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Vol. 18, Issue 8, 2883-2892, August 2007

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Quantitative Analysis of Membrane Remodeling at the Phagocytic Cup

Warren L. Lee^*,,, David Mason^*, Alan D. Schreiber, and Sergio Grinstein^*

*Programme in Cell Biology, Hospital for Sick Children, Interdepartmental Division of Critical Care Medicine, and the Division of Respirology, Department of Medicine, University of Toronto, Toronto, Ontario, M5G 1X8 Canada; and Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA 19104

Submitted May 23, 2006; Revised April 16, 2007; Accepted May 7, 2007
Monitoring Editor: Ralph Isberg

Nascent phagosomes, which are derived from the plasma membrane, acquire microbicidal properties through multiple fusion and fission events collectively known as maturation. Here we show that remodeling of the phagosomal membrane is apparent even before sealing, particularly when large particles are ingested. Fluorescent probes targeted to the plasma membrane are cleared from the region lining the particle before engulfment is completed. Extensive clearance was noted for components of the inner as well as outer monolayer of the plasmalemma. Segregation of lipid microdomains was ruled out as the mechanism underlying membrane remodeling, because markers residing in rafts and those that are excluded were similarly depleted. Selective endocytosis was also ruled out. Instead, several lines of evidence indicate that endomembranes inserted by exocytosis at sites of ingestion displace the original membrane constituents from the base of the phagosomal cup. The Fc receptors that trigger phagocytosis remain associated with their ligands. By contrast, Src-family kinases that are the immediate effectors of receptor activation are flushed away from the cup by the incoming membranes. Together with the depletion of phosphoinositides required for signal transduction, the disengagement of receptors from their effectors by bulk membrane remodeling provides a novel means to terminate receptor signaling.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Sergio Grinstein (sga{at}sickkids.ca).

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