Downstream signalling

• Interaction of ternary complex with CD3
• Downstream effects of this interaction involving ZAP-70

Ternary complex-CD3 interactions

The shape of the TCR-MHC-CD4 complex restricts the downstream interactions possible for the complex. As there is a large gap, of roughly 70 angstroms, between the TCR and CD4 D4 domain in the membrane this allows enough space for the CD3 epsilon gamma, epsilon delta, and zeta zeta subunits to be inserted. These domains then cause activation of the T cell via ITAMs. Mutation studies of the TCR and CD3 have determined the their mode of interaction. It has been shown that two of the CD3 subunits are found underneath the ternary complex arch. When the ternary complex is formed this causes the displacement of these CD3 subunits from the membrane. This subsequently causes exposure of CD3 ITAMs, which can now interact with Src kinases This reaction is important for downstream signalling.


The structure of the ternary complex restricts the orientation of CD3 relative to Lck. Some of the evidence for the ternary complex structure comes from the fact that if the docking polarity of TCR-pMHC was reversed then Lck would no longer be able to phosphorylate CD3 ITAMs. Lck is found bound to CD4, which enforces this view. Although the binding complex is rigid there must be some flexibility to permit variation in the TCR-pMHC docking geometry. It has been postulated that this flexibility is conferred by the residues causing the interaction between CD3 and CD4/Lck.


Downstream phosphorylation pathway
 

Figure 7: Cartoon representation of the activation of Zap-70. It is slightly incorrect as the CD3 subunits should be found underneath the TCR-pMHC-CD4 arch. (Au-Yeung, 2009)

The structure, regulation, and function of ZAP-70


Signalling downstream of the TCR-pMHC-CD4 interaction is mostly by protein phosphorylation meaning that many different protein tyrosine kinases and phosphatases are used. The initial kinase used is known as Lck and this phosphorylates the ITAMs found on CD3 zeta subunits. This phosphorylation then allows the binding of the zeta-associated protein of 70 kDa (ZAP-70) shown in Fig. 7. This causes the ZAP-70 to be localised to a stimulated TCR and also causes a conformational change in the ZAP-70 from an autoinhibited form. For ZAP-70 to become totally active it is further phosphorylated by Lck or by trans-autophosphorylation of the kinase domain. ZAP-70 then phosphorylates two adaptor proteins, LAT and SLP-76, which can then recruit many other signalling molecules causing various different downstream effects. These can lead to transcriptional changes or secretory responses. For further information of the effects of ZAP-70 follow the link above.

Obviously diseases caused by problems with proteins in the downstream phosphorylation reactions can have a catastrophic effect. A ZAP-70 deficiency in humans and mice results in severe combined immuno-deficiency (SCID). This disorder is due to the inability to produce functional T cells and results in severely immunodeficient individuals.