Siegmund Fischer

The multiple endocrine neoplasia type 2B point mutation switches the specificity of the Ret tyrosine kinase towards cellular substrates that are susceptible to interact with Crk and Nck

The RET proto-oncogene encodes a Tyrosine Kinase Receptor (RTK) which plays an important function in the proliferation and/or differentiation of neuroectodermic cells. Germline mutation of a methionine to a threonine within the RET TK domain predisposes to the Multiple Endocrine Neoplasia type 2B (MEN 2B). It has been demonstrated that, unlike c-Ret, the MEN 2B mutated Ret displays constitutive TK activity, tyrosine autophosphorylation and transforms fibroblasts. However, this oncoprotein is more than a fully activated wild-type (WT) Ret TK since it also displays modified substrate specificity. Change in substrate specificity leads to the tyrosine autophosphorylation of MEN 2B Ret on new sites as well as the phosphorylation of several novel downstream targets. But, none of these substrates have been identified and the ability of MEN 2B Ret phosphoprotein to interact with Src Homology 2 (SH2) domain containing molecules has been poorly investigated. In this report, using a constitutively activated Ret TK form, Ret-ptc 2, we demonstrate that the MEN 2B as the activated WT Ret TK binds to several SH2 signalling proteins such as Shc, Grb-2, Phospholipase Cγ, Crk and Nck. However, in contrast to the activated WT form, expression of the MEN 2B mutated Ret-ptc 2 results in the tyrosine phosphorylation of a panel of proteins which interestingly interact with Crk and Nck. We identified Paxillin, a cytoskeletal protein as one of the Crk associated proteins that is dramatically phosphorylated in MEN 2B but not in WT Ret expressing cells. These data suggest that MEN 2B mutated Ret triggers distinct signalling pathways that might be related to its transforming power.

p95vav associates with the nuclear protein Ku-70.

The proto-oncogene vav is expressed solely in hematopoietic cells and plays an important role in cell signaling, although little is known about the proteins involved in these pathways. To gain further information, the Src homology 2 (SH2) and 3 (SH3) domains of Vav were used to screen a lymphoid cell cDNA library by the yeast two-hybrid system. Among the positive clones, we detected a nuclear protein, Ku-70, which is the DNA-binding element of the DNA-dependent protein kinase. In Jurkat and UT7 cells, Vav is partially localized in the nuclei, as judged from immunofluorescence and confocal microscopy studies. By using glutathione S-transferase fusion proteins derived from Ku-70 and coimmunoprecipitation experiments with lysates prepared from human thymocytes and Jurkat and UT7 cells, we show that Vav associates with Ku-70. The interaction of Vav with Ku-70 requires only the 150-residue carboxy-terminal portion of Ku-70, which binds to the 25 carboxy-terminal residues of the carboxy SH3 domain of Vav. A proline-to-leucine mutation in the carboxy SH3 of Vav that blocks interaction with proline-rich sequences does not modify the binding of Ku-70, which lacks this motif. Therefore, the interaction of Vav with Ku-70 may be a novel form of protein-protein interaction. The potential role of Vav/Ku-70 complexes is discussed.

Structure and function of vav

The proto-oncogene vav is expressed solely in cells of hematopoietic origin regardless of their differentiation lineage. However, recently an homologue of vav, which is widely expressed (vav2) has been identified. Vav is a complicated and interesting molecule that contains a number of structural features found in proteins involved in cell signaling. Vav has a leucine-rich region, a leucine-zipper, a calponin homology domain, an acidic domain, a Dbl-homology domain, a pleckstrin homology domain, a cysteine-rich domain, two Src homology 3 domains, with a proline-rich region in the amino-SH3 domain, and finally an Src homology 2 domain. These domains have been implicated in protein protein interactions and strongly suggest that vav is involved in signaling events. vav is also rapidly and transiently tyrosine phosphorylated through the activation of multiple receptors on hematopoietic cells. Furthermore, vav is tyrosine phosphorylated upon the activation of several cytokines and growths factors. Recently, the generation of nice vav-/- showed that vav has an essential role in proliferation/activation of T and B cells. The purpose of this review is to summarize the current knowledge on vav and to evaluate the roles of vav in cellular functions.

Transferrin receptors in developing murine erythroid cells.

Summary. Techniques of cell separation were used to isolate murine erythroid cells at different stages of maturation. The number of transferrin receptors in these cell populations was assayed by measuring binding of 125I‐labelled transferrin. Nearly 23 times as many receptors were found in the least mature cells, chiefly pronormoblasts, as in reticulocytes. Iron transport, determined by measurement of the rate of 59Fe uptake from 59Fe‐labelled transferrin, was proportional to the number of receptors at all stages of differentiation. Electron microscope radioautographic studies of the interaction of 125I‐labelled transferrin with erythroid precursor cells demonstrated that 15–35% of cell associated transferrin was intracellular in erythroid precursors.

hSiah2 Is a New Vav Binding Protein Which Inhibits Vav-Mediated Signaling Pathways

ABSTRACT The hematopoietic proto-oncogene vav has been characterized as a Rac1-GDP/GTP exchanger protein which regulates cytoskeletal reorganization as well as signaling pathways leading to the activation of stress-activated protein kinases (SAPK/JNKs). Furthermore, vav overexpression enhances basal and T-cell receptor (TCR)-mediated stimulation of the nuclear factor of activated T cells (NFAT). We report here the interaction between Vav and hSiah2, a mammalian homolog of Drosophila Seven in absentia (Sina) that has been implicated in R7 photoreceptor cell formation during Drosophila eye development via the proteasome degradation pathway. Vav and hSiah2 interact in vitro and in vivo and colocalize in the cytoplasm of hematopoietic cells. The Src homology domain of Vav and the C-terminal region of hSiah2 are required for this interaction. We provide evidence for a negative regulation by hSiah2 of Vav-induced basal and TCR-mediated NFAT-dependent transcription. Overexpression of hSiah2 also inhibits the onco-Vav-induced JNK activation. Although the Vav-interacting domain is located in the C-terminal portion of hSiah2, the N-terminal region of hSiah2 is necessary for the inhibitory role that seems to be independent of the proteasome degradation.

Association of the protein tyrosine phosphatase PTP1C with the protein tyrosine kinase c-Src in human platelets

Protein tyrosine phosphatase 1C (PTP1C), highly expressed in hematopoietic cells, is a soluble protein tyrosine phosphatase containing two Src homology 2 (SH2) domains at the N‐terminus and two putative sites of tyrosine phosphorylation at the C‐terminus. This paper reports that PTP1C and c‐Src could be coimmunoprecipitated during thrombin‐induced platelet activation. Moreover, association between the two signalling proteins occurred only after PTP1C had been tyrosine phosphorylated. In in vitro experiments, PTP1C bound to the SH2 domain of c‐Src, suggesting that association between tyrosine phosphorylated PTP1C and c‐Src was mediated by the SH2 domain of c‐Src. Finally, in resting platelets, PTP1C was mainly found in the Nonidet P‐40 soluble fraction whereas following thrombin‐induced activation, around 17% of PTP1C was associated with the insoluble fraction.

Vav Binding to Heterogeneous Nuclear Ribonucleoprotein (hnRNP) C EVIDENCE FOR Vav-hnRNP Interactions in an RNA-dependent Manner

The vav proto-oncogene is exclusively expressed in hematopoietic cells and encodes a 95-kDa protein that contains multiple structural domains. Vav is involved in the expansion of T and B cells, in antigen-mediated proliferative responses, and in the induction of intrathymic T cell maturation. It becomes rapidly and transiently tyrosine-phosphorylated upon triggering of a large number of surface receptors and catalyzes GDP/GTP exchange on Rac-1. We now provide evidence for the specific interaction of Vav with heterogeneous nuclear ribonucleoprotein (hnRNP) C. Vav and hnRNP C interact bothin vivo and in vitro mediated through the carboxyl Src homology 3 domain of Vav and the proline-rich motif located in the nuclear retention sequence of hnRNP C. More importantly, Vav-hnRNP C complexes are present in living hematopoietic cells and both proteins localize in the nuclei, mainly on perichromatic fibrils but also on clusters of interchromatin granules. The Vav-hnRNP C interaction is regulated by poly(U) RNA, although a basal association is still detected in the absence of RNA. Furthermore, RNA homopolymers differentially alter the binding affinity of Vav to hnRNP C and hnRNP K. We propose that Vav-hnRNP interactions may be established in an RNA-dependent manner.

CSF-1 stimulation induces the formation of a multiprotein complex including CSF-1 receptor, c-Cbl, PI 3-kinase, Crk-II and Grb2.

Recently c-Cbl has been reported to be phosphorylated upon CSF-1 stimulation. The product of the c-cbl proto-oncogene (c-Cbl) is a 120 kDa protein harboring several docking sites for Src homology 2 (SH2) domain containing proteins and proline-rich regions that have been shown to allow its constitutive association with the SH3 domains of Grb2. We demonstrate here that CSF-1 exposure of stable transfectant CHO cells expressing the CSF-1 receptor induced the sustained tyrosine phosphorylation of c-Cbl and its subsequent association with Crk-II and the p85 kDa subunit of the PI 3-kinase, while it constitutively associates with Grb2. We demonstrate by in vitro experiments that these associations require the SH2 domain of Crk-II and both the C- and N-terminal SH2 domains of the p85 subunit of the PI 3-kinase. c-Cbl is the major PI 3-kinase-containing protein in c-Fms expressing CHO cells upon CSF-1 stimulation. Thus c-Cbl behaves as a core protein, allowing the formation of a quaternary complex including, Crk-II, PI 3-kinase and Grb2. We provide evidence that this multiprotein complex can interact with the tyrosine phosphorylated CSF-1 receptor through the unoccupied SH2 domain of Grb2.

A spin label study of the erythrocyte membranes in duchenne muscular dystrophy

Red blood cells and freshly prepared erythrocyte membranes of 15 patients with Duchenne muscular dystrophy (DMD) as well as age-matched controls were studied by the spin label method. No significant modifications appeared for spin-labelled proteins of ghost membranes. With the two fatty acid spin labels, 5-nitroxide stearate and 16-nitroxide stearate, we have confirmed previous results of Sato et al. concerning the thermal behaviour of the erythrocyte membranes, i.e. no change near the polar part probed by 5-nitroxide stearate and a linearization of the fluidity versus temperature variation around 12 degrees C, as explored by 16-nitroxide stearate. Furthermore we studied in the whole erythrocyte the amplitude of the 5-nitroxide stearate electron spin resonance signal as a function of the microwave power. This saturation effect was observed in 12 out of 15 controls and only in 1 out of 13 DMD cases studied. In erythrocyte membranes labelled with 16-nitroxide stearate the penetration of the label inside membranes was statistically different between DMD and controls. These new findings furnish further arguments in favour of a structural alteration of the phospholipid organization of erythrocyte membranes in DMD. Associated together, these different sets of tests obtained by spin labelling permit good statistical discrimination between DMD and normal subjects.

Tyrosine phosphorylation in human T lymphoma cells.

A high level of tyrosine protein kinase (TPK) has been recently detected in the murine lymphoma LSTRA. The main substrate for tyrosine phosphorylation in this cell line is a Mr 55 000 protein associated with the insoluble matrix of the cell. These findings prompted the search for TPK activities in human lymphoid cells. Three human T lymphoma cell lines (i.e. Molt. 4, JM, and Ke 37) and control lymphocytes were examined. After in vitro phosphorylation of detergent insoluble extracts from human T lymphoma cells, 2 major phosphotyrosine containing proteins with Mr 55 000 and 35 000 can be detected in all three T lymphoma lines, whereas an additional species with Mr 78 000 is present only in the Ke 37 cell line. Similar size proteins are weakly phosphorylated in normal lymphocytes. Tyrosine phosphorylation in these proteins proceeds actively at 0 degrees C, and is dramatically stimulated by Mn++ ions. Partial proteolysis mappings of the Mr 55 000 phosphoproteins from murine and human lymphomas revealed a strong homology among these molecules. The function of this protein in transformed lymphocytes is discussed.