N-TERMINUS OF SOS1 RAS EXCHANGE FACTOR - CRITICAL ROLES FOR THE DBL AND PLECKSTRIN HOMOLOGY DOMAINS

We have studied the functional importance of the N terminus of mouse S os1 (mSos1), a ubiquitously expressed Pas-specific guanine nucleotide exchange factor whose C-terminal sequences bind Grb-2, Consistent with previous reports, addition of a myristoylation signal to mSos1 (MyrSo s1) rendered it transforming for NIH 3T3 cells and deletion of the mSo s C terminus (MyrSos1-Delta C) did not interfere with this activity. H owever, an N-terminally deleted myristoylated mSos1 protein (MyrSos1-D elta N) was transformation defective, although the protein was stable and localized to the membrane. Site-directed mutagenesis was used to e xamine the role of the Dbl and pleckstrin homology (PH) domains locate d in the N terminus, When mutations in the PH domain were introduced i nto two conserved amino acids either singly or together in MyrSos1 or MyrSos1-Delta C, the transforming activity was severely impaired, An a nalogous reduction in biological activity,vas seen when a cluster of p oint mutations was engineered into the Dbl domain, The mitogen-activat ion protein (MAP) kinase activities induced by the various Dbl and PH mutants of MyrSos1 correlated with their biological activities, When N IH 3T3 cells were transfected with a myristoylated Sos N terminus, the ir growth response to epidermal growth factor (EGF), platelet-derived growth factor, lysophosphatidic acid or serum was greatly impaired, Th e dominant inhibitory biological activity of the N terminus correlated with its ability to impair EGF-dependent activation of GTP-Ras and of MAP kinase, as well with the ability of endogenous Sos to form a stab le complex with activated EGF receptors, The N terminus with mutations in the Dbl and PH domains was much less inhibitory in these biologica l and biochemical assays, In contrast to wild-type Sos1, nonmyristoyla ted versions of Sos1-Delta N and Sos1-Delta C did not form a stable co mplex with activated EGF receptors, We conclude that the Dbl and PH do mains are critical for Sos function and that stable association of Sos with activated EGF receptors requires both the Sos N and C termini.