trkB encodes a receptor tyrosine kinase activated by three neurotrophins--brain-derived neurotrophic factor (BDNF), neurotrophin-3, and neurotrophin-4/5. In vivo, three isoforms of the receptor are generated by differential splicing--gp145trkB or the full-length trkB receptor, and trkB.T1 and trkB.T2, two cytoplasmically truncated receptors that lack kinases, but contain unique C termini. Although the truncated receptors appear to be precisely regulated during nervous system development and regeneration, their role in neurotrophin signaling has not been directly tested. In this paper, we studied the signaling properties and interactions of gp145trkB, trkB.T1, and trkB.T2 by expressing the receptors in a Xenopus oocyte microinjection assay. We found that oocytes expressing gp145trkB, but not trkB.T1 or trkB.T2, were capable of eliciting 45Ca efflux responses (a phospholipase C-gamma-mediated mechanism) after stimulation by BDNF. When trkB.T1 and trkB.T2 were coexpressed with gp145trkB, they acted as dominant negative receptors, inhibiting the BDNF signal by forming nonfunctional heterodimers with the full-length receptors. An ATP-binding mutant of gp145trkB had similar dominant inhibitory effects. Our data suggest that naturally occurring truncated trkB receptors function as inhibitory modulators of neurotrophin responsiveness. Furthermore, the homodimerization of gp145trkB appears to be an essential step in activation of the BDNF signaling cascade.

A number of cellular processes, such as proliferation, differentiation, and transformation, are regulated by cell-extracellular matrix interactions. Previous studies have identified a novel tyrosine kinase, the focal adhesion kinase p125 AEI] that fits the consensus sequence for the binding of Src.It has been well established that certain proteins encoded by retroviruses are able to produce cellular transformation (reviewed in reference 1). However, the processes that mediate this transformation are still poorly understood. Early studies indicated that cells infected with the Rous sarcoma virus (RSV; a transforming retrovirus encoding the Src oncogene, pp60 v-src ) showed an increase in the level of tyrosine-phosphorylated proteins (9,23,32,39). This suggested that a correlation might exist between the transformed phenotype induced by RSV and the tyrosine phosphorylation of cellular substrates.One approach used to identify the cellular substrates for v-Src has been the preparation of monoclonal antibodies against tyrosine-phosphorylated proteins from RSV-infected cells (24). One antibody developed by this method was found to recognize a 125-kDa tyrosine-phosphorylated protein in the lysates of RSV-transformed chicken embryo fibroblasts (36). Using this antibody, investigators identified a positive clone from a gt11 chicken embryo cDNA expression library that encoded a novel 125-kDa tyrosine kinase. This protein was composed of a central tyrosine kinase domain and two 400-amino-acid flanking regions that had no recognizable structural motifs or regions of significant homology with other known proteins, except a consensus sequence for phosphorylation by protein kinase C and two tyrosine-containing sequences, one of which forms a potential site for binding of Src and the other of which forms a potential site for binding of phosphatidylinositol 3-kinase. Since this protein was found by immunohistochemistry to be localized to cellular focal adhesions, it was given the name focal adhesion kinase or p125 FAK (23,46).A number of interesting findings have since been made regarding p125FAK and its cellular function. It has been shown that treatment of Swiss 3T3 fibroblasts with any of a number of substances increases the level of p125 FAK tyrosine phosphorylation. Such substances include small peptide mitogens such as vasopressin, bombesin, endothelin, and bradykinin (26,42,47); bioactive lipids such as lysophosphatidic acid and sphingosine (40, 41); peptide compounds such as Alzheimer's A␤ peptide (48); antigens for immunoglobulin E receptors (17); and growth factors such as hepatocyte growth factor scatter factor (28) and platelet-derived growth factor (33). Recent studies have also suggested that p125 FAK may play a role in the biology of cellular focal adhesions. When cell surface integrins bind to their ligands in the extracellular matrix (reviewed in reference 21), incorporation of phosphotyrosine by p125 FAK is stimulated, and when cells are detached from the extracellular matrix by trypsin or EDTA, p125FAK is rapi...

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Comparison of cDNA sequences from multiple sources predicts a genus of highly homologous but structurally distinct G protein α‐subunits, designated as Giα, that may include the α‐subunit of the functionally defined adenylate cyclase inhibitory G protein. Using specific oligonucleotide probes on Northern blots, we show that Giα−2 and Giα−3, but not Giα−1, are expressed in HL60 cells. Antisera raised against synthetic peptides from regions predicted to be conserved (AS6) and divergent (LE3) among Giα subtypes bind to a 40 kDa protein in Western blots of HL60 membranes. AS6 identifies an additional protein at 41 kDa. Thus, Northern blot and immunoblot results show that at least two Giα subtypes, a 40 kDa protein likely to correspond to Giα−2 and a 41 kDa protein possibly representing Giα−3, may be expressed in a single cell type.