distinct Tiam1 mutation (A441G) was identified in several human RCCs. This mutation induced transformation of NIH3T3 cells and, hence, might play a major role in the progression of human RCCs. Further analyses on Tiam1 mutations in human tumors might give new clues to their role in tumor progression.

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.

Regulation of the Rac1-specific exchange factor Tiam1 involves both phosphoinositide 3-kinase-dependent and -independent components

The small GTPase Rac1 is involved in regulating membrane ruffling, gene transcription, cell-cycle progression and cell transformation, and some of these events are blocked by inhibitors of phosphoinositide 3-kinase (PI 3-kinase). Moreover, Rac1 can be activated by several guanine nucleotide exchange factors, which facilitate the release of GDP. We therefore investigated the ability of PI 3-kinase lipid products to regulate Tiam1, a Rac1-specific exchange factor. Tiam1 bound to polyphosphorylated inositol lipids in the rank order PtdIns(3,4,5)P(3)>PtdIns(3,4)P(2) >>PtdIns(4,5)P(2), and this binding could be attributed to the N-terminal pleckstrin-homology (N-PH) domain. Both PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) enhanced Tiam1 guanine nucleotide exchange activity in vitro, but PtdIns(4,5)P(2) had no effect. Co-expression of a constitutively active PI 3-kinase with Tiam1 increased the amount of GTP-bound Rac1 in vivo, a response which required the N-PH domain of Tiam1. Ectopic expression of Tiam1 caused membrane ruffling in Swiss 3T3 cells that was characterized by wortmannin-sensitive and -insensitive components, which required the N-PH domain and the C-terminal PH domain of Tiam1 respectively. These results reveal novel facets of Tiam1-dependent regulation of Rac1 function.

Myosin-X, a novel myosin with pleckstrin homology domains, associates with regions of dynamic actin

Myosin-X is the founding member of a novel class of unconventional myosins characterized by a tail domain containing multiple pleckstrin homology domains. We report here the full-length cDNA sequences of human and bovine myosin-X as well as the first characterization of this protein's distribution and biochemical properties. The 235 kDa myosin-X contains a head domain with <45% protein sequence identity to other myosins, three IQ motifs, and a predicted stalk of coiled coil. Like several other unconventional myosins and a plant kinesin, myosin-X contains both a myosin tail homology 4 (MyTH4) domain and a FERM (band 4.1/ezrin/radixin/moesin) domain. The unique tail domain also includes three pleckstrin homology domains, which have been implicated in phosphatidylinositol phospholipid signaling, and three PEST sites, which may allow cleavage of the myosin tail. Most intriguingly, myosin-X in cultured cells is present at the edges of lamellipodia, membrane ruffles, and the tips of filopodial actin bundles. The tail domain structure, biochemical features, and localization of myosin-X suggest that this novel unconventional myosin plays a role in regions of dynamic actin.

New role for Shc in activation of the phosphatidylinositol 3-kinase/Akt pathway

Most, if not all, cytokines activate phosphatidylinositol 3-kinase (PI-3K). Although many cytokine receptors have direct binding sites for the p85 subunit of PI-3K, others, such as the interleukin-3 (IL-3) receptor beta common chain (betac) and the IL-2 receptor beta chain (IL-2Rbeta), lack such sites, leaving the mechanism by which they activate PI-3K unclear. Here, we show that the protooncoprotein Shc, which promotes Ras activation by recruiting the Grb2-Sos complex in response to stimulation of cytokine stimulation, also signals to the PI-3K/Akt pathway. Analysis of Y-->F and "add-back" mutants of betac shows that Y577, the Shc binding site, is the major site required for Gab2 phosphorylation in response to cytokine stimulation. When fused directly to a mutant form of IL-2Rbeta that lacks other cytoplasmic tyrosines, Shc can promote Gab2 tyrosyl phosphorylation. Mutation of the three tyrosyl phosphorylation sites of Shc, which bind Grb2, blocks the ability of the Shc chimera to evoke Gab2 tyrosyl phosphorylation. Overexpression of mutants of Grb2 with inactive SH2 or SH3 domains also blocks cytokine-stimulated Gab2 phosphorylation. The majority of cytokine-stimulated PI-3K activity associates with Gab2, and inducible expression of a Gab2 mutant unable to bind PI-3K markedly impairs IL-3-induced Akt activation and cell growth. Experiments with the chimeric receptors indicate that Shc also signals to the PI-3K/Akt pathway in response to IL-2. Our results suggest that cytokine receptors lacking direct PI-3K binding sites activate Akt via a Shc/Grb2/Gab2/PI-3K pathway, thereby regulating cell survival and/or proliferation.

A specific intermolecular association between the regulatory domains of a Tec family kinase

Interleukin-2 tyrosine kinase (Itk), is a T-cell specific tyrosine kinase of the Tec family. We have examined a novel intermolecular interaction between the SH3 and SH2 domains of Itk. In addition to the interaction between the isolated domains, we have found that the dual SH3/SH2 domain-containing fragment of Itk self-associates in a specific manner in solution. Tec family members contain the SH3, SH2 and catalytic domains common to many kinase families but are distinguished by a unique amino-terminal sequence, which contains a proline-rich stretch. Previous work has identified an intramolecular regulatory association between the proline-rich region and the adjacent SH3 domain of Itk. The intermolecular interaction between the SH3 and SH2 domains of Itk that we describe provides a possible mechanism for displacement of this intramolecular regulatory sequence, a step that may be required for full Tec kinase activation. Additionally, localization of the interacting surfaces on both the SH3 and SH2 domains by chemical shift mapping has provided information about the molecular details of this recognition event. The interaction involves the conserved aromatic binding pocket of the SH3 domain and a newly defined binding surface on the SH2 domain. The interacting residues on the SH2 domain do not conform to the consensus motif for an SH3 proline-rich ligand. Interestingly, we note a striking correlation between the SH2 residues that mediate this interaction and those residues that, when mutated in the Tec family member Btk, cause the hereditary immune disorder, X-linked agamaglobulinemia.

Mechanism of N-WASP activation by CDC42 and phosphatidylinositol 4, 5-bisphosphate

Neuronal Wiskott-Aldrich Syndrome protein (N-WASP) transmits signals from Cdc42 to the nucleation of actin filaments by Arp2/3 complex. Although full-length N-WASP is a weak activator of Arp2/3 complex, its activity can be enhanced by upstream regulators such as Cdc42 and PI(4,5)P(2). We dissected this activation reaction and found that the previously described physical interaction between the NH(2)-terminal domain and the COOH-terminal effector domain of N-WASP is a regulatory interaction because it can inhibit the actin nucleation activity of the effector domain by occluding the Arp2/3 binding site. This interaction between the NH(2)- and COOH termini must be intramolecular because in solution N-WASP is a monomer. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) influences the activity of N-WASP through a conserved basic sequence element located near the Cdc42 binding site rather than through the WASp homology domain 1. Like Cdc42, PI(4,5)P(2) reduces the affinity between the NH(2)- and COOH termini of the molecule. The use of a mutant N-WASP molecule lacking this basic stretch allowed us to delineate a signaling pathway in Xenopus extracts leading from PI(4, 5)P(2) to actin nucleation through Cdc42, N-WASP, and Arp2/3 complex. In this pathway, PI(4,5)P(2) serves two functions: first, as an activator of N-WASP; and second, as an indirect activator of Cdc42.

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).

A phospholipid kinase regulates actin organization and intercellular bridge formation during germline cytokinesis

The endgame of cytokinesis can follow one of two pathways depending on developmental context: resolution into separate cells or formation of a stable intercellular bridge. Here we show that the four wheel drive (fwd) gene of Drosophila melanogaster is required for intercellular bridge formation during cytokinesis in male meiosis. In fwd mutant males, contractile rings form and constrict in dividing spermatocytes, but cleavage furrows are unstable and daughter cells fuse together, producing multinucleate spermatids. fwd is shown to encode a phosphatidylinositol 4-kinase (PI 4-kinase), a member of a family of proteins that perform the first step in the synthesis of the key regulatory membrane phospholipid PIP2. Wild-type activity of the fwd PI 4-kinase is required for tyrosine phosphorylation in the cleavage furrow and for normal organization of actin filaments in the constricting contractile ring. Our results suggest a critical role for PI 4-kinases and phosphatidylinositol derivatives during the final stages of cytokinesis.

Regulation of Tiam1 nucleotide exchange activity by pleckstrin domain binding ligands

Rho family GTPases play roles in cytoskeletal organization and cellular transformation. Tiam1 is a member of the Dbl family of guanine nucleotide exchange factors that activate Rho family GTPases. These exchange factors have in common a catalytic Dbl homology and adjacent pleckstrin homology domain. Previous structural studies suggest that the pleckstrin domain, a putative phosphoinositide-binding site, may serve a regulatory function. We identified ascorbyl stearate as a compound that binds to the pleckstrin domain of p120 Ras GTPase-activating protein. Furthermore, ascorbyl stearate appears to be a general pleckstrin domain ligand, perhaps by mimicking an endogenous amphiphilic ligand. Tiam1 nucleotide exchange activity was greatly stimulated by ascorbyl stearate. Certain phosphoinositides also stimulated Tiam1 activity but were less potent than ascorbyl stearate. Tiam1 contains an additional N-terminal pleckstrin domain, but only the C-terminal pleckstrin domain was required for activation. Our results suggest that the pleckstrin domains of Dbl-type proteins may not only be involved in subcellular localization but may also directly regulate the nucleotide exchange activity of an associated Dbl homology domain. In addition, this paper introduces ascorbyl stearate as a pleckstrin domain ligand that can modulate the activity of certain pleckstrin domain-containing proteins.

Functional analysis of a human homologue of the Drosophila actin binding protein anillin suggests a role in cytokinesis

We have characterized a human homologue of anillin, a Drosophila actin binding protein. Like Drosophila anillin, the human protein localizes to the nucleus during interphase, the cortex following nuclear envelope breakdown, and the cleavage furrow during cytokinesis. Anillin also localizes to ectopic cleavage furrows generated between two spindles in fused PtK(1) cells. Microinjection of antianillin antibodies slows cleavage, leading to furrow regression and the generation of multinucleate cells. GFP fusions that contain the COOH-terminal 197 amino acids of anillin, which includes a pleckstrin homology (PH) domain, form ectopic cortical foci during interphase. The septin Hcdc10 localizes to these ectopic foci, whereas myosin II and actin do not, suggesting that anillin interacts with the septins at the cortex. Robust cleavage furrow localization requires both this COOH-terminal domain and additional NH(2)-terminal sequences corresponding to an actin binding domain defined by in vitro cosedimentation assays. Endogenous anillin and Hcdc10 colocalize to punctate foci associated with actin cables throughout mitosis and the accumulation of both proteins at the cell equator requires filamentous actin. These results indicate that anillin is a conserved cleavage furrow component important for cytokinesis. Interactions with at least two other furrow proteins, actin and the septins, likely contribute to anillin function.

A mutation in the pleckstrin homology (PH) domain of the FGD1 gene in an Italian family with faciogenital dysplasia (Aarskog-Scott syndrome)

Aarskog-Scott Syndrome (AAS) is an X-linked disorder characterised by short stature and multiple facial, limb and genital abnormalities. A gene, FGD1, altered in a patient with AAS phenotype, has been identified and found to encode a protein with homology to Rho/Rac guanine nucleotide exchange factors (Rho/Rac GEF). However, since this original report on identification of a mutated FGD1 gene in an AAS patient, no additional mutations in the FGD1 gene have been described. We analysed 13 independent patients with clinical diagnosis of AAS. One patient presented a mutation that results in a nucleotide change in exon 10 of the FGD1 gene (G2559>A) substituting a Gln for Arg in position 610. The mutation was found to segregate with the AAS phenotype in affected males and carrier females in the family of this patient. Interestingly, Arg-610 is located within one of the two pleckstrin homology (PH) domains of the FGD1 gene and it corresponds to a highly conserved residue which has been involved in InsP binding in PH domains of other proteins. The same residue is often mutated in the Bruton's tyrosine kinase (Btk) gene in patients with an X-linked agammaglobulinemia. The Arg610Gln mutation represents the first case of a mutation in the PH domain of the FGD1 gene and additional evidence that mutations in PH domains can be associated to human diseases.

Negative regulation of PI 3-kinase by Ruk, a novel adaptor protein

Class I(A) phosphatidylinositol 3-kinase (PI 3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk(l), which forms complexes with the PI 3-kinase holoenzyme in vitro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 alpha regulatory subunit of the class I(A) PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kinase, interaction with Ruk(l) substantially inhibits the lipid kinase activity of the enzyme. Overexpression of Ruk(l) in cultured primary neurons induces apoptosis, an effect that could be reversed by co-expression of constitutively activated forms of the p110 alpha catalytic subunit of PI 3-kinase or its downstream effector PKB/Akt. Our data provide evidence for the existence of a negative regulator of the PI 3-kinase signalling pathway that is essential for maintaining cellular homeostasis. Structural similarities between Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel family of adaptor molecules that are involved in various intracellular signalling pathways.

Structural basis of 3-phosphoinositide recognition by pleckstrin homology domains

Lipid second messengers generated by phosphoinositide (PI) 3-kinases regulate diverse cellular functions through interaction with pleckstrin homology (PH) domains in modular signaling proteins. The PH domain of Grp1, a PI 3-kinase-activated exchange factor for Arf GTPases, selectively binds phosphatidylinositol 3,4,5-trisphosphate with high affinity. We have determined the structure of the Grp1 PH domain in the unliganded form and bound to inositol 1,3,4,5-tetraphosphate. A novel mode of phosphoinositide recognition involving a 20-residue insertion within the beta6/beta7 loop explains the unusually high specificity of the Grp1 PH domain and the promiscuous 3-phosphoinositide binding typical of several PH domains including that of protein kinase B. When compared to other PH domains, general determinants of 3-phosphoinositide recognition and specificity can be deduced.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

T cell activation and the cytoskeleton

Ligation of the T cell antigen receptor (TCR) stimulates protein tyrosine kinases (PTKs), which regulate intracellular calcium and control the activity of protein kinase C (PKC) isozymes. PTKs activated by antigen receptors and costimulatory molecules also couple to phosphatidylinositol-3 kinase (PI3K) and control the activity of Ras- and Rho-family GTPases. T cell signal transduction is triggered physiologically by antigen in the context of antigen presenting cells (APC). The formation of stable and prolonged contacts between T cells and APCs is not necessary to initiate T cell signaling but is required for effective T cell proliferation and differentiation. The stabilization of the T cell/ APC conjugate is regulated by intracellular signals induced by antigen receptors and costimulators. These coordinate the regulation of the actin and microtubule cytoskeleton and organize a specialized signaling zone that allows sustained TCR signaling.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

Molecular modelling and site-directed mutagenesis of the inositol 1,3,4,5-tetrakisphosphate-binding pleckstrin homology domain from the Ras GTPase-activating protein GAP1IP4BP

GAP1(IP4BP) is a Ras GTPase-activating protein (GAP) that in vitro is regulated by the cytosolic second messenger inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P(4)]. We have studied Ins(1,3,4,5)P(4) binding to GAP1(IP4BP), and shown that the inositol phosphate specificity and binding affinity are similar to Ins(1,3,4,5)P(4) binding to Bruton's tyrosine kinase (Btk), evidence which suggests a similar mechanism for Ins(1,3,4,5)P(4) binding. The crystal structure of the Btk pleckstrin homology (PH) domain in complex with Ins(1,3,4,5)P(4) has shown that the binding site is located in a partially buried pocket between the beta 1/beta 2- and beta 3/beta 4-loops. Many of the residues involved in the binding are conserved in GAP1(IP4BP). Therefore we generated a model of the PH domain of GAP1(IP4BP) in complex with Ins(1,3,4,5)P(4) based on the Btk-Ins(1,3,4,5)P(4) complex crystal structure. This model had the typical PH domain fold, with the proposed binding site modelling well on the Btk structure. The model has been verified by site-directed mutagenesis of various residues in and around the proposed binding site. These mutations have markedly reduced affinity for Ins(1,3,4,5)P(4), indicating a specific and tight fit for the substrate. The model can also be used to explain the specificity of inositol phosphate binding.

The isolation and characterization of a cDNA encoding phospholipid-specific inositol polyphosphate 5-phosphatase

We report the cDNA cloning and characterization of a novel human inositol polyphosphate 5-phosphatase (5-phosphatase) that has substrate specificity unlike previously described members of this large gene family. All previously described members hydrolyze water soluble inositol phosphates. This enzyme hydrolyzes only lipid substrates, phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 4,5-bisphosphate. The cDNA isolated comprises 3110 base pairs and predicts a protein product of 644 amino acids and M(r) = 70,023. We designate this 5-phosphatase as type IV. It is a highly basic protein (pI = 8.8) and has the greatest affinity toward phosphatidylinositol 3,4,5-trisphosphate of known 5-phosphatases. The K(m) is 0.65 micrometer, 1/10 that of SHIP (5.95 micrometer), another 5-phosphatase that hydrolyzes phosphatidylinositol 3,4,5-trisphosphate. The activity of 5-phosphatase type IV is sensitive to the presence of detergents in the in vitro assay. Thus the enzyme hydrolyzes lipid substrates in the absence of detergents or in the presence of n-octyl beta-glucopyranoside or Triton X-100, but not in the presence of cetyltriethylammonium bromide, the detergent that has been used in other studies of the hydrolysis of phosphatidylinositol 4,5-bisphosphate. Remarkably SHIP, a 5-phosphatase previously characterized as hydrolyzing only substrates with d-3 phosphates, also readily hydrolyzed phosphatidylinositol 4,5-bisphosphate in the presence of n-octyl beta-glucopyranoside but not cetyltriethylammonium bromide. We used antibodies prepared against a peptide predicted by the cDNA to identify the 5-phosphatase type IV enzyme in human tissues and find that it is highly expressed in the brain as determined by Western blotting. We also performed Western blotting of mouse tissues and found high levels of expression in the brain, testes, and heart with lower levels of expression in other tissues. mRNA was detected in many tissues and cell lines as determined by Northern blotting.

Enzymatic activity of the Src homology 2 domain-containing inositol phosphatase is regulated by a plasma membrane location

The negative regulatory role of the Src homology 2 domain-containing inositol 5-phosphatase (SHIP) has been invoked in a variety of receptor-mediated signaling pathways. In B lymphocytes, co-clustering of antigen receptor surface immunoglobulin with FcgammaRIIb promotes the negative effects of SHIP, but how SHIP activity is regulated is unknown. To explore this issue, we investigated the effect of SHIP phosphorylation, receptor tyrosine engagement by its Src homology 2 domain, and membrane recruitment of SHIP on its enzymatic activity. We examined two SHIP phosphorylation kinase candidates, Lyn and Syk, and observed that the Src protein-tyrosine kinase, Lyn is far superior to Syk in its ability to phosphorylate SHIP both in vitro and in vivo. However, we found a minimal effect of phosphorylation or receptor tyrosine engagement of SHIP on its enzymatic activity, whereas membrane localization of SHIP significantly reduced cellular phosphatidylinositol 3,4, 5-triphosphate levels. Based on our results, we propose that a membrane localization of SHIP is the crucial event in the induction of its phosphatase effects.

Phosphatidylinositol 3,4,5-trisphosphate regulates Ca(2+) entry via btk in platelets and megakaryocytes without increasing phospholipase C activity

The role of phosphatidylinositol 3,4,5-trisphosphate (PI3,4,5P(3)) and Btk in signalling by the collagen receptor glycoprotein VI was investigated. PI3,4,5P(3) was increased in platelets from mice deficient in the SH2 domain-containing inositol 5-phosphatase (SHIP), in response to collagen related peptide (CRP). Tyrosine phosphorylation and activation of phospholipase Cgamma2 (PLCgamma2) were unaltered in SHIP(-/-) platelets, whereas Btk was heavily tyrosine phosphorylated under basal conditions and maximally phosphorylated by low concentrations of CRP. There was an increase in basal Ca(2+), maximal expression of P-selectin, and potentiation of Ca(2+) and aminophospholipid exposure to CRP in SHIP(-/-) platelets in the presence of Ca(2+) (1 mM). Microinjection of PI3,4, 5P(3) into megakaryocytes caused a 3-fold increase in Ca(2+) in response to CRP, which was absent in X-linked immunodeficiency (Xid) mice, which have a mutation in the PH domain of Btk. There was a corresponding partial reduction in the sustained level of intracellular Ca(2+) in response to CRP in Xid mice but no change in PLC activity. These results demonstrate a novel pathway of Ca(2+) entry that involves PI3,4,5P(3) and Btk, and which is independent of increased PLC activity.

A sensitized genetic system for the analysis of murine B lymphocyte signal transduction pathways dependent on Bruton's tyrosine kinase

Modifier screens have been powerful genetic tools to define signaling pathways in lower organisms. The identification of modifier loci in mice has begun to allow a similar dissection of mammalian signaling pathways. Transgenic mice (Btk(lo)) expressing 25% of endogenous levels of Bruton's tyrosine kinase (Btk) have B cell functional responses between those of wild-type and Btk(-/-) mice. We asked whether reduced dosage or complete deficiency of genes previously implicated as Btk regulators would modify the Btk(lo) phenotype. We used two independent assays of Btk-dependent B cell function. Proliferative response to B cell antigen receptor cross-linking in vitro was chosen as an example of a relatively simple, well-defined signaling system. In vivo response to type II T-independent antigens (TI-II) measures complex interactions among multiple cell types over time and may identify additional Btk pathways. All modifiers identified differentially affected these two assays, indicating that Btk mediates these processes via distinct mechanisms. Loss of Lyn, PTEN (phosphatase and tensin homolog), or SH2-containing inositol phosphatase suppressed the Btk(lo) phenotype in vitro but not in vivo, whereas CD19 and the p85alpha form of phosphoinositide 3-kinase behaved as Btk(lo) enhancers in vivo but not in vitro. Effects of Lyn, PTEN, or p85alpha haploinsufficiency were observed. Haploinsufficiency or complete deficiency of protein kinase C beta, Fyn, CD22, Galphaq, or Galpha11 had no detectable effect on the function of Btk(lo) B cells. A transgenic system creating a reduction in dosage of Btk can therefore be used to identify modifier loci that affect B cell responses and quantitatively rank their contribution to Btk-mediated processes.

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.

Novel functional PI 3-kinase antagonists inhibit cell growth and tumorigenicity in human cancer cell lines

New efforts in cancer therapy are being focused at various levels of signaling pathways. With phosphoinositide 3-kinase (PI3-K) potentially being necessary for a range of cancer-related functions, we have investigated the influence of selected inositol tris- to hexakisphosphates on cell growth and tumorigenicity. We show that micromolar concentrations of inositol 1,3,4,5,6-pentakisphosphate and inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P(4)] inhibit IGF-1-induced [(3)H]-thymidine incorporation in human breast cancer (MCF-7) cells and the ability to grow in liquid medium and form colonies in agarose semisolid medium by small cell lung cancer (SCLC) cells, a human cancer cell line containing a constitutively active PI3-K. In an ovarian cancer cell line that also contains a constitutively active PI3-K (SKOV-3 cells), Ins(1,4,5,6)P(4) again inhibited liquid medium growth. Furthermore, when applied extracellularly, inositol 1,3,4,5-tetrakisphosphate was shown indeed to enter SCLC cells. These effects appeared specifically related to PH domains known to bind to phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P(2)] and phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], indicating involvement of the PI3-K downstream target protein kinase B (PKB/Akt). This was further supported by inhibition of PKB/Akt PH domain membrane targeting in COS-7 cells by Ins(1,4,5,6)P(4). Thus, we propose that specific inositol polyphosphates inhibit PI3-K by competing with PtdIns(3,4, 5)P(3)-binding PH domains and that this occurs mainly at the level of the downstream PI3-K target, PKB/Akt.

Rapid lamellipodia formation in nerve growth factor-stimulated PC12 cells is dependent on Rac and PI3K activity

Neuronal differentiation of PC12 cells is achieved by stimulation with nerve growth factor (NGF) but not by epidermal growth factor (EGF). However, features of differentiation such as neurite outgrowth are observable at the earliest after several hours. Using actin staining of the cells, we show here that NGF stimulation leads to lamellipodia formation within only 3 min at the periphery of the PC12 cells. EGF stimulation or microinjection of differentiation-inducing c-Crk I protein does not cause lamellipodia. The actin reorganization after NGF stimulation is blocked by microinjecting dominant negative Rac protein. The lamellipodia formation is also abolished by inhibitors of phosphatidylinositol 3-kinase, wortmannin and LY 294002 in a concentration-dependent manner. Phase-contrast time-lapse microscopy was used to analyze membrane dynamics in real time and to confirm the induction of lamellipodia by NGF and their inhibition by pretreatment with both wortmannin and LY 294002. The results indicate that NGF, but not EGF, leads to rapid lamellipodia formation in PC12 cells via phosphatidylinositol 3-kinase and the small GTPase Rac, thereby defining a novel role for these factors in early NGF signaling.

Identification of a plasma membrane-associated guanine nucleotide exchange factor for ARF6 in chromaffin cells. Possible role in the regulated exocytotic pathway

ADP-ribosylation factors (ARFs) constitute a family of structurally related proteins that forms a subset of the Ras superfamily of regulatory GTP-binding proteins. Like other GTPases, activation of ARFs is facilitated by specific guanine nucleotide exchange factors (GEFs). In chromaffin cells, ARF6 is associated with the membrane of secretory granules. Stimulation of intact cells or direct elevation of cytosolic calcium in permeabilized cells triggers the rapid translocation of ARF6 to the plasma membrane and the concomitant activation of phospholipase D (PLD) in the plasma membrane. Both calcium-evoked PLD activation and catecholamine secretion in permeabilized cells are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6, suggesting that the ARF6-dependent PLD activation near the exocytotic sites represents a key event in the exocytotic reaction in chromaffin cells. In the present study, we demonstrate the occurrence of a brefeldin A-insensitive ARF6-GEF activity in the plasma membrane and in the cytosol of chromaffin cells. Furthermore, reverse transcriptase-polymerase chain reaction and immunoreplica analysis indicate that ARNO, a member of the brefeldin A-insensitive ARF-GEF family, is expressed and predominantly localized in the cytosol and in the plasma membrane of chromaffin cells. Using permeabilized chromaffin cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits, in a dose-dependent manner, both PLD activation and catecholamine secretion in calcium-stimulated cells. Furthermore, co-expression in PC12 cells of a catalytically inactive ARNO mutant with human growth hormone as a marker of secretory granules in transfected cells resulted in a 50% inhibition of growth hormone secretion evoked by depolarization with high K(+). The possibility that the plasma membrane-associated ARNO participates in the exocytotic pathway by activating ARF6 and downstream PLD is discussed.

Rho GTPases and their effector proteins

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, gene transcription, cell-cycle progression and cell adhesion. About 30 potential effector proteins have been identified that interact with members of the Rho family, but it is still unclear which of these are responsible for the diverse biological effects of Rho GTPases. This review will discuss how Rho GTPases physically interact with, and regulate the activity of, multiple effector proteins and how specific effector proteins contribute to cellular responses. To date most progress has been made in the cytoskeleton field, and several biochemical links have now been established between GTPases and the assembly of filamentous actin. The main focus of this review will be Rho, Rac and Cdc42, the three best characterized mammalian Rho GTPases, though the genetic analysis of Rho GTPases in lower eukaryotes is making increasingly important contributions to this field.

Control of intramolecular interactions between the pleckstrin homology and Dbl homology domains of Vav and Sos1 regulates Rac binding

Vav and Sos1 are Dbl family guanine nucleotide exchange factors, which activate Rho family GTPases in response to phosphatidylinositol 3-kinase products. A pleckstrin homology domain adjacent to the catalytic Dbl homology domain via an unknown mechanism mediates the effects of phosphoinositides on guanine nucleotide exchange activity. Here we tested the possibility that phosphatidylinositol 3-kinase substrates and products control an interaction between the pleckstrin homology domain and the Dbl homology domain, thereby explaining the inhibitory effects of phosphatidylinositol 3-kinase substrates and stimulatory effects of the products. Binding studies using isolated fragments of Vav and Sos indicate phosphatidylinositol 3-kinase substrate promotes the binding of the pleckstrin homology domain to the Dbl homology domain and blocks Rac binding to the DH domain, whereas phosphatidylinositol 3-kinase products disrupt the Dbl homology/pleckstrin homology interactions and permit Rac binding. Additionally, Lck phosphorylation of Vav, a known activating event, reduces the affinities between the Vav Dbl homology and pleckstrin homology domains and permits Rac binding. We also show Vav activation in cells, as monitored by phosphorylation of Vav, Vav association with phosphatidylinositol 3,4,5-trisphosphate, and Vav guanine nucleotide exchange activity, is blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. These results suggest the molecular mechanisms for activation of Vav and Sos1 require disruption of inhibitory intramolecular interactions involving the pleckstrin homology and Dbl homology domains.

Binding of the PH and polybasic C-terminal domains of ARNO to phosphoinositides and to acidic lipids

The activity on ARF of the guanine nucleotide exchange factor ARNO depends on its membrane recruitment, induced by binding of its PH domain to phosphoinositides. A polycationic C-terminal extension to the PH domain might also contribute to its specific binding to phosphatidylinositol 4,5-bisphosphate [(4,5)PIP2] and to phosphatidylinositol 3,4,5-trisphosphate [(3,4,5)PIP3], and to ionic binding to other acidic lipids. We have analyzed in vitro the relative contributions to phospholipid binding of the PH domain and C-terminal extension by cosedimentation of "PH+C domain" and "nominal PH domain" protein constructs including or not including the polycationic C-terminus, with sucrose-loaded unilamellar vesicles made of equal proportions of the neutral lipids phosphatidylcholine and phosphatidylethanolamine, and supplemented or not with 30% acidic phosphatidylserine (PS) and 2% of various phosphoinositides. Binding was measured as a function of the vesicle concentration and of the medium ionic strength. Both proteins bound with higher affinity to (3,4,5)PIP3 than to (4,5)PIP2, the selectivity for (3,4,5)PIP3 being highest for the nominal PH domain. We observed also a clear selectivity of (3,4,5)PIP3 over (4,5)PIP2 for stimulating the activity of ARNO on ARF with vesicles containing 10% PS and 1% PIP2 or PIP3. Our data suggest that the PH domain provides the specific phosphoinositide binding site and some unspecific ionic interaction with acidic PS, whereas the polybasic C domain contributes to binding mainly by unspecific ionic interactions vith PS. Phosphorylation by protein kinase C of a serine in the C domain reduces the ionic affinity of the PH+C domain for PS, but does not affect the phosphoinositide specificity.

Identification and characterization of a new family of guanine nucleotide exchange factors for the ras-related GTPase Ral

Guanine nucleotide exchange factors (GEFs) are responsible for coupling cell surface receptors to Ras protein activation. Here we describe the characterization of a novel family of differentially expressed GEFs, identified by database sequence homology searching. These molecules share the core catalytic domain of other Ras family GEFs but lack the catalytic non-conserved (conserved non-catalytic/Ras exchange motif/structurally conserved region 0) domain that is believed to contribute to Sos1 integrity. In vitro binding and in vivo nucleotide exchange assays indicate that these GEFs specifically catalyze the GTP loading of the Ral GTPase when overexpressed in 293T cells. A central proline-rich motif associated with the Src homology (SH)2/SH3-containing adapter proteins Grb2 and Nck in vivo, whereas a pleckstrin homology (PH) domain was located at the GEF C terminus. We refer to these GEFs as RalGPS 1A, 1B, and 2 (Ral GEFs with PH domain and SH3 binding motif). The PH domain was required for in vivo GEF activity and could be functionally replaced by the Ki-Ras C terminus, suggesting a role in membrane targeting. In the absence of the PH domain RalGPS 1B cooperated with Grb2 to promote Ral activation, indicating that SH3 domain interaction also contributes to RalGPS regulation. In contrast to the Ral guanine nucleotide dissociation stimulator family of Ral GEFs, the RalGPS proteins do not possess a Ras-GTP-binding domain, suggesting that they are activated in a Ras-independent manner.

Biotinylated phosphatidylinositol 3,4,5-trisphosphate as affinity ligand

Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), a primary output signal of phosphoinositide (PI) 3-kinase, plays a crucial role in diverse cellular processes. Evidence indicates that PIP(3) exerts downstream signaling, in part, by recruiting effector proteins to plasma membranes. Consequently, identification of signaling enzymes with PIP(3)-binding motifs represents a viable approach to understand the mechanism by which specificity of the PI 3-kinase-mediated signaling network is maintained. To address this issue, we have developed biotinylated derivatives of PIP(3) as affinity probes for the purification and characterization of PIP(3)-binding proteins. Considering the relaxed requirement for the acyl moiety in PIP(3) recognition, these biotinylated PIP(3) analogues display two structural features. First, they contain short acyl side chains (C(4) and C(8)), allowing them to be soluble in aqueous milieu. This desirable feature avoids the formation of lipid aggregates, which minimizes nonspecific hydrophobic interactions with proteins. Second, the appended biotin is located at the terminus of the sn-1 acyl side chain, thereby maintaining the integrity of the phosphoinositol head group essential for selective recognition. The utility of these affinity ligands is validated by the purification of recombinant PIP(3)-binding proteins, expressed as GST fusion proteins, to homogeneity from bacterial lysates. These include the C-terminal SH2 domain of the p85 subunit of PI 3-kinase and the N-terminal PH domain of PLCgamma1. The efficiency of biotinylated PIP(3) analogues in the purification of these recombinant proteins was approximately 20% of that of glutathione beads

A novel B lymphocyte-associated adaptor protein, Bam32, regulates antigen receptor signaling downstream of phosphatidylinositol 3-kinase

We have identified and characterized a novel src homology 2 (SH2) and pleckstrin homology (PH) domain-containing adaptor protein, designated Bam32 (for B cell adaptor molecule of 32 kD). cDNAs encoding the human and mouse Bam32 coding sequences were isolated and the human bam32 gene was mapped to chromosome 4q25-q27. Bam32 is expressed by B lymphocytes, but not T lymphocytes or nonhematopoietic cells. Human germinal center B cells show increased Bam32 expression, and resting B cells rapidly upregulate expression of Bam32 after ligation of CD40, but not immunoglobulin M. Bam32 is tyrosine-phosphorylated upon B cell antigen receptor (BCR) ligation or pervanadate stimulation and associates with phospholipase Cgamma2. After BCR ligation, Bam32 is recruited to the plasma membrane through its PH domain. Membrane recruitment requires phosphatidylinositol 3-kinase (PI3K) activity and an intact PI(3,4, 5)P(3)-binding motif, suggesting that membrane association occurs through binding to 3-phosphoinositides. Expression of Bam32 in B cells leads to a dose-dependent inhibition of BCR-induced activation of nuclear factor of activated T cells (NF-AT), which is blocked by deletion of the PH domain or mutation of the PI(3,4,5)P(3)-binding motif. Thus, Bam32 represents a novel B cell-associated adaptor that regulates BCR signaling downstream of PI3K.