| Description: |
A key event in the regulation of the adaptive immune response is the bindingof major histocompatibility complex (MHC)-peptide complexes to T cellantigen receptors (TCRs). The formation of such ternary complexes inducessignificant biochemical changes within T cells of the host animal. The firstdetectable response of the T cell is the rapid accumulation of numeroustyrosine-phosphorylated proteins within the cell. Increased phosphotyrosineoccurs as a consequence of the activation of several different TCR-associated, hematopoietic-specific protein kinases (PTKs), thereby perturbingthe balance between those enzymes that add, and those that remove,phosphates from key tyrosine residues. These initial phosphorylation eventsare required for the subsequent activation of the small guanosinetriphosphatase (GTPase) proteins Ras and Rac, the lipid kinase P13K and PLC-gamma1. Activation of these cytoplasmic signalling proteins ultimately leadsto activation of various transcription factors (NF-AT, NF-kB, and AP-1) andincreased transcription from genes that have an important role in initiatingT cell proliferation, such as interleukin-2 (IL-2) [].An unresolved question in the field has been which molecules and whatsequence of events tie together the early tyrosine phosphorylation eventswith the activation of these downstream signalling molecules. A likelycandidate for linking the proximal and distal portions of the TCR signallingpathway is a 36kDa transmembrane protein termed LAT. LAT becomesphosphorylated after TCR engagement, thereby creating binding sites for theSrc homology 2 (SH2) domains of other proteins, including PLC-gamma1, Grb2,Gads, Grap, 3BP2 and Shb. It also indirectly binds SOS, c-Cbl, Vav, SLP-76and Itk.LAT is expressed in peripheral blood lymphocytes, thymus and spleen, aswell as in other blood cells, including megakaryocytes, platelets, naturalkiller cells and mast cells. It is excluded from the cytosol, and is foundat the plasma membrane and in the perinuclear compartment. The cellularlocalisation of LAT seems to be extremely sensitive to alternations in theintracellular redox balance. Reduced intracellular levels of antioxidantsresult in the membrane displacement of LAT (a consequence of aconformational change interfering with its insertion into the membrane),abrogation of TCR-mediated signalling and, consequently, hyporesponsivenessof T lymphocytes.The amino acid sequence of LAT contains no recognised functional domains,but, consistent with its strong tyrosine phosphorylation upon TCRstimulation, its predicted cytoplasmic tail contains 10 tyrosines, 9 ofwhich are conserved between mouse, rat and human proteins. In addition,LAT also has 2 cysteine residues (Cys26 and Cys29 in human) that areconserved among human, rat, mouse and bovine proteins. These residues lieproximal to the inner face of the plasma membrane: Cys26 within the TMregion andCys29 located juxtamembrane. These membrane-proximal residuesare thought to play a vital role in LAT function. In fact, LAT is subject topost-translational palmitoylation of these residues, which appears to benecessary to target LAT to lipid rafts in the membrane, where it canrecruit key cytosolic signalling proteins to the aggregated rafts upon TCRstimulation. Raft membrane domains are envisaged as lateral assemblies ofsphingolipids and cholesterol that form ordered membrane phases []. |
