An increasing amount of experimental data suggest that cross-talk exists between pathways involving tyrosine kinases and heterotrimeric G proteins. In a previous study, we demonstrated that bradykinin (BK) increases the intracellular accumulation of cAMP in the human epidermoid carcinoma cell line A431 by stimulating adenylate cyclase activity via a stimulatory G protein (G s␣ There is mounting evidence indicative of complex, probably cell-specific interactions between signaling pathways involving heterotrimeric G proteins and tyrosine kinases. For example, the stimulation of G protein-coupled receptors modulates key proteins of the mitogen-activated protein kinase pathway via protein kinase C-or protein kinase A-mediated phosphorylation on serine or threonine residues (1-3). Furthermore, several isoforms of ␣ subunits of G proteins were shown to be phosphorylated in vitro on tyrosine residues by tyrosine kinase receptors such as the epidermal growth factor (EGF) 1 receptor and the insulin receptor or by non-receptor tyrosine kinases of the Src kinase family. EGF was shown to activate cardiac adenylate cyclase via a mechanism requiring both G s␣ and the EGF receptor tyrosine kinase (4, 5). EGF was also found to stimulate phospholipase C in rat hepatocytes (6) or phospholipase A 2 in rat kidney (7) in a pertussis toxin-sensitive manner, suggesting an involvement of G i proteins. The molecular mechanism of these receptor tyrosine kinase-mediated effects remained unclear. In reconstituted phospholipid vesicles, the insulin receptor was found to catalyze tyrosine phosphorylation of G i␣ and G o␣ , suggesting the possibility of a direct interaction of receptor tyrosine kinases and G proteins (8). Further progress in this field came from in vitro studies of Hausdorff et al. (9) on direct interactions between the non-receptor tyrosine kinase pp60 c-src and purified G protein ␣ subunits. pp60 c-src was shown to phosphorylate recombinant G s␣ as well as other G ␣ isoforms on tyrosine residues almost stoichiometrically (9). In 1995, the in vitro sites of phosphorylation of G s␣ by pp60 c-src were identified (10). The phosphorylated tyrosine residues are located at N-terminal position 37 and at C-terminal position 377 of G s␣ and thus in regions known to participate in nucleotide exchange and receptor interaction (10, 11). Very recently, using purified EGF receptor and recombinant G s␣ , Poppleton et al. (12) demonstrated an activation of G s␣ via phosphorylation on tyrosine residues by EGF receptor kinase. However, little is known about tyrosine phosphorylation of G s␣ in intact cells and how such phosphorylation might affect the function of the G protein. We have recently shown that bradykinin (BK) activates dual pathways in A431 human epidermoid carcinoma cells, i.e. the phospholipase C-␤/protein kinase C pathway and, independently via G s␣ , the cyclic AMP/protein kinase A pathway, which represents a negative feedback loop to the BKinduced protein kinase C activation (13). At the same time, as observed earlier by...