sters (Tseng et al., 2008). Blocking TCR signaling with anti-MHC-peptide antibodies rapidly eliminated the bright CD28-CD80 foci, consistent with a continual role for TCR signaling in maintaining this component of the cSMAC. These studies demonstrate that the cSMAC consists of two compartments: one a stable structure enriched in non-signaling TCR destined for shedding or degradation (a) and one highly dynamic structure rich in F-actin, CD28 and PKC-θ (b) (table 1). These compartments are difficult to resolve in cSMACs formedin cell-cell interfaces. The mechanisms by which CD28 and TCR are segregated into these two cSMAC compartments is not known.
The mechanism of TCR triggering has been an enduring problem. The prevailing model has been based on TCR dimerization or clustering as a triggering modality (Germain, 1997; Weiss and Littman, 1994). This is largely based on models from receptor tyrosine kinases and observations that antibody crosslinking triggers signaling. However, the is an adapter that recruits non-receptor tyrosine kinases to 10 docking sites, the immunotyrosine based activation motifs (ITAMs), in response to ligand binding. With all these docking sites, it is not clear that dimerizing this receptor would increase its activity. Recent studies have provided indirect evidence that monovalent ligands can trigger TCR microclusters and signaling events without directly cross-linking the receptor to form dimers e.g.(Varma et al., 2006). It is interesting at this point to think about how the TCR would sense such a monovalent engagement event? Recent data from the BCR and TCR systems provide insights into accessibility changes in cytoplasmic domains of the receptor during triggering and provide raw material for a draft model that incorporates an early role for F-actin.
The BCR has two signal transduction subunits with one ITAM in each. BCR with a FRET donor at the C-terminus of transmembrane immunoglobulin (sIg) and a FRET acceptor on the C-terminus of Igα have a high FRET signal in the basal, non-signaling 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 recepto