Fig. 7.13 Structure of the autoinhibited ZAP-70 kinase. The structure of the inactive autoinhibited conformation of Zap-70 is shown with the protein domains color-coded according to the domain map shown at the bottom; the dashed red line indicates a region of the protein that was not detected in the structural analysis. prior to T-cell receptor stimulation, the Zap-70 kinase is in this inactive conformation, based on interactions between the tandem Sh2 domain-kinase domain linker region (thick red line) and the kinase domain. This interaction stabilizes Zap-70 in a catalytically inactive state, referred to as the autoinhibited form of Zap-70, by locking the kinase domain in an inactive conformation. following T-cell receptor stimulation, Lck phosphorylates two tyrosine residues in this linker region, y315 and y319, shown in yellow. Lck also phosphorylates a tyrosine residue in the catalytic (kinase) domain. When y315 and y319 are phosphorylated, the linker region no longer binds to the kinase domain, allowing the phosphorylated kinase domain to adopt an active conformation. courtesy of arthur Weiss.
Fc˜RIII (CD16) Fc˜RIV Fc°RI ˜˛˜˝˙NKG2C, D, E (CD94) DAP12or Receptors other than antigen receptors also associate with ITAM-containing chains that deliver activating signals NK cells Macrophages Neutrophils NK cells Mast cells Basophils 7-10 Phosphorylated ITAMs recruit and activate the tyrosine kinase ZAP-70.
The precise spacing of the two YXXL/I motifs in an ITAM suggests that the ITAM is a binding site for a signaling protein with two SH2 domains. In the case of the T-cell receptor, this protein is the tyrosine kinase ZAP-70 (ζ-chainassociated protein), which carries the activation signal onward. ZAP-70 has two tandem SH2 domains that can be simultaneously engaged by the two phosphorylated tyrosines in an ITAM (see Fig. 7.9). The affinity of the phosphorylated YXXL sequence for a single SH2 domain is low; binding of both SH2 domains to the ITAM is significantly stronger and confers specificity on ZAP-70 binding. Thus, when Lck has sufficiently phosphorylated an ITAM in the T-cell receptor, ZAP-70 binds to it. Once bound, ZAP-70 is phosphorylated by Lck at three tyrosine residues, two in the linker region between the tandem SH2 domains and the kinase domain, and a third residue in the catalytic domain. Together these phosphorylations activate ZAP-70 by disrupting the autoinhibited form of inactive ZAP-70, allowing the ZAP-70 kinase domain to adopt the active conformation (Fig. 7.13). ZAP-70 can also be activated by autophosphorylation.
7-11 ITAMs are also found in other receptors on leukocytes that signal for cell activation.