Imaging signal transduction during phagocytosis: phospholipids, surface charge, and electrostatic interactions.

Together with the development of genetically encoded fluorescent probes, digital imaging has provided great impetus to the study of cell signaling by providing enhanced sensitivity and much-improved spatial and temporal resolution. We have used phagocytosis as a paradigm of signal transduction, taking advantage of the generous size of phagosomes and of their comparatively leisurely rate of formation. Aided by the design of specific probes, we demonstrated a highly localized and elegantly choreographed sequence of changes in the level of several phosphoinositides and were able to also monitor the fate of phosphatidylserine. The net changes in the content of these anionic phospholipids are accompanied by marked alterations in the surface charge of the membrane of nascent phagosomes. These, in turn, cause the relocation of proteins that associate with the membrane by electrostatic interactions. Our studies suggest that anionic lipids control protein targeting not only through stereospecific recognition by specialized domains but also by electrostatic association mediated by polycationic motifs. The "electrostatic switch" can be turned on or off by altering the charge of the protein ligand (e.g., by phosphorylation) or, alternatively, by modifying the lipid composition of the target membrane.