Highly localized changes in membrane phosphoinositides are critical to phagocytosis, a process that can take several minutes. Wondering how changes in membrane lipids are confined to such a restricted region, Corbett-Nelson et al. used fluorescence recovery after photobleaching (FRAP) analysis to visualize the mobility of lipid-anchored chimeric proteins. Acylated fluorescent proteins targeted to the inner leaflet of the plasma membrane [PM-GFP, which contains the Lyn myristoylation and palmitoylation sequence, and GFP-tH, which contains the C-terminal 9 amino acids of H-Ras] diffused freely in the membrane of unstimulated RAW264.7 macrophages. Their mobility was substantially restricted at phagocytic sites but not at unengaged regions of cells phagocytosing IgG-opsonized latex beads. In contrast, the mobility of glycosylphosphatidylinositol (GPI) constructs targeted to the outer membrane was the same at phagocytic sites as at unengaged regions. The restriction of PM-GFP mobility at phagocytic sites was not impeded by inhibition of actin polymerization, nor was it blocked by cholesterol depletion. Moreover, mobility of fluorescently labeled linker for activation of T cells (LAT), which associates with sphingolipid- and cholesterol-rich lipid rafts, did not differ in phagocytic and unengaged regions. However, the restriction of PM-GFP mobility at phagocytic sites was markedly reduced by pharmacological inhibition of Src-family tyrosine kinases with PP1. Thus, the authors propose that the restriction of lipid mobility at phagocytic sites requires signaling-dependent events and does not involve conventional lipid rafts; they suggest that recruitment of molecules with lipid-binding domains could lead to the formation of an annulus of restricted lipid mobility.
E. F. Corbett-Nelson, D. Mason, J. G. Marshall, Y. Collette, S. Grinstein, Signaling-dependent immobilization of acylated proteins in the inner monolayer of the plasma membrane. J. Cell Biol. 174, 255-265 (2006). [Abstract] [Full Text]