Extracellular matrix survival signals transduced by focal adhesion kinase suppress p53-mediated apoptosis. 1C stable cell lines in response to fibronectin adhesion and that expression of constitutively active Ras, Ras 61 (L), rescued 1C-mediated down-regulation of ERK2 activation. Inhibition of cell proliferation in 1C stable cell lines was attributable to an inhibitory effect of 1C on the Ras/MAP kinase pathway because expression of activated MAPK kinase rescued 1C antiproliferative effect. These findings show that the 1C variant, by means of a unique signaling mechanism, selectively inhibits the MAP kinase pathway by preventing Ras activation without affecting either survival signals stimulated by integrins or cellular interactions ST7612AA1 with the extracellular matrix. These findings highlight a role for 1-specific cytodomain sequences in maintaining an intracellular balance of proliferation and survival signals. INTRODUCTION Integrins are a large family of heterodimeric transmembrane receptors composed of and subunits (Hynes, 1992 ). In addition to their role as adhesion receptors, integrins have been shown to regulate intracellular signaling pathways and cellular functions such as cell migration, proliferation, and survival (Schwartz for 15 min at 4C. FAK was immunoprecipitated from 500 g of total cell lysate with 0.5 g of C-20, an affinity-purified antibody to FAK. Immunocomplexes were collected with protein ACSepharose, washed five times with lysis buffer, and resuspended in loading buffer. Proteins were separated by 10% SDS-PAGE, and FAK phosphorylation on Tyr397 was analyzed by immunoblotting with a rabbit affinity-purified antibody that recognizes FAK only when phosphorylated on Y397. FAK protein levels were analyzed by immunoblotting with C-20 rabbit affinity-purified antibody to FAK as described (Zheng (1995) . Briefly, 50 g of detergent cell extracts were cleared by centrifugation at 14,000 for 15 min at 4C. AKT was immunoprecipitated with 0.1 g of affinity-purified antibody to AKT. Immunocomplexes were collected with protein ACSepharose and washed three times with lysis buffer, once with 20 mM HEPES, pH 7.5, and once with kinase buffer (20 mM HEPES, pH 7.5, 1 mM DTT, 10 mM MnCl2, 10 mM MgCl2). The AKT kinase activity was assayed with kinase buffer containing 10 Ci of [-32P]ATP (3000 Ci/mmol; Amersham Life Sciences, ST7612AA1 Arlington Heights, IL), 5 M ATP (Boehringer Mannheim), and 100 g/ml histone H2B (Boehringer Mannheim) as a substrate for 20 min at 30C. The reactions were terminated ST7612AA1 with loading buffer. Phosphorylated histone H2B was viewed by autoradiography. ERK2 Analysis CHO stable cell lines were lysed as described for analysis of AKT activation. Analysis of ERK2 phosphorylation by immunoblotting was performed with 0.5 g/ml E10, a mAb that recognizes ERK2 only when phosphorylated at Thr202/Tyr204, according to the manufacturer’s instructions (DH5 was induced with 1 mM isopropyl-1-thio–d-galactopyranoside (American Bioanalytical) for 2 h at 37C. The cells were then washed once with ice-cold 20 mM HEPES, pH 7.5, 150 mM NaCl and lysed by sonication in the following buffer: 20 mM HEPES, pH 7.5, 120 mM NaCl, 10% glycerol, 2 mM EDTA, 100 mg/ml lysozyme (American Bioanalytical), 1 mM PMSF, 10 g/ml aprotinin, 10 g/ml leupeptin, 1 g/ml pepstatin. The lysate was clarified by centrifugation and incubated with glutathione Sepharose (Amersham Pharmacia Biotech, Piscataway, NJ) for 30 min at 4C. The Sepharose beads were then washed six times with lysis buffer containing 0.5% NP-40 and stored in the same buffer at 4C. For affinity precipitation, cells were washed twice with ice-cold 20 mM HEPES, pH 7.5, 150 mM NaCl and lysed with the following buffer (25 mM HEPES, pH 7.5, 150 mM NaCl, 1% NP-40, 0.25% deoxycholate, 10% glycerol, 10 mM MgCl2, 25 mM sodium fluoride, 1 ST7612AA1 mM EDTA, 1 mM sodium vanadate, 1 mM PMSF, 10 g/ml aprotinin, 10 g/ml leupeptin) for 30 min at 4C. One milligram of whole cell lysate was incubated with CRF (human, rat) Acetate GST-RBD bound to glutathione Sepharose for 30 min at 4C. Bound proteins were washed three times with lysis buffer, eluted with loading buffer, and separated by SDS-PAGE (12%). Proteins were visualized by immunoblotting with 2 g/ml anti-pan Ras.