Modular PH and C2 domains in membrane attachment and other functions

Binding of Ca2+-independent C2 domains to lipid membranes: A multi-scale molecular dynamics study

C2 domains facilitate protein interactions with lipid bilayers in either a Ca²⁺-dependent or -independent manner. We used molecular dynamics (MD) simulations to explore six Ca²⁺-independent C2 domains, from KIBRA, PI3KC2α, RIM2, PTEN, SHIP2, and Smurf2. In coarse-grained MD simulations these C2 domains formed transient interactions with zwitterionic bilayers, compared with longer-lived interactions with anionic bilayers containing phosphatidylinositol bisphosphate (PIP₂). Type I C2 domains bound non-canonically via the front, back, or side of the β sandwich, whereas type II C2 domains bound canonically, via the top loops. C2 domains interacted strongly with membranes containing PIP₂, causing bound anionic lipids to cluster around the protein. Binding modes were refined via atomistic simulations. For PTEN and SHIP2, CG simulations of their phosphatase plus C2 domains with PIP₂-containing bilayers were also performed, and the roles of the two domains in membrane localization compared. These studies establish a simulation protocol for membrane-recognition proteins.

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Binding of Ca²⁺-independent C2 domains to membranes was explored by MD simulation

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C2 domains from KIBRA, PI3KC2α, RIM2, PTEN, SHIP2, and Smurf2 were compared

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C2 domains formed longer-lived interactions with lipid bilayers containing PIP₂

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For PTEN and SHIP2, simulations of their phosphatase plus C2 domains were performed

The trypanosome transcriptome is remodelled during differentiation but displays limited responsiveness within life stages

BACKGROUND

Trypanosomatids utilise polycistronic transcription for production of the vast majority of protein-coding mRNAs, which operates in the absence of gene-specific promoters. Resolution of nascent transcripts by polyadenylation and trans-splicing, together with specific rates of mRNA turnover, serve to generate steady state transcript levels that can differ in abundance across several orders of magnitude and can be developmentally regulated. We used a targeted oligonucleotide microarray, representing the strongly developmentally-regulated T. brucei membrane trafficking system and approximately 10% of the Trypanosoma brucei genome, to investigate both between-stage, or differentiation-dependent, transcriptome changes and within-stage flexibility in response to various challenges.

RESULTS

6% of the gene cohort are developmentally regulated, including several small GTPases, SNAREs, vesicle coat factors and protein kinases both consistent with and extending previous data. Therefore substantial differentiation-dependent remodeling of the trypanosome transcriptome is associated with membrane transport. Both the microarray and qRT-PCR were then used to analyse transcriptome changes resulting from specific gene over-expression, knockdown, altered culture conditions and chemical stress. Firstly, manipulation of Rab5 expression results in co-ordinate changes to clathrin protein expression levels and endocytotic activity, but no detectable changes to steady-state mRNA levels, which indicates that the effect is mediated post-transcriptionally. Secondly, knockdown of clathrin or the variant surface glycoprotein failed to perturb transcription. Thirdly, exposure to dithiothreitol or tunicamycin revealed no evidence for a classical unfolded protein response, mediated in higher eukaryotes by transcriptional changes. Finally, altered serum levels invoked little transcriptome alteration beyond changes to expression of ESAG6/7, the transferrin receptor.

CONCLUSION

While trypanosomes regulate mRNA abundance to effect the major changes accompanying differentiation, a given differentiated state appears transcriptionally inflexible. The implications of the absence of a transcriptome response in trypanosomes for both virulence and models of life cycle progression are discussed.

The primate-specific protein TBC1D3 is required for optimal macropinocytosis in a novel ARF6-dependent pathway

The generation of novel genes and proteins throughout evolution has been proposed to occur as a result of whole genome and gene duplications, exon shuffling, and retrotransposition events. The analysis of such genes might thus shed light into the functional complexity associated with highly evolved species. One such case is represented by TBC1D3, a primate-specific gene, harboring a TBC domain. Because TBC domains encode Rab-specific GAP activities, TBC-containing proteins are predicted to play a major role in endocytosis and intracellular traffic. Here, we show that the TBC1D3 gene originated late in evolution, likely through a duplication of the RNTRE locus, and underwent gene amplification during primate speciation. Despite possessing a TBC domain, TBC1D3 is apparently devoid of Rab-GAP activity. However, TBC1D3 regulates the optimal rate of epidermal growth factor-mediated macropinocytosis by participating in a novel pathway involving ARF6 and RAB5. In addition, TBC1D3 binds and colocalize to GGA3, an ARF6-effector, in an ARF6-dependent manner, and synergize with it in promoting macropinocytosis, suggesting that the two proteins act together in this process. Accordingly, GGA3 siRNA-mediated ablation impaired TBC1D3-induced macropinocytosis. We thus uncover a novel signaling pathway that appeared after primate speciation. Within this pathway, a TBC1D3:GGA3 complex contributes to optimal propagation of signals, ultimately facilitating the macropinocytic process.

Neuronal calcium sensor-1 and phosphatidylinositol 4-kinase beta stimulate extracellular signal-regulated kinase 1/2 signaling by accelerating recycling through the endocytic recycling compartment

We demonstrate that recycling through the endocytic recycling compartment (ERC) is an essential step in Fc epsilonRI-induced activation of extracellular signal-regulated kinase (ERK)1/2. We show that ERK1/2 acquires perinuclear localization and colocalizes with Rab 11 and internalized transferrin in Fc epsilonRI-activated cells. Moreover, a close correlation exists between the amount of ERC-localized ERK1/2 and the amount of phospho-ERK1/2 that resides in the nucleus. We further show that by activating phosphatidylinositol 4-kinase beta (PI4Kbeta) and increasing the cellular level of phosphatidylinositol(4) phosphate, neuronal calcium sensor-1 (NCS-1), a calmodulin-related protein, stimulates recycling and thereby enhances Fc epsilonRI-triggered activation and nuclear translocation of ERK1/2. Conversely, NCS-1 short hairpin RNA, a kinase dead (KD) mutant of PI4Kbeta (KD-PI4Kbeta), the pleckstrin homology (PH) domain of FAPP1 as well as RNA interference of synaptotagmin IX or monensin, which inhibit export from the ERC, abrogate Fc epsilonRI-induced activation of ERK1/2. Consistently, NCS-1 also enhances, whereas both KD-PI4Kbeta and FAPP1-PH domain inhibit, Fc epsilonRI-induced release of arachidonic acid/metabolites, a downstream target of ERK1/2 in mast cells. Together, our results demonstrate a novel role for NCS-1 and PI4Kbeta in regulating ERK1/2 signaling and inflammatory reactions in mast cells. Our results further identify the ERC as a crucial determinant in controlling ERK1/2 signaling.

Regulation of Phospholipase C-δ1 through Direct Interactions with the Small GTPase Ral and Calmodulin

Second messengers generated from membrane lipids play a critical role in signaling and control diverse cellular processes. Despite being one of the most evolutionarily conserved of all the phosphoinositide-specific phospholipase C (PLC) isoforms, a family of enzymes responsible for hydrolysis of the membrane lipid phosphatidylinositol bisphosphate, the mechanism of PLC-delta1 activation is still poorly understood. Here we report a novel regulatory mechanism for PLC-delta1 activation that involves direct interaction of the small GTPase Ral and the universal calcium-signaling molecule calmodulin (CaM) with PLC-delta1. In addition, we have identified a novel IQ type CaM binding motif within the catalytic region of PLC-delta1 that is not found in other PLC isoforms. Binding of CaM at the IQ motif inhibits PLC-delta1 activity, while addition of Ral reverses the inhibition. The overexpression of various Ral mutants in cells potentiates PLC-delta1 activity. Thus, the Ral-CaM complex defines a multifaceted regulatory mechanism for PLC-delta1 activation.

The C2 domains of the class I Rab11 family of interacting proteins target recycling vesicles to the plasma membrane

The Rab11 family of interacting proteins (Rab11-FIP) is a recently identified protein family composed of, to date, six members that interact with Rab11. They all share a highly homologous Rab11-binding domain (RBD) at their C-termini. However, apart from the RBD, they vary in their domain organization. Rab11-FIP3 and Rab11-FIP4 possess an ezrin-radixin-moesin (ERM) domain in their C-terminal half and EF hands in their N-terminal region. They have been termed class II Rab11-FIPs. The class I Rab11-FIPs, Rab coupling protein (RCP), Rip11 and Rab11-FIP2, each have a C2 phospholipid-binding domain near their N-termini. Although they are still membrane associated, truncation mutants of the class I Rab11-FIPs that lack their C2 domains display an altered subcellular distribution in vivo, indicating that this domain plays an important role in specifying their correct intracellular localization. To determine the phospholipids to which they bind, a protein phospholipid overlay assay was performed. Our results indicate that the class-I Rab11-FIPs bind preferentially to phosphatidylinositol-(3,4,5)-trisphosphate [PtdIns(3,4,5)P3] and the second messenger phosphatidic acid. Stimulation of PtdIns(3,4,5)P3 or phosphatidic acid synthesis results in the translocation of the Rab11-FIPs from a perinuclear location to the periphery of the cell. By contrast, the transferrin receptor does not translocate to the plasma membrane under these conditions. This translocation is dependent on the presence of the C2 domain, because class I Rab11-FIP green-fluorescent-protein fusions that lack the C2 domain cannot translocate to the plasma membrane. We propose that the C2 domains of the class I Rab11-FIPs function to target these proteins to `docking sites' in the plasma membrane that are enriched in PtdIns(3,4,5)P3 and phosphatidic acid.

Phosphorylation-independent membrane relocalization of ezrin following association with Dbl in vivo

Ezrin, a widespread protein involved in cell migration, morphogenesis and cell adhesion, belongs to a large family of proteins known as ERM (ezrin, radixin, moesin). These three closely related proteins are thought to function as linkers between plasma membrane and actin cytoskeleton and their function is regulated by the small GTP-binding protein Rho. It has been previously shown that the active form of radixin can bind in vitro to Dbl, a Rho-specific guanine nucleotide exchange factor, although an in vivo interaction has not yet been demonstrated. In this paper, we attempted to investigate whether ezrin can also associate with Dbl. We show here that Dbl protein can effectively bind both in vitro and in vivo to the N-terminal region (amino acids 1-531) of a constitutively active mutant of ezrin and with the full-length molecule. We found that this binding is mediated by the Dbl pleckstrin homology domain, responsible for the proper subcellular localization of the Dbl protein. Moreover, we show that Dbl induces localization to the plasma membrane of both the active deletion mutant and the full-length ezrin proteins. Finally, we show that the relocalization of ezrin is independent of Dbl GEF activity. These results indicate that Dbl could induce translocation of ezrin to the plasma membrane through a mechanism that does not require ezrin C-terminus phosphorylation by Rho-associated kinases.

Requirements for distinct steps of phospholipase Cgamma2 regulation, membrane-raft-dependent targeting and subsequent enzyme activation in B-cell signalling

Studies of PLCgamma (phospholipase Cgamma) have identified a number of regulatory components required for signalling; however, molecular mechanisms and the relationship between events leading to translocation and an increase of substrate hydrolysis have not been well defined. The addition of a membrane-targeting tag to many signal transducers results in constitutive activation, suggesting that these processes could be closely linked and difficult to dissect. The present study of PLCgamma2 regulation by cross-linking of the BCR (B-cell antigen receptor) or H2O2 stress in DT40 B-cells, demonstrated that the membrane targeting is a separate step from further changes that result in enzyme activation and substrate hydrolysis. Furthermore, we have defined the roles of different domains of PLCgamma2 and, using a panel of cell lines deficient in components linked to PLCgamma2 regulation, the involvement of signalling molecules with respect to each of the steps. We have found that only the lipid-raft-targeted Lyn-PLCgamma2 construct, unlike non-specific membrane targeting, overcame the requirement for the adapter protein BLNK (B-cell linker). The stable expression of Lyn-PLCgamma2 was not accompanied by an increase in substrate hydrolysis in resting cells, which followed stimulation and specifically required the presence and/or activation of Syk, Btk, phosphoinositide 3-kinase but not BLNK, as established using deficient cell lines or specific inhibitors. Based on mutational analysis of the specific tyrosine residues [Tyr753-->Phe (Y753F)/Y759F] and SH2 (Src homology 2) domains (R564A/R672A) in the context of Lyn-PLCgamma2, we found that Tyr753/Tyr759 were essential, whereas the PLCgamma2 SH2 domains did not have an important role in the transient activation of Lyn-PLCgamma2 but may serve to stabilize an activated form in sustained activation.

Regulation of proto-Dbl by intracellular membrane targeting and protein stability

The pleckstrin homology (PH) domain of onco-Dbl, a guanine nucleotide exchange factor (GEF) for Cdc42 and RhoA GTPases, interacts with phosphoinositides (PIPs). This interaction modulates both the GEF activity and the targeting to the plasma membrane of onco-Dbl. Conversely, we have previously shown that in proto-Dbl an intramolecular interaction between the N-terminal domain and the PH domain imposes a negative regulation on both the DH and PH functions, suppressing its transforming activity. Here we have further investigated the mode of regulation of proto-Dbl by generating proto-Dbl mutants deleted of the last C-terminal 50 amino acids, which contain a PEST motif, and/or unable to bind to PIPs due to substitutions of the positively charged residues of the PH domain. The PH mutants of proto-Dbl retained a relative weak GEF activity toward Cdc42 and RhoA in vitro, but their RhoA activating potential was impaired in vivo. Further, these mutants lost both the plasma membrane targeting and the transforming activities, contrary to the PH mutants of onco-Dbl that retained the exchange activity both in vitro and in vivo and showed significant, but partially, reduced transforming activity. Deletion of the C-terminal sequences from onco-Dbl did not affect its function, whereas similar deletion of proto-Dbl led to an increase of transforming activity. Analysis of the half-life of the proto-Dbl mutants revealed that deletion of the C-terminal sequences increases the stability of the protein. Overall, the transformation potential of proto-Dbl mutants was associated with an augmented localization of the protein to the plasma membrane and a strong activation of Jun N-terminal kinase activity and transcription of cyclin D1. Together with previous observations, these data suggest that the biological activity of proto-Dbl is tightly regulated by a combination of mechanisms that involve intramolecular interaction, PH binding to PIPs, and the N- and C-terminal domain-dependent turnover of the protein.

Identification of a new polyphosphoinositide in plants, phosphatidylinositol 5-monophosphate (PtdIns5P), and its accumulation upon osmotic stress

Polyphosphoinositides play an important role in membrane trafficking and cell signalling. In plants, two PtdInsP isomers have been described, PtdIns3P and PtdIns4P. Here we report the identification of a third, PtdIns5P. Evidence is based on the conversion of the endogenous PtdInsP pool into PtdIns(4,5)P(2) by a specific PtdIns5P 4-OH kinase, and on in vivo (32)P-labelling studies coupled to HPLC head-group analysis. In Chlamydomonas, 3-8% of the PtdInsP pool was PtdIns5P, 10-15% was PtdIns3P and the rest was PtdIns4P. In seedlings of Vicia faba and suspension-cultured tomato cells, the level of PtdIns5P was about 18%, indicating that PtdIns5P is a general plant lipid that represents a significant proportion of the PtdInsP pool. Activating phospholipase C (PLC) signalling in Chlamydomonas cells with mastoparan increased the turnover of PtdIns(4,5)P(2) at the cost of PtdIns4P, but did not affect the level of PtdIns5P. This indicates that PtdIns(4,5)P(2) is synthesized from PtdIns4P rather than from PtdIns5P during PLC signalling. However, when cells were subjected to hyperosmotic stress, PtdIns5P levels rapidly increased, suggesting a role in osmotic-stress signalling. The potential pathways of PtdIns5P formation are discussed.

Retinal phospholipase C from squid is a regulator of Gq alpha GTPase activity

The phospholipase C (PLC) pathway is the major signaling mechanism of photoactivation in invertebrate photoreceptors. Here we report the cloning of a cDNA encoding a 140-kDa retinal PLC that is uniquely expressed in squid photoreceptors. This cDNA encodes a protein with multiple distinct modular domains: PH, X and Y catalytic, and C2 domains, as well as G- and P-box motifs and two GTP/ATP binding motifs. The PLC was stimulated by activated squid Gq alpha but not by squid Gq beta gamma or mammalian beta gamma subunits. The PLC was inhibited by monophosphate, diphosphate and triphosphate nucleotides but not cyclic nucleosides. We also tested the ability of PLC-140 to regulate the GTPase activity of Gq alpha in the rhabdomeric membranes. Depletion of PLC-140 from the rhabdomeric membranes decreased the GTP hydrolysis but not GTP gamma S binding to the membranes. Reconstitution of purified PLC-140 with membranes accelerated Gq alpha GTPase activity by fivefold at a concentration of 2.5 microM. Our data suggest that PLC-140 plays an important role in both the activation and inactivation pathways of invertebrate visual transduction.

EGF-dependent translocation of green fluorescent protein-tagged PLC-gamma1 to the plasma membrane and endosomes

Growth factor-dependent translocation of phospholipase C-gamma1 (PLC-gamma1) was investigated using a green fluorescent protein-tagged PLC-gamma1 (PLC-gamma1-GFP) expressed in human epidermoid carcinoma A-431 cells. In the absence of growth factors, PLC-gamma1-GFP was present throughout the cytoplasm of A-431 cells. Treatment of the cells with epidermal growth factor (EGF) produced a very rapid redistribution of PLC-gamma1-GFP to the plasma membrane in a nonuniform manner. This translocation to the plasma membrane was insensitive to an inhibitor of phosphatidylinositol 3-kinase and was independent of cell adhesion. However, the translocation was disrupted by an agent which depolymerizes the actin cytoskeleton. At later times following the addition of EGF, PLC-gamma1-GFP appeared associated with intracellular vesicles. Stimulation of A-431 cells by Texas red-conjugated EGF for more than 10 min resulted in punctate intracellular PLC-gamma1-GFP distribution that colocalized with Texas red-conjugated EGF. This suggests that PLC-gamma1 is translocated to endosomes after EGF treatment, probably by associating with the internalized and autophosphorylated EGF receptor. Fractionation studies demonstrated that the EGF-induced plasma membrane-localized PLC-gamma1 is concentrated in caveolae microdomains. Disruption of caveolae with methyl-beta-cyclodextrin resulted in the ablation of EGF-induced, but not bradykinin-induced, mobilization of intracellular Ca(2+). This treatment, however, only partially decreased PLC-gamma1 membrane translocation.

Real time fluorescence imaging of PLC gamma translocation and its interaction with the epidermal growth factor receptor

The translocation of fluorescently tagged PLC gamma and requirements for this process in cells stimulated with EGF were analyzed using real time fluorescence microscopy applied for the first time to monitor growth factor receptor--effector interactions. The translocation of PLC gamma to the plasma membrane required the functional Src homology 2 domains and was not affected by mutations in the pleckstrin homology domain or inhibition of phosphatidylinositol (PI) 3-kinase. An array of domains specific for PLC gamma isoforms was sufficient for this translocation. The dynamics of translocation to the plasma membrane and redistribution of PLC gamma, relative to localization of the EGF receptor and PI 4,5-biphosphate (PI 4,5-P(2)), were shown. Colocalization with the receptor was observed in the plasma membrane and in membrane ruffles where PI 4,5-P(2) substrate could also be visualized. At later times, internalization of PLC gamma, which could lead to separation from the substrate, was observed. The data support a direct binding of PLC gamma to the receptor as the main site of the plasma membrane recruitment. The presence of PLC gamma in membrane structures and its access to the substrate appear to be transient and are followed by a rapid incorporation into intracellular vesicles, leading to downregulation of the PLC activity.

The Dictyostelium Bcr/Abr-related protein DRG regulates both Rac- and Rab-dependent pathways

Dictyostelium discoideum DdRacGap1 (DRG) contains both Rho-GEF and Rho-GAP domains, a feature it shares with mammalian Bcr and Abr. To elucidate the physiological role of this multifunctional protein, we characterized the enzymatic activity of recombinant DRG fragments in vitro, created DRG-null cells, and studied the function of the protein in vivo by analysing the phenotypic changes displayed by DRG-depleted cells and DRG-null cells complemented with DRG or DRG fragments. Our results show that DRG-GEF modulates F-actin dynamics and cAMP-induced F-actin formation via Rac1-dependent signalling pathways. DRG's RacE-GAP activity is required for proper cytokinesis to occur. Additionally, we provide evidence that the specificity of DRG is not limited to members of the Rho family of small GTPases. A recombinant DRG-GAP accelerates the GTP hydrolysis of RabD 30-fold in vitro and our complementation studies show that DRG-GAP activity is required for the RabD-dependent regulation of the contractile vacuole system in Dictyostelium.

Structural characterization of the C2 domain of novel protein kinase Cepsilon

Infrared spectroscopy (IR) and differential scanning calorimetry (DSC) were used to study the biophysical properties of the PKCepsilon-C2 domain, a C2 domain that possess special characteristics as it binds to acidic phospholipids in a Ca2+-independent manner and no structural information about it is available to date. When the secondary structure was determined by IR spectroscopy in H2O and D2O buffers, beta sheet was seen to be the major structural component. Spectroscopic studies of the thermal denaturation in D2O showed a broadening in the amide I' band starting at 45 degrees C. Curve fitting analysis of the spectra demonstrated that two components appear upon thermal denaturation, one at 1623 cm(-1) which was assigned to aggregation and a second one at 1645 cm(-1), which was assigned to unordered or open loop structures. A lipid binding assay has demonstrated that PKCepsilon-C2 domain has preferential affinity for PIP2 although it exhibits maximal binding activity for phosphatidic acid when 100 mol% of this negatively charged phospholipid was used. Thus, phosphatidic acid containing vesicles were used to characterize the effect of lipid binding on the secondary structure and thermal stability. These experiments showed that the secondary structure did not change upon lipid binding and the thermal stability was very high with no significant changes occurring in the secondary structure after heating. DSC experiments demonstrated that when the C2-protein was scanned alone, it showed a Tm of 49 degrees C and a calorimetric denaturation enthalpy of 144.318 kJ x mol(-1). However, when phoshatidic acid vesicles were included in the mixture, the transition disappeared and further IR experiments demonstrated that the protein structure was not modified under these conditions.

Mutations in ARHGEF6, encoding a guanine nucleotide exchange factor for Rho GTPases, in patients with X-linked mental retardation

X-linked forms of mental retardation (XLMR) include a variety of different disorders and may account for up to 25% of all inherited cases of mental retardation. So far, seven X-chromosomal genes mutated in nonspecific mental retardation (MRX) have been identified: FMR2, GDI1, RPS6KA3, IL1RAPL, TM4SF2, OPHN1 and PAK3 (refs 2-9). The products of the latter two have been implicated in regulation of neural plasticity by controlling the activity of small GTPases of the Rho family. Here we report the identification of a new MRX gene, ARHGEF6 (also known as alphaPIX or Cool-2), encoding a protein with homology to guanine nucleotide exchange factors for Rho GTPases (Rho GEF). Molecular analysis of a reciprocal X/21 translocation in a male with mental retardation showed that this gene in Xq26 was disrupted by the rearrangement. Mutation screening of 119 patients with nonspecific mental retardation revealed a mutation in the first intron of ARHGEF6 (IVS1-11T-->C) in all affected males in a large Dutch family. The mutation resulted in preferential skipping of exon 2, predicting a protein lacking 28 amino acids. ARHGEF6 is the eighth MRX gene identified so far and the third such gene to encode a protein that interacts with Rho GTPases.

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 role for phospholipase D in GLUT4 glucose transporter translocation

Based on recent studies showing that phospholipase D (PLD)1 is associated with intracellular membranes and promotes membrane budding from the trans-Golgi, we tested its possible role in the membrane trafficking of GLUT4 glucose transporters. Using immunofluorescence confocal microscopy, expressed Myc epitope-tagged PLD1 was found to associate with intracellular vesicular structures by a mechanism that requires its N-terminal pleckstrin homology domain. Partial co-localization with expressed GLUT4 fused to green fluorescent protein in both 3T3-L1 adipocytes and Chinese hamster ovary cells was evident. Furthermore, microinjection of purified PLD into cultured adipocytes markedly potentiated the effect of a submaximal concentration of insulin to stimulate GLUT4 translocation to cell surface membranes. Insulin stimulated PLD activity in cells expressing high levels of insulin receptors but no such insulin effect was detected in 3T3-L1 adipocytes. Taken together, these results are consistent with the hypothesis that PLD1 associated with GLUT4-containing membranes acts in a constitutive manner to promote the mechanism of GLUT4 translocation by insulin.

Phospholipase C-gamma as a signal-transducing element

A ubiquitous signaling event in hormonal responses is the phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4, 5-bisphosphate to produce the metabolite second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol. The former provokes a transient increase in intracellular free Ca(2+), while the latter serves as a direct activator of protein kinase C. In tyrosine kinase-dependent signaling pathways this reaction is mediated by the PLC-gamma isozymes. These are direct substrates of many tyrosine kinases in a wide variety of cell types. The mechanism of PLC-gamma activation involves its association with and phosphorylation by receptor and non-receptor tyrosine kinases, as well as interaction with specialized adaptor molecules and, perhaps, other second messenger molecules. However, the biochemistry of PLC-gamma is at a more advanced state than a clear understanding of exactly how this signaling element functions in the generation of a mitogenic response.