Twice upon a time: PI3K's secret double life exposed

Class I phosphoinositide 3-kinases (PI3Ks) are heterodimeric enzymes involved in signal transduction triggered by growth factors and G-protein-coupled receptors. The catalytic function of PI3Ks is well known to promote a wide variety of biological processes, including proliferation, survival and migration, but a new layer of complexity in the function of PI3Ks has recently emerged, indicating that these proteins function not only as kinases but also as scaffold proteins. Knockout mice that lack PI3K protein expression show a different phenotype from knock-in mice expressing PI3K mutants that have lost their kinase activity, providing evidence for this novel role of PI3Ks. We will discuss such findings, highlighting the crucial scaffold function of PI3K gamma in cAMP homeostasis and PI3K beta in receptor recycling.