Biology of the JAK3 kinase
Janus kinases (JAKs) belong to the family of cytoplasmic tyrosine kinases. In human, the JAK family itself consist of 4 members (JAK1, JAK2, JAK3 and TYK). JAK family members are multi-domain proteins of about 130 kDa, which are highly homologous with respect to their domain structure and residue conservation within their structured domains.
All JAK family members possess a catalytic kinase domain (KD) located at the C-terminus, an adjacent pseudokinase domain (PKD) flanked by a Src homology 2 (SH2) domain. The N-terminal FERM (four-point-one, ezrin, radixin and moesin homology) domain (B41) serves to mediate the interaction between the JAK and the cytokine receptor .
JAK kinases are effectors of the JAK-STAT signaling pathway, which is triggered by ligand binding to a cognate receptor resulting in activation of JAK by phosphorylation of key tyrosine residues within the catalytic domain. After activation, tyrosine resides in the receptor intracellular region are also phosphorylated which triggers recruitment and phosphorylation of the principal downstream effectors, the STATs. Phosphorylated STATs dimerize and translocate to the nucleus where they initiate transcription of specific responsive genes [1, 2, 3].
Contrary to the other JAK family members, JAK3 expression is restricted to the hematopoietic system, where it plays a specific role in the development of immune-competent cells . This key function of JAK3 has been confirmed by loss-of-function mutations of JAK3, which cause severe combined immunodeficiency syndrome (SCID) . Aberrant activation in kinase domain has been described in lymphoproliferative disorders (T-ALL; T-PLL). JAK3 is required for cytokine signalling downstream of receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21 and it acts via activation of the gamma chain receptors (γc). JAK1 and JAK3 co-localise on cytokine receptor dimers suggesting that dual JAK1 and JAK3 inhibition may be required for efficient suppression of cytokine signaling . In order to study JAK3 specific functions we developed in collaboration with the laboratory of Stefan Laufer a JAK3 specific reversible covalent inhibitor.
We recommend using FM-381 at 100-300 nM for specific JAK3 inhibition. The inactive control FM-479 has no activity on JAK3 or other kinases when used at similar concentration.
As a non-covalent control inhibitor we recommend NIBR3049 (Novartis) at 1 µM (this potent reversible compound is selective for JAK3 within the JAK family, but shows also potent inhibition of other kinases such as GSK3, PKCα, PKCθ).