small wing


EVOLUTIONARY HOMOLOGS part 2/3

Interaction of Phospholipase C gamma with growth factor receptors and other tyrosine kinases

Binding of macrophage colony stimulating factor (M-CSF) to its receptor (Fms) induces dimerization and activation of the tyrosine kinase domain of the receptor, resulting in autophosphorylation of cytoplasmic tyrosine residues used as docking sites for SH2-containing signaling proteins that relay growth and development signals. To determine whether a distinct signaling pathway is responsible for the Fms differentiation signal versus the growth signal, a search for new molecules involved in Fms signaling was carried out employing a two-hybrid screen in yeast, using the autophosphorylated cytoplasmic domain of the wild-type Fms receptor as bait. Clones containing SH2 domains of phospholipase C-gamma2 (PLC-gamma2) are frequently isolated and have been shown to interact with the phosphorylated Tyr721 of the Fms receptor, which is also the binding site of the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase). At variance with previous reports, M-CSF induces rapid and transient tyrosine phosphorylation of PLC-gamma2 in myeloid FDC-P1 cells; this activation requires the activity of the PI3-kinase pathway. The Fms Y721F mutation strongly decreases this activation. Moreover, the Fms Y807F mutation decreases both binding and phosphorylation of PLC-gamma2 but not that of p85. Since the Fms Y807F mutation abrogates the differentiation signal when expressed in FDC-P1 cells and since this phenotype could be reproduced by a specific inhibitor of PLC-gamma, it is proposed that a balance between the activities of PLC-gamma2 and PI3-kinase in response to M-CSF is required for cell differentiation (Bourette, 1997).

The neuregulins comprise a subfamily of epidermal growth factor (EGF)-like growth factors that elicit diverse cellular responses by activating members of the ErbB family of receptor tyrosine kinases. Although neuregulin-1 and neuregulin-2 are both binding ligands for the ErbB3 and ErbB4 receptors, they exhibit distinct biological activities depending on cellular context. In MDA-MB-468 human mammary tumor cells, neuregulin-2beta (NRG2beta) inhibits cell growth, whereas neuregulin-1beta (NRG1beta) does not. In these cells, NRG2beta appears to preferentially act through the EGF receptor, stimulating receptor tyrosine phosphorylation and the recruitment of phospholipase C-gamma, Cbl, SHP2, and Shc to that receptor. NRG1beta preferentially acts through ErbB3 in these cells by stimulating the tyrosine phosphorylation and recruitment of phosphatidylinositol 3-kinase and Shc to that receptor. In MDA-MB-453 cells, both NRG1beta and NRG2beta stimulate the tyrosine phosphorylation of the ErbB2 and ErbB3 receptors to similar extents, but only NRG1beta potently stimulates morphological changes consistent with cellular differentiation. The profiles of SH2 domain-containing proteins that are efficiently recruited to activated receptors differ for the two factors. These observations indicate that despite their overlapping receptor specificity, the neuregulins exhibit distinct biological and biochemical properties. Since both of these cell lines express only two of the known ErbB receptors, these results imply that EGF-like ligands might elicit differential signaling within the context of a single receptor heterodimer (Crovello, 1998).

TCR engagement activates phospholipase C gamma 1 (PLC gamma 1) via a tyrosine phosphorylation-dependent mechanism. PLC gamma 1 contains a pair of Src homology 2 (SH2) domains that promote protein interactions by binding phosphorylated tyrosine and adjacent amino acids. The role of the PLC gamma 1 SH2 domains in PLC gamma 1 phosphorylation was explored by mutational analysis of an epitope-tagged protein transiently expressed in Jurkat T cells. Mutation of the amino-terminal SH2 domain [SH2(N) domain] results in defective tyrosine phosphorylation of PLC gamma 1 in response to TCR/CD3 perturbation. In addition, the PLC gamma 1 SH2(N) domain mutants fail to associate with either Grb2 or a 36- to 38-kDa phosphoprotein (p36-38), which has previously been recognized to interact with PLC gamma 1, Grb2, and other molecules involved in TCR signal transduction. Conversely, mutation of the carboxyl-terminal SH2 domain [SH2(C) domain] does not affect TCR-induced tyrosine phosphorylation of PLC gamma 1. Furthermore, binding of p36-38 to PLC gamma 1 is not abrogated by mutations of the SH2(C) domain. In contrast to TCR/CD3 ligation, treatment of cells with pervanadate induced tyrosine phosphorylation of either PLC gamma 1 SH2(N) or SH2(C) domain mutants to a level comparable with that of the wild-type protein, indicating that pervanadate treatment induces an alternate mechanism of PLC gamma 1 phosphorylation. These data indicate that the SH2(N) domain is required for TCR-induced PLC gamma 1 phosphorylation, presumably by participating in the formation of a complex that promotes the association of PLC gamma 1 with a tyrosine kinase (Stoica, 1998).

Interactions and biological functions of Phospholipase C gamma's SH3 domains

Microinjection of a truncated form of the c-kit tyrosine kinase present in mouse spermatozoa (tr-kit) activates mouse eggs parthenogenetically, and tr-kit-induced egg activation is inhibited by preincubation with an inhibitor of phospholipase C (PLC). Co-injection of glutathione-S-transferase (GST) fusion proteins containing the src-homology (SH) domains of the gamma1 isoform of PLC (PLCgamma1) competitively inhibits tr-kit-induced egg activation. A GST fusion protein containing the SH3 domain of PLCgamma1 inhibits egg activation as efficiently as the whole SH region, while a GST fusion protein containing the two SH2 domains is much less effective. A GST fusion protein containing the SH3 domain of the Grb2 adaptor protein does not inhibit tr-kit-induced egg activation, showing that the effect of the SH3 domain of PLCgamma1 is specific. Tr-kit-induced egg activation is also suppressed by co-injection of antibodies raised against the PLCgamma1 SH domains, but not against the PLCgamma1 COOH-terminal region. In transfected COS cells, coexpression of PLCgamma1 and tr-kit increases diacylglycerol and inositol phosphate production, and the phosphotyrosine content of PLCgamma1, with respect to cells expressing PLCgamma1 alone. These data indicate that tr-kit activates PLCgamma1, and that the SH3 domain of PLCgamma1 is essential for tr-kit-induced egg activation (Sette, 1998).

SH3 domains are protein modules that interact with proline-rich polypeptide fragments. Cbl is an adapter-like protein known to interact with several SH3 domains, including the PLCgamma1 SH3 domain and the Grb2 amino terminal SH3 domain. Do sequences surrounding the PLCgamma1 SH3 domain core sequence (aa.796-851) affect the binding to Cbl, a target used as a prototypical ligand? A weak binding of Cbl to GST fusion proteins that strictly encompass the structural core of the PLCgamma1 SH3 domain has been demonstrated but a high-avidity binding occurs to the Grb2 amino-terminal SH3 domain. Inclusion of amino acids immediately flanking the PLCgamma1 SH3 core domain, however, substantially increase binding of Cbl to a level comparable to that of the Grb2 amino-terminal SH3 domain. The interaction of this extended PLCgamma1 SH3 domain fusion protein with Cbl depends entirely on the interaction of the domain with a proline-rich motif in Cbl, ruling out the possibility that amino acids adjacent to the core SH3 domain of PLCgamma1 provide independent Cbl binding. These data suggest that sequences surrounding the SH3 domain of PLCgamma1 may contribute to or stabilize the association of the domain with the target protein, thus increasing its binding efficiency (Graham, 1998).

Phospholipase C gamma 1 (PLC-gamma 1) hydrolyses phosphatidylinositol-4,5-bisphosphate to the second messengers inositol-1,4,5-trisphosphate and diacylglycerol. PLC-gamma 1 also has mitogenic activity upon growth-factor-dependent tyrosine phosphorylation; however, this activity is not dependent on the phospholipase activity of PLC-gamma 1, but requires an SH3 domain. PLC-gamma 1 acts as a guanine nucleotide exchange factor (GEF) for PIKE (phosphatidylinositol-3-OH kinase [PI(3)K] enhancer). PIKE is a nuclear GTPase that activates nuclear PI(3)K activity, and mediates the physiological activation by nerve growth factor (NGF) of nuclear PI(3)K activity. This enzymatic activity accounts for the mitogenic properties of PLC-gamma 1 (Ye, 2002).

Other interactions of PLCgamma

Epidermal growth factor (EGF)-induced autophosphorylation of the EGF receptor results in high-affinity binding of the adaptor protein GRB2, which serves as a convergence point for multiple signaling pathways. Present studies demonstrate that in WB cells EGF induces the co-immunoprecipitation of phospholipase C (PLC)-gamma1 with the adaptor protein GRB2 and the guanine nucleotide exchange factor Sos, but not with the adaptor protein SHC. Inhibition of PLC-gamma1 tyrosine phosphorylation by phenylarsine oxide reduces the co-immunoprecipitation of PLC-gamma1 with GRB2. Furthermore, angiotensin II, a G protein-coupled receptor agonist, also induces the tyrosine phosphorylation of PLC-gamma1 and its co-immunoprecipitation with GRB2 in WB cells. Interestingly, angiotensin II stimulation also causes tyrosine phosphorylation of the EGF receptor, suggesting that angiotensin II-induced PLC-gamma1 tyrosine phosphorylation in WB cells may be via EGF receptor tyrosine kinase activation. In addition, there is some level of association between PLC-gamma1 and GRB2 that is independent of the tyrosine phosphorylation of PLC-gamma1 in both in vivo and in vitro studies. In vitro studies further demonstrate that the Tyr771 and Tyr783 region of PLC-gamma1 and the SH2 domain of GRB2 are potentially involved in the tyrosine phosphorylation-dependent association between PLC-gamma1 and GRB2. The association of PLC-gamma1 with GRB2 and Sos suggests that PLC-gamma1 may be directly involved in the Ras signaling pathway and that GRB2 may be involved in the translocation of PLC-gamma1 from cytosol to the plasma membrane as a necessary step for its effect on inositol lipid hydrolysis (Pei, 1997).

Tyrosine phosphorylation of cellular proteins mediates the assembly and localization of effector proteins through interactions facilitated by modular Src homology 2 (SH2) and phosphotyrosine binding domains. Two tyrosine-phosphorylated proteins with Mr values of 70,000 and 68,000 are described that have been found to interact with Grb2, phospholipase C (PLCgamma1 and PLCgamma2), and Vav -- this occurs after B cell receptor cross-linking. The interaction of pp70 and pp68 with PLC and Vav is mediated by the carboxyl-terminal SH2 domain of PLC and the SH2 domain of Vav. In contrast, the interaction of pp70 and pp68 with Grb2 requires cooperative binding of the SH2 and SH3 domains of Grb2. Western blot analysis demonstrates that neither pp70 nor pp68 represent the recently described linker protein SLP-76, which binds Grb2, PLC, and Vav in T cells after T cell receptor activation. Moreover, SLP-76 protein is not detected in a number of B cell lines or in normal mouse B cells. Hence, it is proposed that pp70 and pp68 likely represent B cell homologs of SLP-76 which facilitate and coordinate B cell activation (Fu, 1997).

The role of the phospholipase C gamma 1 (PLC gamma 1) in signal transduction was investigated by characterizing its interactions with proteins that may represent components of a novel signaling pathway. A 145-kDa protein that binds SH2 domain of PLC gamma 1 was purified from rat brain. The sequence of peptide derived from the purified binding protein now identifies it as synaptojanin, a nerve terminal protein that has been implicated in the endocytosis of fused synaptic vesicles and shown to be a member of the inositol polyphosphate 5-phosphatase family. Stable association of PLC gamma 1 with synaptojanin has been demonstrated. Synaptojanin is a protein that not only binds the carboxyl terminal SH2 domain of PLC gamma 1, but also inhibits PLC gamma 1 activity (Ahn, 1998).

continued: small wing Evolutionary homologs
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Interactive Fly, Drosophila small wing: Biological Overview | Developmental Biology | Effects of Mutation | References

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