Figure 1. Molecular interactions leading to T-cell receptor (TCR)–induced actin polymerization. T-cell activation is initiated by antigen presenting cells (APCs) containing stimulatory major histocompatibility complex (MHC)–peptide complexes. Phosphorylation of the TCR (black circles) is mediated by Src family protein tyrosine kinases. ZAP-70 is recruited to the phosphorylated TCR subunits through its SH2 domain. It is phosphorylated and activated by the Src family protein tyrosine kinases. The linker for the activation of T cells (LAT) is tyrosine phosphorylated by ZAP-70. LAT contains nine tyrosine residues that, when phosphorylated, act as docking sites for adapter proteins such as Grb2 and Gads. SLP-76 is recruited by the LAT-nucleated complex through its interaction with the SH3 domains of Gads. SLP-76 associates with the SH2 domain of Nck. Nck binds WASp, which in turn binds the Arp2/3 complex that mediates actin polymerization.