As described in Section 7-10, phosphorylation of the tyrosines in the T-cell receptor ITAMs leads to the recruitment and activation of ZAP-70. This provides ZAP-70 proximity to the cell membrane, where it phosphorylates the scaffold protein LAT (linker for activated T cells), a transmembrane protein with a large cytoplasmic domain (Fig. 7.15). ZAP-70 also phosphorylates another adaptor protein, SLP-76. LAT and SLP-76 can be linked by the adaptor protein Gads; this three-protein complex, referred to as the LAT:Gads:SLP-76 complex, plays a central role in T-cell activation. This is illustrated by the profound TCR signaling and T-cell development defects seen in mice lacking any
MOVIE 7.1
Fig. 7.15 ZAP-70 phosphorylates LAT and SLP-76, initiating four downstream signaling modules. activated Zap-70 phosphorylates the scaffold proteins LaT and SLp-76 and recruits them to the activated T-cell receptor (TcR) complex. an adaptor protein, Gads, holds the tyrosinephosphorylated LaT and SLp-76 together. The multiple binding sites on these scaffold proteins recruit several additional adaptors and enzymes that initiate four essential downstream signaling modules. one key component required for several of these modules is the activation of pi 3-kinase, which phosphorylates pip2 in the plasma membrane to generate pip3. These four modules include the activation of the serine/ threonine kinase akt, which promotes enhanced cellular metabolic activity; the activation of pLc-γ, which leads to transcription factor activation; the activation of vav, which induces actin polymerization and cytoskeletal reorganization; and the recruitment of adap, an adaptor protein that promotes enhanced integrin adhesiveness and clustering.
Akt activation leads to increased cellular metabolic activity PIP3AktPHdomainADAP recruitment leads to enhanced integrin adhesiveness and clustering ADAPVav activation leads to actin polymerization and cytoskeletal reorganization PLC-˜activation leads to transcription factor activation PLC-˜GadsVavZAP-70LATSLP-76Activated ZAP-70 phosphorylates LAT and SLP-76, initiating four signaling modules
Fig. 7.16 The recruitment of phospholipase C-γ by LAT and SLP-76, and its phosphorylation and activation by the protein kinase Itk, are crucial steps in T-cell activation. Zap-70 phosphorylates the scaffold proteins LaT and SLp-76, which are brought together to form a complex at the activated T-cell receptor by the adaptor protein Gads. This complex also promotes the activation of pi 3-kinase, leading to the production of pip3 (formed by the phosphorylation of pip2 by pi 3-kinase). phospholipase c-γ (pLc-γ) is recruited to the membrane by its ph domain binding to pip3 and then binds to phosphorylated sites in LaT and to the proline-rich domain of SLp-76. To be activated, pLc-γ must be phosphorylated by the Tec-family kinase itk. itk is recruited to the membrane by interaction of its ph domain with pip3, and by interactions with phosphorylated SLp-76. once phosphorylated by itk, phospholipase c-γ is active.