DIRECT ASSOCIATION OF GRB2 WITH THE P85 SUBUNIT OF PHOSPHATIDYLINOSITOL 3-KINASE

Phosphatidylinositol 3-kinase (PI 3-kinase) has been shown to play a k ey role in growth factor signaling pathways, although its signaling me chanism has not been fully elucidated. Using the yeast interaction tra p system, we have identified Grb2 as a PI 3-kinase interacting protein . Our experiments demonstrate that p85, the regulatory subunit of PI 3 -kinase, interacts with Grb2 in vivo, and this interaction is independ ent of growth factor stimulation. The direct association between Grb2 and p85 was reconstituted in vitro with glutathione S-transferase fusi on proteins. Domain analyses and peptide competition indicate that the association is mediated by the SH3 domains of Grb2 and the proline-ri ch motifs of p85 and that only one SH3 domain is required for minimal binding. The interaction does not displace the catalytic subunit of PI 3-kinase but is exclusive of Sos. Signaling through PI 3-kinase, ther efore, may involve the ubiquitous adapter Grb2, which serves as a conv ergence point for multiple pathways.