Featured Article - September 2012
Short description: Structures of the JH2 pseudokinase domain provide clues to its role in normal and disease-associated JAK signaling.
Human JAK2 tyrosine kinase associates with the cytoplasmic domain of cytokine receptors and is activated by receptor dimerization or rearrangement that is induced by cytokine binding. JAK2 activation is important for the initial innate immune response, as well as for myeloid cell development, proliferation and survival.
JAK2 is comprised of a FERM (band 4.1, ezrin, radixin, moesin) domain, important for association with cytokine receptors, an Src homology-2 (SH2) domain, a pseudokinase domain (JH2) and a C-terminal tyrosine kinase domain (JH1). JH2 is known to regulate the activity of JH1, but how it does so is unclear. Mutations to the JAK genes cause bone marrow diseases called myeloproliferative neoplasms (MPNs), and many of these mutations have been mapped to the JH2 region of JAK2 and result in constitutive JAK2 tyrosine kinase activity, suggesting an inhibitory function for JH2. However, some mutations in JH2 of JAK3 lead to loss of function, suggesting a positive regulatory role for this domain in JAK activity.
Recent work has shown that, surprisingly, JH2 is an active kinase domain that phosphorylates two sites on JH1 that negatively regulate its activity. To gain further insight into the activity of JH2, Silvennoinen, Hubbard and colleagues have determined the structures of wild type JAK2 JH2 (PDB 4FVP)and the V617F mutation (PDB 4FVR), the most commonly identified MPN mutation, in apo and Mg-ATP-bound forms.
The structures show that JH2 adopts the prototypical protein kinase fold. Similar to other kinases, Mg-ATP binds in the cleft formed between the N- and C-terminal lobes. However, the authors note a non-canonical mode of Mg-ATP binding that explains its lower rate of basal activity compared to JH1. Of greater interest is the effect the V617F mutation has on the structure of the JH2 domain. Val617 is located in a loop in the N lobe. While the V617F mutation does not affect nucleotide binding, it does result in rigidification of the N lobe αC helix that simulations suggest would facilitate activation of the JH1 domain.
While the structures don't fully delineate the mechanism by which JH2 regulates JAK signaling, they provide a starting point for further investigations. As the JH2 domain is a mutation hot spot, it may also provide an alternative target for small molecule therapeutics.
R.M. Bandaranayake et al. Crystal structures of the JAK2 pseudokinase domain and the pathogenic mutant of V617F.
Nat Struct Mol Biol. 19, 754-759 (2012). doi:10.1038/nsmb.2348