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

Ivan Gout^1,2, Gayle Middleton³, Jimi Adu³, Natalia N. Ninkina^3,4, Ludmila B. Drobot⁵, Valery Filonenko², Gennady Matsuka², Alun M. Davies³, Michael Waterfield¹, and Vladimir L. Buchman^3,4,6

¹Ludwig Institute for Cancer Research, Courtauld Building, 91 Riding House Street, London W1P 8BT, ³School of Biology, University of St Andrews, Bute Medical Buildings, St Andrews, Fife KY16 9TS, UK, ²Institute of Molecular Biology and Genetics, 150 Zabolotnogo Str., 252143 Kiev, ⁵Division of Regulatory Cell Systems, H.V.Palladin Institute of Biochemistry, 14/16 Drahomanov St., 290005 Lviv, Ukraine and ⁴Institute of Gene Biology, Russian Academy of Sciences, Vavilov Str. 34/6, B334 Moscow, Russia ⁶Corresponding author e-mail: vlb{at}st-and.ac.uk

Abstract: Class I_A phosphatidylinositol 3-kinase (PI 3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk_l, which forms complexes with the PI 3-kinase holoenzyme in vitro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 regulatory subunit of the class I_A PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kinase, interaction with Ruk_l substantially inhibits the lipid kinase activity of the enzyme. Overexpression of Ruk_l in cultured primary neurons induces apoptosis, an effect that could be reversed by co-expression of constitutively activated forms of the p110 catalytic subunit of PI 3-kinase or its downstream effector PKB/Akt. Our data provide evidence for the existence of a negative regulator of the PI 3-kinase signalling pathway that is essential for maintaining cellular homeostasis. Structural similarities between Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel family of adaptor molecules that are involved in various intracellular signalling pathways.

Key Words: Keywords: adaptor protein/neuronal apoptosis/phosphoinositide3-kinase/signal transduction