Introduction myo-Inositol-containing phospholipids form a minor, but metabolically highly active, component of eukaryotic membranes, where they play a central role in signal-transduction pathways. Hydrolysis of PtdIns(4,5)P2 by phospholipase C generates the two second messengers Ins( 1,4,5)P3 and diacylglycerol, which function in mobilization of intracellular Ca2+ and activation of protein kinase C respectively [l]. Ins( 1,4,5)P3 is an intermediate in the generation of a newly discovered second messenger, Ins( 1 ,3,4,5)P4, by Ins( 1,4,5)P3-specific 3-kinase isozymes [2]. The role of Ins( 1,3,4,5)P4 in Ca2+ mobilization remains undefined. Initially, it was implicated in the regulation of intracellular Ca2+ levels in synergy with Ins(1,4,5)P3, but recently the possibility was raised that Ins( 1,3,4,5)P4 may inhibit Ins( 1,4,5)P3-induced Ca2+ signalling in mouse lacrimal acinar cells [3]. An Ins( 1,3,4,5)P4-binding protein has been identified as a member of the GTPase-activating protein family with Ins( 1,3,4,5)P4-stimulated activity against Ras, suggesting a link between phospholipase Cderived and Ras signalling pathways [4]. Phosphoinositide (PI) 3-kinase phosphorylates the D-3 position of the inositol ring of phosphatidylinositols, generating the putative second messengers, PtdIns(3)P, PtdIns(3,4)P2 and PtdIns(3,4,5)P3 [ 5 ] . Although PI 3-kinase activation has been demonstrated to be involved in various cellular responses, such as mitogenic signalling, membrane trafficking and secretion, much less is known about the function of the 3-phosphorylated phosphoinositides. Potential targets of PtdIns(3,4,5)P3, the major PI 3-kinase product in vivo in mammalian cells [6], are Ca2+-independent protein kinase C isoforms [7] and proteins containing Src homology 2 (SH2) domains [8]. This suggests a role for PtdIns(3,4,5)P3 in recruiting proteins with SH2 domains to the membrane. Furthermore 3-phos-