Analysis of steady-state Förster resonance energy transfer data by avoiding pitfalls: interaction of JAK2 tyrosine kinase with N-methylanthraniloyl nucleotides

A versatile strategy to synthesize N-methyl-anthranilic acid-labelled glycoprobes for fluorescence-based screening assays

Here we propose a general strategy to label carbohydrates with N-methyl-anthranilic acid to generate glycotools for fluorescence-based screening and carbohydrate–protein interaction studies.

Hyperactivation of Oncogenic JAK3 Mutants Depend on ATP Binding to the Pseudokinase Domain

Janus kinase 3 (JAK3) tyrosine kinase has a central role in the control of lymphopoiesis, and mutations in JAK3 can lead to either severe combined immunodeficiency or leukemia and lymphomas. JAK3 associates with the common gamma chain (γc) receptor and functions in a heteromeric signaling pair with JAK1. In IL-2 signaling JAK1 is the effector kinase for STAT5 phosphorylation but the precise molecular regulatory mechanisms of JAK1 and JAK3 and their individual domains are not known. The pseudokinase domain (JAK homology 2, JH2) of JAK3 is of particular interest as approximately half of clinical JAK3 mutations cluster into it.

In this study, we investigated the role of JH2s of JAK1 and JAK3 in IL-2R signaling and show that STAT5 activation requires both JH1 and JH2 of JAK1, while both JH1 and JH2 in JAK3 are specifically required for the cytokine-induction of cellular signaling. Characterization of recombinant JAK3 JH2 in thermal shift assay shows an unstable protein domain, which is strongly stabilized by ATP binding. Unexpectedly, nucleotide binding to JAK3 JH2 was found to be cation-independent. JAK3 JH2 showed higher nucleotide binding affinity in MANT-ATP and fluorescent polarization competition assays compared to the other JAK JH2s. Analysis of the functional role of ATP binding in JAK3 JH2 in cells and in zebrafish showed that disruption of ATP binding suppresses ligand-independent activation of clinical JAK3 gain-of-function mutations residing in either JH2 or JH1 but does not inhibit constitutive activation of oncogenic JAK1. ATP-binding site mutations in JAK3 JH2 do not, however, abrogate normal IL-2 signaling making them distinct from JH2 deletion or kinase-deficient JAK3. These findings underline the importance of JAK3 JH2 for cellular signaling in both ligand-dependent and in gain-of-function mutation-induced activation. Furthermore, they identify the JH2 ATP-binding site as a key regulatory region for oncogenic JAK3 signaling, and thus a potential target for therapeutic modulation.

Hyperinsulinism-Causing Mutations Cause Multiple Molecular Defects in SUR1 NBD1

The sulfonylurea receptor 1 (SUR1) protein forms the regulatory subunit in ATP sensitive K⁺ (K_ATP) channels in the pancreas. SUR proteins are members of the ATP binding cassette (ABC) superfamily of proteins. Binding and hydrolysis of MgATP at the SUR nucleotide binding domains (NBDs) lead to channel opening. Pancreatic K_ATP channels play an important role in insulin secretion. SUR1 mutations that result in increased levels of channel opening ultimately inhibit insulin secretion and lead to neonatal diabetes. In contrast, SUR1 mutations that disrupt trafficking and/or decrease gating of K_ATP channels cause congenital hyperinsulinism, where oversecretion of insulin occurs even in the presence of low glucose levels. Here, we present data on the effects of specific congenital hyperinsulinism-causing mutations (G716V, R842G, and K890T) located in different regions of the first nucleotide binding domain (NBD1). Nuclear magnetic resonance (NMR) and fluorescence data indicate that the K890T mutation affects residues throughout NBD1, including residues that bind MgATP, NBD2, and coupling helices. The mutations also decrease the MgATP binding affinity of NBD1. Size exclusion and NMR data indicate that the G716V and R842G mutations cause aggregation of NBD1 in vitro, possibly because of destabilization of the domain. These data describe structural characterization of SUR1 NBD1 and shed light on the underlying molecular basis of mutations that cause congenital hyperinsulinism.

ATP binding to the pseudokinase domain of JAK2 is critical for pathogenic activation

Pseudokinases lack conserved motifs typically required for kinase activity. Nearly half of pseudokinases bind ATP, but only few retain phosphotransfer activity, leaving the functional role of nucleotide binding in most cases unknown. Janus kinases (JAKs) are nonreceptor tyrosine kinases with a tandem pseudokinase-kinase domain configuration, where the pseudokinase domain (JAK homology 2, JH2) has important regulatory functions and harbors mutations underlying hematological and immunological diseases. JH2 of JAK1, JAK2, and TYK2 all bind ATP, but the significance of this is unclear. We characterize the role of nucleotide binding in normal and pathogenic JAK signaling using comprehensive structure-based mutagenesis. Disruption of JH2 ATP binding in wild-type JAK2 has only minor effects, and in the presence of type I cytokine receptors, the mutations do not affect JAK2 activation. However, JH2 mutants devoid of ATP binding ameliorate the hyperactivation of JAK2 V617F. Disrupting ATP binding in JH2 also inhibits the hyperactivity of other pathogenic JAK2 mutants, as well as of JAK1 V658F, and prevents induction of erythrocytosis in a JAK2 V617F myeloproliferative neoplasm mouse model. Molecular dynamic simulations and thermal-shift analysis indicate that ATP binding stabilizes JH2, with a pronounced effect on the C helix region, which plays a critical role in pathogenic activation of JAK2. Taken together, our results suggest that ATP binding to JH2 serves a structural role in JAKs, which is required for aberrant activity of pathogenic JAK mutants. The inhibitory effect of abrogating JH2 ATP binding in pathogenic JAK mutants may warrant novel therapeutic approaches.