Formation of c-Cbl.phosphatidylinositol 3-kinase complexes on lymphocyte membranes by a p56lck-independent mechanism.
The 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.
Hartley
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Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
Corvera
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D001665:Binding Sites; D002462:Cell Membrane / Q000201:enzymology; D003599:Cytoskeleton / Q000201:enzymology; D003600:Cytosol / Q000201:enzymology; D006801:Humans; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D008214:Lymphocytes / Q000166:cytology / Q000201:enzymology*; D009097:Multienzyme Complexes / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D050721:Proto-Oncogene Proteins c-cbl; D013329:Structure-Activity Relationship; D044767:Ubiquitin-Protein Ligases; D019061:src-Family Kinases / Q000378:metabolism*
D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D009097:Multienzyme Complexes; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins; D050721:Proto-Oncogene Proteins c-cbl; D044767:Ubiquitin-Protein Ligases; D019061:src-Family Kinases; C000715470:MAS1 protein, human; C067419:CBL protein, human