Lipid rafts: cell surface platforms for T cell signaling
The Src family tyrosine kinase Lck is essential for T cell development and T cell receptor (TCR)* signaling. Lck is post-translationally fatty acylated at its N-terminus conferring membrane targeting and concentration in plasma membrane lipid rafts, which are lipid-based organisational platforms. Confocal fluorescence microscopy shows that Lck colocalises in rafts with GPI-linked proteins, the adaptor protein LAT and Ras, but not with non-raft membrane proteins including the protein tyrosine phosphatase CD45. The TCR also associates with lipid rafts and its cross-linking causes coaggregation of raft-associated proteins including Lck, but not of CD45. Cross-linking of either the TCR or rafts strongly induces specific tyrosine phosphorylation of the TCR in the rafts. Remarkably, raft patching alone induces signalling events analogous to TCR stimulation, with the same dependence on expression of key TCR signalling molecules. Our results indicate a mechanism whereby TCR engagement promotes aggregation of lipid rafts, which facilitates colocalisation of signaling proteins including Lck, LAT, and the TCR, while excluding CD45, thereby potentiating protein tyrosine phosphorylation and downstream signaling. We are currently testing this hypothesis as well as using imaging techniques such as fluorescence resonance energy transfer (FRET) microscopy to study the dynamics of proteins and lipids in lipid rafts in living cells undergoing signaling events. Recent data show that the key phosphoinositide PI(4,5)P2 is concentrated in T cell lipid rafts and that on stimulation of the cells it is rapidly converted to PI(3,4,5)P3 and diacylglycerol within rafts. Thus rafts are hotspots for both protein and lipid signalling pathways.