Interleukin-2 Tyrosine Kinase (PDB ID: 2ETZ)
Created by Dean Smith
The interleukin-2 tyrosine kinase, or ITK, is a member of the Tec family of non-receptor tyrosine kinases, and is essential for signaling through T-cell antigen receptors (TCRs) (1). ITK is an unbound protein that drifts through the cytoplasm of T-cells until it becomes associated with a membrane-bound receptor protein that has become tyrosine-phosphorylated (2). The ITK-SH2 domain allows for ITK association with membrane-bound proteins (5). SH2 domains are non-catalytic domains that bind to phosphotyrosine containing proteins contained within TCRs (4). Basically, the ITK-SH2 domain is essential for T-cell growth regulation, as it is the domain that binds to activated receptor proteins in the cell membrane. Initially, T-cell antigen receptors recognize and bind to antigens. The activated T-cell antigen receptors are subsequently tyrosine-phosphorylated. The ITK protein binds to the tyrosine-phophorylated portion of the receptor protein through the SH2-domain. This binding leads to unfolding of the kinase domain of ITK, subsequently leading to protein activation and T-cell growth regulation (2,4).
Binding to TCRs is mediated by a functionally important lysine residue at 67, which ligates with an acetyl group, becoming N-6 acetyllysine, causing subsequent association with the phosphotyrosine of a TCR (6,8). The SH3 and kinase domains of ITK become tyrosine-phosphorylated rapidly after binding (2). Acetyl group ligation occurs in one other place in this domain: an asparagine at 5.
The SH2 domain also contains a functionally important residue that mediates protein activity. A proline at 42 is capable of isomerization between the cis conformer ( pdb id = 1luk) to the trans conformer ( pdb id = 1lun) (7). The trans SH2 conformer mediates phospholigand binding, which, as stated before, is linked to activation of the kinase domain. The cis SH2 conformer leads to binding with the ITK SH3 domain, which causes ITK to self-associate ( pdb id = 2k79) and likely down-regulates kinase activity (6). The method in which the cis conformer regulates SH3 binding is in the fact that the Pro42 isomerization causes the protein to fold in such a way that the proline-rich region that flanks the SH3 domain is positioned next to the phosphotyrosine in the SH3 region (1,5). The two regions interact with each other and cause intramolecular SH3 association, decreasing ITK activity (1,7). This isomerization of Pro42 between cis and trans is not forced in any way, as the proline is located on a loop and not constrained by a rigid secondary structure, and both conformations are populated in solution (5).
Mutations in the protein structure of ITK, particularly the SH2 domain, have been linked to several health issues, including severe immune dysfunction and therapy-resistant Epstein-Barr virus (EBV)-positive B cell proliferation (4). By understanding the relationship between structure and function in this protein, inhibitors and drugs can be developed to curve the ill-effects of mutant proteins (3). The selective inhibition of ITK through inhibitors such as staurosporine can modulate these diseases, especially in cases where undesired activation of the immune system is involved (3).
The significance of the SH2 family of domains can be seen in other protein signaling pathways. In the epithelial and endothelial tyrosine kinase (Etk/Bmx) protein found in humans, a similar SH2 domain (pbd id: 2ekx) can be found. Dali results show a Z score of 10.5 and an rmsd of 2.4, meaning 2ekx has a very similar tertiary structure to 2etz (9). Etk proteins were first identified in prostate cancer cells and discovered to be involved in interleukin 6 signaling in prostate cancer cell lines (10). Interleukin 6 signaling is a very similar type of signaling to that of ITK. In its native “closed” form, the Etk/Bmx kinase is inactive. When the SH2 domain interacts with signaling molecules bearing phosphotyrosine fragments, the intermolecular interactions within the protein become disrupted, resulting in unfolding of the kinase domain and subsequent activation (10). Essentially, both ITK and Etk/Bmx are involved in similar growth modulating, protein-protein signaling pathways triggered by SH2 domain interactions. In this sense, it can be understood that a protein’s overall, as well as domain, structure will dictate its function.