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Molecular Immunogenetics Program, Oklahoma Medical Research Foundation, Oklahoma City, OK 73104
In T cells, glycolipid-enriched membrane (GEM) domains, or lipid rafts, are assembled into immune synapses in response to Ag presentation. However, the properties of T cell GEM domains in the absence of stimulatory signals, such as their size and distribution in the plasma membrane, are less clear. To address this question, we used confocal microscopy to measure GEM domains in unstimulated T cells expressing a GEM-targeted green fluorescent protein molecule. Our experiments showed that the GEM domains were assembled into membrane patches that were micrometers in size, as evidenced by a specific enrichment of GEM-associated molecules and resistance of the patches to extraction by Triton X-100. However, treatment of cells with latrunculin B disrupted the patching of the GEM domains and their resistance to Triton X-100. Similarly, the patches were coenriched with F-actin, and actin occurred in the detergent-resistant GEM fraction of T cells. Live-cell imaging showed that the patches were mobile and underwent translocation in the plasma membrane to immune synapses in stimulated T cells. Targeting of GEM domains to immune synapses was found to be actin-dependent, and required phosphatidylinositol 3-kinase activity and myosin motor proteins. We conclude from our results that T cell GEM domains are constitutively assembled by the actin cytoskeleton into micrometer-sized membrane patches, and that GEM domains and the GEM-enriched patches can function as a vehicle for targeting molecules to immune synapses.
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