The E3 ubiquitin ligase Itch in T cell activation, differentiation, and tolerance

Abstract

Tagging a small molecule ubiquitin to a protein substrate, or protein ubiquitination, plays an important role in the immune responses. This process is catalyzed by a cascade of enzymatic reactions, with the E3 ubiquitin ligases being the critical enzymes that determine the specificity of substrate recognition. The E3 ligase Itch was identified from a mutant mouse which displays skin scratching and abnormal immune disorders. In the past few years, much progress has been made in our understanding of Itch-promoted protein ubiquitination, modulation of its ligase activity by upstream kinases, and the kinase-ligase interaction in T cell differentiation and tolerance induction.

Figure 1

A schematic representation of Itch and its potential substrates. Itch consists of an N-terminal protein-kinase C-related C2 domain, followed by four protein-interacting WW domains, and a C-terminal HECT E3 ligase domain. Positions for serine/threonine (pS/T) or tyrosine phosphorylation (pY) are also indicated. The HECT ligase domain binds to E2-ubiquitin complex and transfers the activated ubiquitin to an active site cysteine residue at its C-terminus. Itch catalyzes the ubiqutin conjugaton to protein substrates by facilitating the ubiquitin transfer from the HECT domain to the lysine residues of the substrates. Several protein substrates that have been characterized for the WW domain interaction are shown.

Figure 2

Regulation mechanisms for Itch-mediated Th2 differentiation. TCR triggering plus CD28-mediated costimulation results in the activation of MEKK1-JNK signaling pathway, which induces the serine/threonine phosphorylation of Itch. This phosphorylation event causes the increase of Itch ligase activity and the subsequent ubiquitination and degradation of JunB, a transcription factor involved in the production of a Th2 cytokine, IL-4. On the other hand, T cell stimulation also activates the Src kinase Fyn and induces Fyn-mediated tyrosine phosphosphorylation of Itch. Phosphorylation of Itch on the tyrosine residue inhibits the interaction between Itch WW domain and JunB, thus negatively regulating Itch-promoted JunB ubiquitination. Ndfip1 is a membrane-bound protein which may change the subcellular localization of Itch.