Peptide-based NTA(Ni)-nanodiscs for studying membrane enhanced FGFR1 kinase activities

Tyrosine autophosphorylation plays a crucial regulatory role in the kinase activities of fibroblast growth factor receptors (FGFRs), and in the recruitment and activation of downstream intracellular signaling pathways. Biophysical and biochemical investigations of FGFR kinase domains in membrane environments offer key insights into phosphorylation mechanisms. Hence, we constructed nickel chelating nanodiscs based on a 22-residue peptide. The spontaneous anchoring of N-terminal His6-tagged FGFR1c kinase domain (FGFR1K) onto peptide nanodiscs grants FGFR1K orientations occurring on native plasma membranes. Following membrane incorporation, the autophosphorylation of FGFR1K, as exemplified by Y653 and Y654 in the A-loop and the total tyrosine phosphorylation, increase significantly. This in vitro reconstitution system may be applicable to studies of other membrane associated phenomena.

Four tyrosine residues of the rice immune receptor XA21 are not required for interaction with the co-receptor OsSERK2 or resistance to Xanthomonas oryzae pv. oryzae

Tyrosine phosphorylation has emerged as an important regulator of plasma membrane-localized immune receptors activity. Here, we investigate the role of tyrosine phosphorylation in the regulation of rice XANTHOMONAS RESISTANCE 21 (XA21)-mediated immunity. We demonstrate that the juxtamembrane and kinase domain of Escherichia coli–expressed XA21 (XA21JK) autophosphorylates on tyrosine residues. Directed mutagenesis of four out of the nine tyrosine residues in XA21JK reduced autophosphorylation. These sites include Tyr698 in the juxtamembrane domain, and Tyr786, Tyr907, and Tyr909 in the kinase domain. Rice plants expressing XA21-GFP fusion proteins or proteins with these tyrosine residues individually mutated to phenylalanine (XA21YF-GFP), which prevents phosphorylation at these sites, maintain resistance to Xanthomonas oryzae pv. oryzae. In contrast, plants expressing phosphomimetic XA21 variants with tyrosine mutated to aspartate (XA21YD-GFP) were susceptible. In vitro purified XA21JKY698F, XA21JKY907F, and XA21JKY909F variants are catalytically active, whereas activity was not detected in XA21JKY768F and the four XA21JKYD variants. We previously demonstrated that interaction of XA21 with the co-receptor OsSERK2 is critical for biological function. Four of the XA21JKYF variants maintain interaction with OsSERK2 as well as the XA21 binding (XB) proteins XB3 and XB15 in yeast, suggesting that these four tyrosine residues are not required for their interaction. Taken together, these results suggest that XA21 is capable of tyrosine autophosphorylation, but the identified tyrosine residues are not required for activation of XA21-mediated immunity or interaction with predicted XA21 signaling proteins.

Fibroblast Growth Factor Receptor 1 (FGFR1) Tyrosine Phosphorylation Regulates Binding of FGFR Substrate 2α (FRS2α) But Not FRS2β to the Receptor

Binding of the fibroblast growth factor (FGF) to the FGF receptor (FGFR) tyrosine kinase leads to receptor tyrosine autophosphorylation as well as phosphorylation of multiple downstream signaling molecules that are recruited to the receptor either by direct binding or through adaptor proteins. The FGFR substrate 2 (FRS2) family consists of two members, FRS2α and FRS2β, and has been shown to recruit multiple signaling molecules, including Grb2 and Shp2, to FGFR1. To better understand how FRS2 interacted with FGFR1, in vivo binding assays with coexpressed FGFR1 and FRS2 recombinant proteins in mammalian cells were carried out. The results showed that the interaction of full-length FRS2α, but not FRS2β, with FGFR1 was enhanced by activation of the receptor kinase. The truncated FRS2α mutant that was comprised only of the phosphotyrosine-binding domain (PTB) bound FGFR1 constitutively, suggesting that the C-terminal sequence downstream the PTB domain inhibited the PTB-FGFR1 binding. Inactivation of the FGFR1 kinase and substitutions of tyrosine phosphorylation sites of FGFR1, but not FRS2α, reduced binding of FGFR1 with FRS2α. The results suggest that although the tyrosine autophosphorylation sites of FGFR1 did not constitute the binding sites for FRS2α, phosphorylation of these residues was essential for optimal interaction with FRS2α. In addition, it was demonstrated that the Grb2-binding sites of FRS2α are essential for mediating signals of FGFR1 to activate the FiRE enhancer of the mouse syndecan 1 gene. The results, for the first time, demonstrate the specific signals mediated by the Grb2-binding sites and further our understanding of FGF signal transmission at the adaptor level.

Identification and Characterization of a Novel Jak2 Tyrosine Kinase Inhibitor.

Jak2 is a cytoplasmic tyrosine kinase that has been linked to hematological malignancies. Recently, a somatic Jak2 mutation (Jak2-V617F) has been identified in several myeloproliferative disorders such as polycythemia vera, essential thrombocythemia and myeloid metaplasia with myelofibrosis. These myeloproliferative disorders are characterized by unregulated expansion of one or more cells in the blood. The presence of the Jak2-V617F mutation in cells results in uncontrolled cell division and resistance to the negative feedback mechanisms that govern normal cell growth. The availability of a Jak2 tyrosine kinase specific inhibitor would facilitate our understanding of these Jak2-related disorders and perhaps serve a clinical benefit for patients. However, the most widely used Jak2 inhibitor, AG490, suffers from a general lack of specificity. Here, we used a novel approach to identify Jak2-specific inhibitors. We used in silico homology modeling of the Jak2 kinase domain to identify solvent accessible exposed pockets on the surface of the Jak2 protein. We then used a high-throughput program called DOCK to predict the ability of 20,000 small molecules to interact with a structural pocket adjacent to the ATP binding site of murine Jak2. The predicted binding energies of interaction between each compound and the Jak2 kinase domain were calculated and the top six scoring compounds were tested for their ability to inhibit Jak2 tyrosine kinase function in vitro. One of these compounds, 2-methyl-1-phenyl-4-pyridin-2-yl-2-(2-pyridin-2-ylethyl)butan-1-one (Z3) effectively inhibited Jak2-V617F and Jak2-WT autophosphorylation. We were able to show that Z3 inhibits total Jak2 tyrosine phosphorylation as well as Jak2 phosphorylation at the critical tyrosine 1007 residue in both a dose-and time-dependent manner. Z3 is able to reduce growth hormone-dependent Jak2 activation and is a direct inhibitor of Jak2-WT and Jak2-V617F tyrosine kinase autophosphorylation as measured by an in vitro kinase assay. Moreover, we found that Z3 inhibits proliferation of human erythroleukemia (HEL) cells, which express the Jak2-V617F mutation. In summary, this work demonstrates proof-of-principle concept that in silico molecular modeling can be used as a means to identify specific tyrosine kinase inhibitors. Z3 Inhibits Jak2 Tyrosine Autophosphorylation in a Dose-Dependent Manner Z3 Inhibits Jak2 Tyrosine Autophosphorylation in a Dose-Dependent Manner Z3 Inhibits Growth Hormone Mediated Jak2 Phosphorylation at Tyrosine 1007 Z3 Inhibits Growth Hormone Mediated Jak2 Phosphorylation at Tyrosine 1007