Phosphoinositide 3-kinase (PI3K) and its downstream kinase effector Akt/PKB are important regulators of cell survival and apoptosis in vertebrates, and of cell motility and chemotaxis in Dictyostelium and mammalian cells. Studies using pleckstrin-homology (PH) domains – including those of Akt/PKB – which bind to the PI3K products PtdIns(3,4,5)P3 and PtdIns(3,4)P2, reveal PI3K-dependent transient localization of PH-domain proteins to the leading edge of chemotaxing Dictyostelium, neutrophils and fibroblasts. Such studies have led to a model of chemotaxis in which the localized activation of PI3K is one of the initial steps that specifies the front of the cell and positions signaling components that are required for directional cell movement.A recent paper by Rupper et al. [1xSequential activities of phosphoinositide 3-kinase, PKB/Akt, and Rab7 during macropinosome formation in Dictyostelium. Rupper, A. et al. Mol. Biol. Cell. 2001; 12: 2813–2824Crossref | PubMedSee all References[1] has now expanded our understanding of the role of PI3K and Akt/PKB in a distinct pathway, macropinocytosis, an important mechanism that allows cells to internalize fluid, cell-surface molecules and extracellular components, including pathogenic organisms. Previous studies had demonstrated that inhibition of PI3K activity in macrophages with Wortmannin or LY294002 caused the arrest of macropinosome formation. By analyzing knockout mutants of two Class I PI3Ks (PI3K1 and PI3K2) and of Akt/PKB, Rupper et al. now provide direct genetic evidence that both PI3K and Akt/PKB are required for macropinocytosis in Dictyostelium. Furthermore, single-cell imaging of a green-fluorescent protein (GFP) fusion with the PH domain of Akt/PKB [to assay localized formation of PtdIns(3,4,5)P3 and PtdIns(3,4)P2] and of GFP fusion with the F-actin-binding domain of ABP-120 (to visualize F-actin) demonstrated that Akt/PKB and F-actin localize contemporaneously (within the time limits of microscopic resolution, ∼5 seconds) to the forming macropinocytotic cup at the cell surface. Inhibition of PI3K with LY294002 did not alter the initial formation of these F-actin-rich cup structures at the plasma membrane but it did prevent Akt/PKB recruitment to these cups and their subsequent fusion into the large rings characteristic of normal macropinosomes.These authors had demonstrated previously that a dominant–negative form of Dictyostelium Rab7 significantly impaired phagocytosis and fluid-phase endocytosis in Dictyostelium cells and now confirm a specific defect in macropinocytosis. However, a GFP fusion with wild-type Rab7 is not recruited to the forming macropinocytotic cups but is only associated with the F-actin-rich ring structures, subsequent to macropinosome formation and internalization.Together, these studies provide a compelling model for certain temporal events that are essential to regulate macropinosome formation. The authors have confirmed a requirement for PI3K in macropinosome formation and fluid uptake, and they also demonstrate an essential role for Akt/PKB in these processes. It will now be crucial to identify potential Akt/PKB substrates and to determine whether other PI3K effectors are also involved. It will also be of significant interest to understand how similar the PI3K-dependent pathways are that control formation of macropinosomes and that are required for pseudopod/lamellipod formation during chemotaxis.