Chemotactic cells generate localized accumulation of phosphoinositides at the leading edge of the cell. The lipids are then bound by lipid-binding domains of various signaling proteins. Dormann et al. used fusion proteins of green fluorescent protein (GFP) with the pleckstrin homology (PH) domains of various signaling proteins to monitor localized abundance of phosphoinositides during chemotaxis or phagocytosis in Dictyostelium. The specificity of the various PH domains for binding particular phosphoinositides enabled them to compare lipid signaling during the two processes. Concentrations of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] accumulated at sites of engulfment during phagocytosis, as they do at the tip of cells facing an increasing gradient of chemoattractant. During chemotaxis, the phosphatase PTEN is thought to decrease concentrations of PtdIns(3,4,5)P3, but studies with cells lacking PTEN indicated it was not providing the same function during phagocytosis. PtdIns(3,4)P2 was rapidly formed during phagocytosis, indicating that PtdIns(3,4,5)P3 was probably degraded by another phosphatase. The authors went on to show that the properties of the PH domains are critical to how localized signals are recognized in polarized cells. During chemotaxis, they found that the PH domain from CRAC (cytosolic regulator of adenylyl cyclase), which recognizes both PtdIns(3,4,5)P3 and PtdIns(3,4)P2, showed spatial and temporal patterns of association with the plasma membrane very distinct from those of the PH domain of GRP1 (general receptor of phosphoinositides), which recognizes primarily PtdIns(3,4,5)P3. The authors suggest that understanding the detailed binding properties of the various lipid-binding domains will be important in understanding the dynamics of such signaling in vivo.
D. Dormann, G. Weijer, S. Dowler, C. J. Weijer, In vivo analysis of 3-phosphoinositide dynamics during Dictyostelium phagocytosis and chemotaxis. J. Cell Sci. 117, 6497-6509 (2004). [Abstract] [Full Text]