| Antigen engagement by the T cell receptor (TCR) leads to a complex array of biochemical changes within the cell, culminating in transcriptional activation and proliferation of the responding T cell. Signals through the TCR are required for T cell development and function, and thus a dissection of the molecules which participate in receptor signaling is essential for a complete understanding of T cell-mediated immunity.; The tyrosine phosphorylation of cellular substrates is one of the earliest discernable biochemical events following TCR ligation. Two families of protein tyrosine kinases (PTKs) facilitate receptor-driven phosphorylation, and hence are required for TCR function. The Src family PTKs mediate phosphorylation of receptor-associated subunits, providing docking sites for the recruitment of the Syk family PTKs, ZAP-70 and Syk. The activation of these kinases, in turn, regulates multiple distinct signaling pathways, permitting elevated concentrations of cytoplasmic calcium, upregulation of mitogen activated protein kinases cascades, and cytoarchitectural changes.; This thesis identifies the SLP-76 protein as an in-vivo substrate of ZAP-70. The phosphorylation of SLP-76 by ZAP-70 is demonstrated to positively regulate cytokine synthesis. In addition, the SH2 domain of SLP-76 is likewise required for cytokine synthesis, suggesting the importance of SLP-76 as a linker molecule which bridges receptor-activated kinases with the downstream signaling machinery.; Further defining the mechanism of SLP-76 function as a substrate of ZAP-70, this thesis describes the association of tyrosine phosphorylated SLP-76 with both the nck adapter protein and Vav, a guanine nucleotide exchange factor for the Rho family GTPases. This function of SLP-76 as a scaffold is required for the enhanced catalytic function of the nck-associated p21 activated kinase, Pak1. Similarly, the interaction of SLP-76 with both Vav and nck facilitates reorganization of the actin cytoskeleton. As the enzymatic activation of Pak1 and cytoskeletal reorganization are dependent on Rho family GTPases, the assembly of this trimolecular complex permits the colocalization of Vav enzymatic function with target effector molecules that interact through nck. This study therefore highlights the pivotal role of SLP-76 as a linker molecule which integrates distinct signaling complexes to regulate T cell function. |
