state. Remarkably, triggering results in a rapid decrease in the FRET signal, suggesting an “opening” of the complex (Tolar et al., 2005). It is not known what conformational change causes this decreased FRET. One interpretation is based on the model that the ITAMs may interact with the membrane prior to triggering, making the tyrosine inaccessible to kinases and preventing signaling (Aivazian and Stern, 2000). The interaction with the membrane would confine the FRET acceptor associated with the C-terminus of the ITAM bearing subunit to the membrane in proximity to the FRET donor that is kept close to the membrane by the short sIg cytoplasmic domain. Triggering would then involve dissociation of the ITAM from the membrane. A recent study using live cell FRET imaging and structure determination supports the idea that ITAM displacement from the membrane is a critical step in triggering (Xu et al., 2008). This study found that each ITAM in the TCR has a positively charged motif on its N-terminal side that interacts with acidic phospholipids in the inner leaflet and facilitates the docking of both tyrosines in the lipid bilayer where they cannot be phosphorylated. The results suggest that a change in the lipid environment to a more neutral composition is needed for ITAM access by Src family kinases (Douglass and Vale, 2005). How could a single MHC-peptide complex change the lipid environment of the TCR?
Our understanding of the basal distribution of the TCR is constrained by two seemingly contradictory results: evidence for preclustering from electron microscopy (Schamel et al., 2005) and non-correlated monomeric diffusion using fluorescence correlation spectroscopy (James et al., 2007). Long term tracking of Fce receptor movements in the context of actin bundles on rat basophilic leukemia cells shows that receptors can be corralled by actin and diffuse independently at the same time (Andrews et al., 2008). These results suggest that the clusters seen in EM maybe a result of transient confinement on monomeric receptors. Basal fluctuations in TCR density were observed as highly dynamic structures similar in size to microclusters, but which could not be tracked from one frame to the next (Varma et al., 2006). In order to maintain low basal activity the membrane domains in which the TCR moves prior to ligand binding will likely be highly acidic and fluid.
The lamellipodium is a relatively flat membrane region with a dense mesh of branched actin filaments translocating away from the leading edge (Svitkina et al., 1997). The mesh size of the actin gel if relatively fine, with interstitial areas on the order of 100 nm based on electron microscopy analysis of model lamellipodia (Svitkina et al., 1997). This F-actin mesh is similar in scale to the lipids domains that have been identified in membranes as lipid rafts (Sharma et al., 2004). The spatial distribution of acidic lipids like PS on the inner leaflet is not known, but structural constrains suggest that liquid ordered domains should be enriched in neutral phosphatidylethanolamine (Brown and London, 2000). I would speculate that TCR maybe confined to approximately 10% of the plasma membrane composed of disordered, fluid phase and acidic lipids, which are corralled by the mobile actin elements. In the basal state the TCR cannot be individually transported into less acidic liquid ordered domains (Fig 2). Clustering of the receptors by serial ligand binding can stabilize the interaction with the F-actin meshwork and create a sufficient force to overcome resistance to entry into less acidic domains, in which the ITAMs are exposed in the presence of Src family kinases and with depletion of CD45 together lead to triggering of the signaling cascade (Fig 2). It is likely the the “lipid rafts” also anchored to disctinct cortical actin networks and this actin may anchor these domains against movement by the dendritic actin network inivolved in the retrograde flow (Chichili and Rodgers, 2007). This model invokes shearing action of the lamellipodial actin to generate force to overcome ener