one of these components, and in humans lacking ZAP-70. A second essential event that occurs rapidly following ZAP-70 activation is the recruitment and activation of the enzyme PI 3-kinase (see Section 7.4); while the detailed mechanism linking PI 3-kinase activation to T-cell receptor stimulation is not well understood, current evidence suggests a role for the small GTPase Ras. In this case, Ras may be activated by recruitment of the Ras-GEF Sos to LAT via Sos binding to the small adaptor protein Grb2, forming a second three-protein complex containing LAT and Sos, bridged by Grb2.
Following formation of the LAT:Gads:SLP-76 complex and the activation of PI 3-kinase, the T-cell receptor signaling pathway branches into several downstream modules, each of which induces cellular changes that contribute to optimal T-cell activation (see Figure 7.15). Each module is initiated by the recruitment of a key intermediate to the active signaling complexes, either via binding to the LAT:Gads:SLP-76 complex, to the PIP3 generated by PI 3-kinase, or to both. In brief, these modules lead to the activation of phospholipase C-γ (PLC-γ), which affects transcription; the activation of the serine/threonine kinase Akt, which affects metabolism, among other things; the recruitment of the adaptor protein ADAP, which upregulates cell adhesion; and the activation of the protein Vav, which initiates actin polymerization. Each of these modules will be described in detail in the sections below.
7-13 Activated PLC-γ generates the second messengers diacylglycerol and inositol trisphosphate that lead to transcription factor activation.
One important module of T-cell receptor signaling is the activation of the enzyme phospholipase C-γ (PLC-γ). First, PLC-γ is brought to the inner face of the plasma membrane by the binding of its PH domain to the PIP3 that has been formed by the phosphorylation of PIP2 by PI 3-kinase; it then binds to phosphorylated LAT and SLP-76. The actions of PLC-γ produce two second messengers that activate three distinct terminal branches of the T-cell receptor pathway leading to transcription factor activation.
Due to this crucial role in T-cell activation, PLC-γ activation is controlled at several different levels. Recruitment to the membrane, while necessary, is not sufficient to activate PLC-γ. PLC-γ activation requires phosphorylation by Itk, a member of the Tec family of cytoplasmic tyrosine kinases. Like PLC-γ, Tec kinases contain PH, SH2, and SH3 domains and are recruited to the plasma membrane by interactions via these domains; specifically, the PH domain interacts with PIP3 on the inner face of the membrane (Fig. 7.16), and the SH2 and SH3 domains interact with SLP-76. These interactions serve to localize Itk in close proximity to its substrate, PLC-γ.