(1/18461) Endocytosis: EH domains lend a hand.
A number of proteins that have been implicated in endocytosis feature a conserved protein-interaction module known as an EH domain. The three-dimensional structure of an EH domain has recently been solved, and is likely to presage significant advances in understanding molecular mechanisms of endocytosis. (+info)
(2/18461) The hematopoietic-specific adaptor protein gads functions in T-cell signaling via interactions with the SLP-76 and LAT adaptors.
BACKGROUND: The adaptor protein Gads is a Grb2-related protein originally identified on the basis of its interaction with the tyrosine-phosphorylated form of the docking protein Shc. Gads protein expression is restricted to hematopoietic tissues and cell lines. Gads contains a Src homology 2 (SH2) domain, which has previously been shown to have a similar binding specificity to that of Grb2. Gads also possesses two SH3 domains, but these have a distinct binding specificity to those of Grb2, as Gads does not bind to known Grb2 SH3 domain targets. Here, we investigated whether Gads is involved in T-cell signaling. RESULTS: We found that Gads is highly expressed in T cells and that the SLP-76 adaptor protein is a major Gads-associated protein in vivo. The constitutive interaction between Gads and SLP-76 was mediated by the carboxy-terminal SH3 domain of Gads and a 20 amino-acid proline-rich region in SLP-76. Gads also coimmunoprecipitated the tyrosine-phosphorylated form of the linker for activated T cells (LAT) adaptor protein following cross-linking of the T-cell receptor; this interaction was mediated by the Gads SH2 domain. Overexpression of Gads and SLP-76 resulted in a synergistic augmentation of T-cell signaling, as measured by activation of nuclear factor of activated T cells (NFAT), and this cooperation required a functional Gads SH2 domain. CONCLUSIONS: These results demonstrate that Gads plays an important role in T-cell signaling via its association with SLP-76 and LAT. Gads may promote cross-talk between the LAT and SLP-76 signaling complexes, thereby coupling membrane-proximal events to downstream signaling pathways. (+info)
(3/18461) The splicing factor-associated protein, p32, regulates RNA splicing by inhibiting ASF/SF2 RNA binding and phosphorylation.
The cellular protein p32 was isolated originally as a protein tightly associated with the essential splicing factor ASF/SF2 during its purification from HeLa cells. ASF/SF2 is a member of the SR family of splicing factors, which stimulate constitutive splicing and regulate alternative RNA splicing in a positive or negative fashion, depending on where on the pre-mRNA they bind. Here we present evidence that p32 interacts with ASF/SF2 and SRp30c, another member of the SR protein family. We further show that p32 inhibits ASF/SF2 function as both a splicing enhancer and splicing repressor protein by preventing stable ASF/SF2 interaction with RNA, but p32 does not block SRp30c function. ASF/SF2 is highly phosphorylated in vivo, a modification required for stable RNA binding and protein-protein interaction during spliceosome formation, and this phosphorylation, either through HeLa nuclear extracts or through specific SR protein kinases, is inhibited by p32. Our results suggest that p32 functions as an ASF/SF2 inhibitory factor, regulating ASF/SF2 RNA binding and phosphorylation. These findings place p32 into a new group of proteins that control RNA splicing by sequestering an essential RNA splicing factor into an inhibitory complex. (+info)
(4/18461) The Gab1 PH domain is required for localization of Gab1 at sites of cell-cell contact and epithelial morphogenesis downstream from the met receptor tyrosine kinase.
Stimulation of the hepatocyte growth factor (HGF) receptor tyrosine kinase, Met, induces mitogenesis, motility, invasion, and branching tubulogenesis of epithelial and endothelial cell lines in culture. We have previously shown that Gab1 is the major phosphorylated protein following stimulation of the Met receptor in epithelial cells that undergo a morphogenic program in response to HGF. Gab1 is a member of the family of IRS-1-like multisubstrate docking proteins and, like IRS-1, contains an amino-terminal pleckstrin homology domain, in addition to multiple tyrosine residues that are potential binding sites for proteins that contain SH2 or PTB domains. Following stimulation of epithelial cells with HGF, Gab1 associates with phosphatidylinositol 3-kinase and the tyrosine phosphatase SHP2. Met receptor mutants that are impaired in their association with Gab1 fail to induce branching tubulogenesis. Overexpression of Gab1 rescues the Met-dependent tubulogenic response in these cell lines. The ability of Gab1 to promote tubulogenesis is dependent on its pleckstrin homology domain. Whereas the wild-type Gab1 protein is localized to areas of cell-cell contact, a Gab1 protein lacking the pleckstrin homology domain is localized predominantly in the cytoplasm. Localization of Gab1 to areas of cell-cell contact is inhibited by LY294002, demonstrating that phosphatidylinositol 3-kinase activity is required. These data show that Gab1 is an important mediator of branching tubulogenesis downstream from the Met receptor and identify phosphatidylinositol 3-kinase and the Gab1 pleckstrin homology domain as crucial for subcellular localization of Gab1 and biological responses. (+info)
(5/18461) The histone acetylase PCAF is a phorbol-ester-inducible coactivator of the IRF family that confers enhanced interferon responsiveness.
Transcription factors of the interferon regulatory factor (IRF) family bind to the type I interferon (IFN)-responsive element (ISRE) and activate transcription from IFN-inducible genes. To identify cofactors that associate with IRF proteins, DNA affinity binding assays were performed with nuclear extracts prepared from tissue culture cells. The results demonstrated that the endogenous IRFs bound to the ISRE are complexed with the histone acetylases, PCAF, GCN5, and p300/CREB binding protein and that histone acetylase activities are accumulated on the IRF-ISRE complexes. By testing recombinant proteins, we show that PCAF directly binds to some but not all members of the IRF family through distinct domains of the two proteins. This interaction was functionally significant, since transfection of PCAF strongly enhanced IRF-1- and IRF-2-dependent promoter activities. Further studies showed that expression of PCAF and other histone acetylases was markedly induced in U937 cells upon phorbol ester treatment, which led to increased recruitment of PCAF to the IRF-ISRE complexes. Coinciding with the induction of histone acetylases, phorbol ester markedly enhanced IFN-alpha-stimulated gene expression in U937 cells. Supporting the role for PCAF in conferring IFN responsiveness, transfection of PCAF into U937 cells led to a large increase in IFN-alpha-inducible promoter activity. These results demonstrate that PCAF is a phorbol ester-inducible coactivator of the IRF proteins which contributes to the establishment of type I IFN responsiveness. (+info)
(6/18461) Mutations of oncoprotein 18/stathmin identify tubulin-directed regulatory activities distinct from tubulin association.
Oncoprotein 18/stathmin (Op18) is a recently identified phosphorylation-responsive regulator of the microtubule (MT) system. It was originally proposed that Op18 specifically regulates dynamic properties of MTs by associating with tubulin, but it has subsequently been proposed that Op18 acts simply by sequestering of tubulin heterodimers. We have dissected the mechanistic action of Op18 by generation of two distinct classes of mutants. One class has interruptions of the heptad repeats of a potential coiled-coil region of Op18, and the other involves substitution at all four phosphorylation sites with negatively charged Glu residues. Both types of mutation result in Op18 proteins with a limited decrease in tubulin complex formation. However, the MT-destabilizing activities of the coiled-coil mutants are more severely reduced in transfected leukemia cells than those of the Glu-substituted Op18 derivative, providing evidence for tubulin-directed regulatory activities distinct from tubulin complex formation. Analysis of Op18-mediated regulation of tubulin GTPase activity and taxol-promoted tubulin polymerization showed that while wild-type and Glu-substituted Op18 derivatives are active, the coiled-coil mutants are essentially inactive. This suggests that Op18-tubulin contact involves structural motifs that deliver a signal of regulatory importance to the MT system. (+info)
(7/18461) BLNK required for coupling Syk to PLC gamma 2 and Rac1-JNK in B cells.
Signaling through the B cell receptor (BCR) is essential for B cell function and development. Despite the key role of Syk in BCR signaling, little is known about the mechanism by which Syk transmits downstream effectors. BLNK (B cell LiNKer protein), a substrate for Syk, is now shown to be essential in activating phospholipase C (PLC)gamma 2 and JNK. The BCR-induced PLC gamma 2 activation, but not the JNK activation, was restored by introduction of PLC gamma 2 membrane-associated form into BLNK-deficient B cells. As JNK activation requires both Rac1 and PLC gamma 2, our results suggest that BLNK regulates the Rac1-JNK pathway, in addition to modulating PLC gamma 2 localization. (+info)
(8/18461) Characterization and partial purification of a novel neutrophil membrane-associated kinase capable of phosphorylating the respiratory burst component p47phox.
The phosphorylation of p47phox is widely viewed as an important step in the activation of the neutrophil respiratory burst oxidase system. The exact nature of the kinase(s) responsible remains to be elucidated. We show here that such a kinase was detected on neutrophil membranes activated by either PMA or formyl-methionyl-leucyl-phenylalanine. This enzyme is not intrinsic to the neutrophil membrane and could be eluted with 0.5 M NaCl. The kinase activity was partially purified and was found not to be due to the presence of previously suggested kinases, including protein kinase C isotypes, mitogen-activated protein kinase and protein kinase B. Gel filtration and renaturation in substrate gels suggest a molecular mass of between 45 and 51 kDa. The kinase activity was independent of calcium and lipids but was potently inhibited by staurosporine. Treatment with protein phosphatase 2Ac suggested that the kinase was activated by serine/threonine phosphorylation. Phosphopeptide maps indicated that the kinase phosphorylated p47phox on similar sites to those found in vivo. These results indicate that activation of neutrophils by PMA results in the activation of a membrane-associated kinase that may play a part in the regulation of neutrophil NADPH oxidase through its ability to phosphorylate p47phox. (+info)