Negative regulation of PI 3-kinase by Ruk, a novel adaptor protein

Class I-A phosphatidylinositol 3-kinase (PI3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk(1), which forms complexes with the PI 3-kinase holoenzyme in v itro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 alpha regulatory subunit of the class I-A PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kin ase, interaction with Ruk(1) substantially inhibits the lipid kinase activi ty of the enzyme. Overexpression of Ruk(1) in cultured primary neurons indu ces apoptosis, an effect that could be reversed by co-expression of constit utively activated forms of the p110 alpha a catalytic subunit of PI 3-kinas e or its downstream effector PKB/Akt, Our data provide evidence for the exi stence of a negative regulator of the PI 3-kinase signalling pathway that i s essential for maintaining cellular homeostasis. Structural similarities b etween Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel fa mily of adaptor molecules that are involved in various intracellular signal ling pathways.