T cell receptor and co-stimulatory signaling (Bos taurus)
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Description
The activation and translocation of transcription factors NFAT, AP-1 and NF-kappa-B via the co-stimulatory signaling cascade triggered by MHC peptide, B7 proteins and PD-L1. The activation of NFAT involves a Ca2+/calcineurin disruption of a massive RNA-protein complex prior to its translocation into the nucleus and ultimate transcription factor activity.
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Function is controlled largely by regulation of its surface expression. Initially CTLA-4 is in the intracellular membrane but moves to the cell surface after T-cell receptor signaling. When CTLA-4 cytoplasmic tail is NOT phosphorylated it binds to AP-2 (clathrin adapter molecule) and is removed from the surface. When the tail is phosphorylated AP-2 cannot bind and CTLA-4 is expressed on the surface. CTLA-4 competes with CD28 for B7 ligand, and it has a higher affinity of B7 in part because CTLA-4 binds B7 in a dimer.
CTLA-4 interfers with the formation of lipid rafts, TCR:ZAP70 microclusters, and central supramolecular activation complex.LcK is activated when the extracellular part of CD8 binds its (MHC:peptide) ligand. Lck is a Src family kinase that is constitutively expressed. It phosphorylates all TCR ITAMS.
Rephosphorylation of this carboxyl-terminal tyrosine by Csk returns Lck to the inactive state.
Basically, Lck is bound to CD8. When CD8 binds MHC:peptide, Lck gets activated and can phosphorylate nearby ITAMs.PLC-g is initially brought to the plasma membrane by binding of its PH domain to membrane lipid PIP3. PLC-g then binds to LAT and SLP-76 and can be activated by Itk mediated phosphorylation.
PLC-g ultimately produces 3 different second messangers to activate 3 paths leading to different TFs that lead to IL-2 transcriptionIt docks at the TCR (requires both ITAM positions to be phosphorylated), is then phosphorylated by Lck, and then recruits other signaling proteins.
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