1994. of Ser608, which results in reduced lipid kinase activity and reduced association of the p110 catalytic subunit with p85. The importance of this phosphorylation was further highlighted by the finding that autophosphorylation on Ser608 was impaired, while Endothelin Mordulator 1 lipid kinase activity was increased, in a p85 mutant recently discovered in human tumors. These results provide the first evidence that phosphorylation of Ser608 Endothelin Mordulator 1 plays a role as a shutoff switch in growth factor signaling and contributes to the differences in functional properties of different PI 3-kinase isoforms in vivo. Numerous studies have documented the fundamental importance of class IA phosphoinositide 3-kinases (PI 3-kinases) for a multitude of cellular functions including cell survival, growth, proliferation, intermediary metabolism, and cytoskeletal rearrangements (7, 37, 45). PI 3-kinases catalyze the transfer of phosphate to the 3-OH position of inositol lipids to produce phosphatidylinositol-3,4-bisphosphate and phosphatidylinositol-3,4,5-trisphosphate (PIP3), which in turn act as second messengers by recruiting proteins containing pleckstrin homology (PH) domains to the plasma membrane to assemble signaling complexes (44). In addition to the lipid kinase activity, in vitro experiments have demonstrated that class I PI 3-kinases possess an intrinsic protein serine kinase activity (9, 15, 40, 42). This protein kinase activity has attracted much interest, but its functional consequences in vivo have not been defined (24). The typical form of class IA PI 3-kinase is a heterodimer with an 85-kDa regulatory subunit and a 110-kDa catalytic subunit (37). Two isoforms of the 85-kDa regulatory subunit have been identified: p85 and p85, which are products of different genes. Also, several splice variants of p85 exist. Furthermore, a third gene product, termed p55, has been identified. Three isoforms of the 110-kDa subunit have been identified in complex with p85: p110, p110, and p110. The and isoforms are widely expressed, whereas the isoform is expressed predominantly in leukocytes. It is well recognized that the activity of class-Ia PI 3-kinase is FEN1 regulated by a range of mechanisms acting via the various modular domains of the subunits. The p85 subunit comprises one SH3 and two SH2 domains, a BH (breakpoint cluster region homology) domain, and two proline-rich domains, all of which appear to contribute to the regulation. The SH3 domain binds and stimulates the activity of the GTP-binding protein dynamin (19), and it may also interact with proline-rich motifs within the p85 protein itself (20, 26). The small GTPases Rac and Cdc42 have been shown to interact with the BH domain and activate the lipid kinase as a result (5, 48). The SH2 domains are of fundamental importance for the enzymatic function of PI 3-kinase, since they bind to phosphotyrosine residues of activated growth factor receptors and adapter molecules such as the platelet-derived growth factor (PDGF) receptors and the insulin receptor substrate IRS-1. In this manner, they dock the Endothelin Mordulator 1 enzyme on the plasma membrane. Synthetic peptides corresponding to tyrosine-phosphorylated regions of either the PDGF receptor or IRS-1 have shown that these activate the enzyme lipid kinase activity (8, 29, 36, 47). However, this activation was evident only when phosphatidylinositol was used as a substrate and not with phosphatidylinositol-4,5-bisphosphate, which is considered to be the main physiological substrate of class I PI 3-kinase. Furthermore, no effect was observed on the protein kinase activity of the enzyme (29). In addition, tyrosine phosphorylation of the regulatory subunit has been reported to activate PI 3-kinase (11, 21, 22, 27, 46). All p110 catalytic subunits contain a domain which interacts with activated Ras (34). In the case of p110, this interaction has been shown to increase the lipid kinase activity above that caused by binding of the regulatory subunit to phosphotyrosine. With respect to the negative regulation of the PI 3-kinase activity, it has been reported that the p85 C-terminal SH2 domain.