Downstream TCR signaling

Downstream TCR signaling Changes in gene expression are required for the T cell to gain full proliferative competence and to produce effector cytokines. Three transcription factors in particular have been found to play a key role in TCR-stimulated changes in gene expression, namely NFkappaB, NFAT and AP-1. A key step in NFkappaB activation is the stimulation and translocation of PRKCQ. The critical element that effects PRKCQ activation is PI3K. PI3K translocates to the plasma membrane by interacting with phospho-tyrosines on CD28 via its two SH2 domains located in p85 subunit (step 24). The p110 subunit of PI3K phosphorylates the inositol ring of PIP2 to generate PIP3 (steps 25). The reverse dephosphorylation process from PIP3 to PIP2 is catalysed by PTEN (step 27). PIP3 may also be dephosphorylated by the phosphatase SHIP to generate PI-3,4-P2 (step 26). PIP3 and PI-3,4-P2 acts as binding sites to the PH domain of PDK1 (step 28) and AKT (step 29). PKB is activated in response to PI3K stimulation by PDK1 (step 30). PDK1 has an essential role in regulating the activation of PRKCQ and recruitment of CBM complex to the immune synapse. PRKCQ is a member of novel class (DAG dependent, Ca++ independent) of PKC and the only member known to translocate to this synapse. Prior to TCR stimulation PRKCQ exists in an inactive closed conformation. TCR signals stimulate PRKCQ (step 31) and release DAG molecules. Subsequently, DAG binds to PRKCQ via the C1 domain and undergoes phosphorylation on tyrosine 90 by LCK to attain an open conformation (step 32). PRKCQ is further phosphorylated by PDK1 on threonine 538 (step 33). This step is critical for PKC activity. CARMA1 translocates to the plasma membrane following the interaction of its SH3 domain with the 'PxxP' motif on PDK1 (step 34). CARMA1 is phosphorylated by PKC-theta on residue S552 (step 35), leading to the oligomerization of CARMA1. This complex acts as a scaffold, recruiting BCL10 to the synapse by interacting with their CARD domains (step 36). BCL10 undergoes phosphorylation mediated by the enzyme RIP2 (step 37). Activated BCL10 then mediates the ubiquitination of IKBKG by recruiting MALT1 and TRAF6. MALT1 binds to BCL10 with its Ig-like domains and undergoes oligomerization (step 38). TRAF6 binds to the oligomerized MALT1 and also undergoes oligomerization (step 39). Oligomerized TRAF6 acts as a ubiquitin-protein ligase, catalyzing auto-K63-linked polyubiquitination (step 40). This K-63 ubiquitinated TRAF6 activates MAP3K7 kinase bound to TAB2 (step 41) and also ubiquitinates IKBKG in the IKK complex (step 44). MAP3K7 undergoes autophosphorylation on residues T184 and T187 and gets activated (step 42). Activated MAP3K7 kinase phosphorylates IKBKB on residues S177 and S181 in the activation loop and activates the IKK kinase activity (step 43). IKBKB phosphorylates the NFKBIA bound to the NFkappaB heterodimer, on residues S19 and S23 (step 45) and directs NFKBIA to 26S proteasome degradation (step 47). The NFkappaB heterodimer with a free NTS sequence finally migrates to the nucleus to regulate gene transcription (step 46).