Formation of c-Cbl.phosphatidylinositol 3-kinase complexes on lymphocyte membranes by a p56lck-independent mechanism
UMass Chan AffiliationsDepartment of Cell Biology
Program in Molecular Medicine
Document TypeJournal Article
Keywords1-Phosphatidylinositol 3-Kinase; Binding Sites; Cell Membrane; Cytoskeleton; Cytosol; Humans; Lymphocyte Specific Protein Tyrosine Kinase p56(lck); Lymphocytes; Multienzyme Complexes; Phosphotransferases (Alcohol Group Acceptor); Proto-Oncogene Proteins; Proto-Oncogene Proteins c-cbl; Structure-Activity Relationship; *Ubiquitin-Protein Ligases; src-Family Kinases
Medicine and Health Sciences
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AbstractThe proto-oncogene c-Cbl was originally identified as a cellular homologue of the transforming protein expressed by the murine Cas NS-1 retrovirus. The full-length c-Cbl protein is a predominantly cytoplasmic protein, abundant in lymphoid cells, and potentially involved in signal transduction in several cell types. The specific signal transduction pathways in which c-Cbl participates, and its precise role in these pathways, are unclear. Previous studies from our laboratory have shown that c-Cbl is the predominant tyrosine-phosphorylated protein bound to the p85 subunit of phosphatidylinositol (PI) 3-kinase on T lymphocyte and B lymphocyte activation. To further understand the properties of c-Cbl and the significance of its interactions with PI 3-kinase, we have further studied the cellular biological and biochemical responses of c-Cbl to stimulation in lymphoid cells. We show that stimulation induces the association of a highly tyrosine-phosphorylated pool of c-Cbl with lymphocyte membranes and with a detergent-insoluble particulate fraction. Immunoprecipitation of c-Cbl from subcellular fractions reveals that p85 is predominantly associated with the c-Cbl pool recovered from the membrane fraction, despite the fact that this pool represents a small amount of total cellular c-Cbl. The formation of c-Cbl.PI 3-kinase complexes on lymphocyte membranes did not depend on the catalytic activity of PI 3-kinase since it was unaltered by the treatment of cells with wortmannin prior to stimulation. Interestingly, c-Cbl tyrosine phosphorylation and the formation of c-Cbl.PI 3-kinase complexes were also observed in a mutant Jurkat cell line, JCaM1.6, lacking p56(lck) expression. Because p56(lck) is critical for mitogenic signal transduction in response to T cell receptor activation, our results suggest that the activation of c-Cbl and the formation of c-Cbl.PI 3-kinase complexes occur upstream or independently of mitogenic signal transduction pathways in T cells.
J Biol Chem. 1996 Sep 6;271(36):21939-43.