Clusters of glycolipid and glycosylphosphatidylinositol-anchored proteins in lymphoid cells: accumulation of actin regulated by local tyrosine phosphorylation

Lateral cross-linking of glycosylphosphatidylinositol (GPI)-anchored protei ns and glycosphingolipids can trigger a signaling cascade which leads to ac tivation of lymphoid cells. A possible explanation how the signal is transd uced through the plasma membrane has arisen from the concept of raft sphing olipid-cholesterol microdomains in cell membranes. Crosslinking of GPI-anch ored proteins, glycolipids and other raft components leads to the formation of stabilized membrane patches in the plasma membrane which enrich members of the Src-tyrosine kinase family. We have studied cellular responses to r aft patch formation in the Jurkat T cell line and in particular changes in the actin cytoskeleton. We found that raft patches formed by GPI-anchored C D59 protein and the ganglioside GM1 accumulate filamentous actin. Most inte restingly, we observed a strong accumulation of tyrosine-phosphorylated pro teins in raft patches, strongly supporting the view that they can function as centers of signal transduction. Using a Lck kinase-deficient variant of Jurkat cells and a specific Lck and Fyn tyrosine kinase inhibitor we found that enrichment of actin in raft patches is dependent on phosphotyrosine ac cumulation in the patches. These observations show a link between raft-medi ated signaling and the interaction of actin cytoskeleton with raft membrane domains.