Phosphatidylinositol transfer proteins (PITPs) bind one molecule of either phosphatidylinositol (PI) or phosphatidylcholine (PC) and can mediate their transfer between membrane compartments in vitro. The best-characterised mammalian PITPs are PITPα and PITPβ, two highly homologous proteins encoded by distinct genes. Genetic studies indicate that PITPβ is an essential gene, as ablation of the gene is embyronically lethal, whilst ablation of PITPα leads to the birth of live mice which only survive for a couple of weeks (Hamilton et al. 1997; Alb et al. 2002). Biochemical studies involving reconstitution of cytosol-depleted cell preparations have demonstrated the requirement of PITPα and -β in signal transduction and in membrane traffic. From such analysis, a requirement for PITP has been identified in phospholipase C (PLC)-mediated phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) hydrolysis, in the synthesis of 3-phosphorylated lipids by phosphoinositide 3-kinases, in regulated exocytosis and in the biogenesis of vesicles at the Golgi. Studies aimed at elucidating the mechanism of action of PITP in each of these seemingly disparate functions have yielded a singular theme; the activity of PITP stems from its ability to transfer PI from its site of synthesis to sites of cellular activity and to stimulate the local synthesis of phosphorylated forms of PI including PI(4)P, PI(4,5)P₂, PI(3)P and PI(3,4,5)P₃ by delivering PI to the lipid kinases involved in phosphoinositide synthesis (Hsuan & Cockcroft, 2001). Using FLIM (fluorescence lifetime imaging microscopy) to measure FRET (fluorescence resonance energy transfer) between GFP-PITP proteins and fluorescently labelled phospholipids, we report that PITPα and PITPβ can dynamically interact with PI or PC at the plasma membrane only when stimulated with epidermal growth factor (EGF). In addition, PITPβ is localised at the Golgi, and upon EGF addition, the lipid environment is altered, resulting in enhanced FRET (B. Larijani et al. 2002, submitted). Our observations demonstrate that the transfer function of PITPα and PITPβ is a regulated process involving dynamic behaviour in vivo. Previous studies have shown that both PITPα and PITPβ can reconstitute PLC signalling (Cunningham et al. 1996) and the observation that both PITPα and PITPβ are found at the plasma membrane following stimulation emphasises that PITP proteins have overlapping functions in PLC signalling.