Sam68 is a docking protein linking GAP and PI3K in insulin receptor signaling.
The 68 kDa Src substrate associated during mitosis (Sam68) is an RNA binding protein with Src homology (SH) 2 and 3 domain binding sites. We have recently found that Sam68 is a substrate of the insulin receptor (IR) and that Tyr-phosphorylated Sam68 associates with the SH2 domains of p85 PI3K. In the present work, using HTC-IR cells, we have found that insulin stimulation promotes the relocalization of Sam68 from the nucleus to the cytoplasm, and we have further studied the role of Sam68 in insulin receptor signaling complexes, by co-precipitating experiments. Thus, Sam68 is co-precipitated with p85 PI3K, IRS-1 and IR. The association of Sam68 with these complexes is mediated by the SH2 domains of PI3K. Moreover, we have found that Sam68 is a p120GAP associated protein after Tyr-phosphorylation by the IR. This association is mediated by the SH2 domains of GAP (preferentially the C-terminal SH2). Thus, Sam68 is linking p120GAP to PI3K signaling pathway. In fact, PI3K activity was increased in both anti-Sam68 and anti-GAP immmunoprecipitates upon insulin stimulation. We propose that the recruitment of the docking protein Sam68 to the PI3K pathway may serve to allow the association of other signaling molecules, i.e. p120GAP. In this way, these signaling complexes may modulate other signaling cascades of IR, such as p21Ras pathway.
Sánchez-Margalet
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Department of Medical Biochemistry and Molecular Biology, School of Medicine, University of Seville and Investigation Unit, University Hospital Virgen Macarena, Av. Sanchez Pizjuan 4, 41009 Seville, Spain. vsanchez@cica.es
Najib
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D000818:Animals; D016895:Culture Media, Serum-Free; D007328:Insulin / Q000494:pharmacology; D046911:Macromolecular Substances; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D016601:RNA-Binding Proteins / Q000378:metabolism*; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction; D014407:Tumor Cells, Cultured; D020729:p120 GTPase Activating Protein / Q000378:metabolism*; D019061:src-Family Kinases / Q000378:metabolism
D016895:Culture Media, Serum-Free; D007328:Insulin; D046911:Macromolecular Substances; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D020729:p120 GTPase Activating Protein; D019869:Phosphatidylinositol 3-Kinases; D011972:Receptor, Insulin; D019061:src-Family Kinases