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The aminopeptidase DPP9 removes dipeptides from N-termini of substrates having a proline or alanine in second position. Although linked to several pathways including cell survival and metabolism, the molecular mechanisms underlying these outcomes are poorly understood. We identified a novel interaction of DPP9 with Filamin A, which recruits DPP9 to Syk, a central kinase in B-cell signalling. Syk signalling can be terminated by degradation, requiring the ubiquitin E3 ligase Cbl. We show that DPP9 cleaves Syk to produce a neo N-terminus with serine in position 1. Pulse-chases combined with mutagenesis studies reveal that Ser1 strongly influences Syk stability. Furthermore, DPP9 silencing reduces Cbl interaction with Syk, suggesting that DPP9 processing is a prerequisite for Syk ubiquitination. Consistently, DPP9 inhibition stabilizes Syk, thereby modulating Syk signalling. Taken together, we demonstrate DPP9 as a negative regulator of Syk and conclude that DPP9 is a novel integral aminopeptidase of the N-end rule pathway. DOI: http://dx.doi.org/10.7554/eLife.16370.001, eLife digest Proteins are made up of building blocks called amino acids bonded together to form chain-like molecules. Around twenty different amino acids are used to make proteins, and enzymes called proteases can recognize specific pairs of amino acids in proteins and cut the bonds between them. Dipeptidylpeptidase 9 (or DPP9 for short) is a protease that removes two amino acids from the end of a protein, just as long the second amino acid is one of two specific kinds (namely, an alanine or a proline). The DPP9 protease influences a range of processes in the cell including cell death, signaling and survival. Indeed, mice born with an inactive version of DPP9 die shortly after birth, but it is not known why this happens. Justa-Schuch et al. investigated how the protease DPP9 controls processes inside cells and found an unexpected connection between DPP9 and another protein called Syk. The Syk protein is found in immune cells called B cells, and becomes highly activated whenever these cells are stimulated. Once activated Syk changes the activity of many proteins, affecting which genes are switched on and how the B cell moves and divides. By using DPP9 as a kind of bait, Justa-Schuch et al. found human proteins that bind to the protease. This search identified a protein called Filamin A that interacted with DPP9, placing DPP9 close to Syk, which also binds to Filamin A. Further experiments showed that when DPP9 was located close to Syk, it cut the end of Syk. This cut left the Syk protein with a different amino acid exposed at its end, which in turn made it susceptible to being broken down inside the cell. Justa-Schuch et al. went on to show that DPP9 preferentially cleaved the active form of Syk. Since cleaved Syk was subsequently broken down, DPP9 acts as a shut-off mechanism for Syk after the B cell has been stimulated. The findings show that DPP9 can influence how much and how long the B cell responds to stimulation. Inhibitors of DPP9 may therefore be useful for stabilizing Syk, which is known to stop specific tumors from growing. Future work will investigate the mechanisms that control how Filamin A, DPP9 and Syk interact. DOI: http://dx.doi.org/10.7554/eLife.16370.002

Ligation of high-affinity IgE receptor I (Fc epsilon RI) on RBL-2H3 cells leads to recruitment of Fc epsilon RI and type II phosphatidylinositol 4-kinases (PtdIns 4-kinases) into lipid rafts. Lipid raft integrity is required for the activation of type II PtdIns 4-kinases and signal transduction through Fc epsilon RI gamma during RBL-2H3 cell activation. However, the molecular mechanism by which PtdIns 4-kinases are coupled to Fc epsilon RI signaling is elusive. Here, we report association of type II PtdIns 4-kinase activity with Fc epsilon RI gamma subunit in anti-Fc epsilon RI gamma immunoprecipitates. Fc epsilon RI gamma-associated PtdIns 4-kinase activity increases threefold upon Fc epsilon RI ligation in anti-Fc epsilon RI gamma immunoprecipitates. Biochemical characterization of PtdIns 4-kinase activity associated with Fc epsilon RI gamma reveals that it is a type II PtdIns 4-kinases. Canonical tyrosine residues mutation in Fc epsilon RI gamma ITAM (Y65 and Y76) reveals that these two tyrosine residues in gamma subunit are required for its interaction with type II PtdIns 4-kinases.

Rac1 and Rac2 GTPases transduce signals from multiple receptors leading to cell migration, adhesion, proliferation, and survival. In the absence of Rac1 and Rac2, B cell development is arrested at an IgD− transitional B cell stage that we term transitional type 0 (T0). We show that T0 cells cannot enter the white pulp of the spleen until they mature into the T1 and T2 stages, and that this entry into the white pulp requires integrin and chemokine receptor signaling and is required for cell survival. In the absence of Rac1 and Rac2, transitional B cells are unable to migrate in response to chemokines and cannot enter the splenic white pulp. We propose that loss of Rac1 and Rac2 causes arrest at the T0 stage at least in part because transitional B cells need to migrate into the white pulp to receive survival signals. Finally, we show that in the absence of Syk, a kinase that transduces B cell antigen receptor signals required for positive selection, development is arrested at the same T0 stage, with transitional B cells excluded from the white pulp. Thus, these studies identify a novel developmental checkpoint that coincides with B cell positive selection.

International audience; BACKGROUND: The spleen tyrosine kinase (Syk) is recognized as a potential pharmaceutical target for the treatment of type I hypersensitivity reactions including allergic rhinitis, urticaria, asthma, and anaphylaxis because of its critical position upstream of immunoreceptor signaling complexes that regulate inflammatory responses in leukocytes. OBJECTIVE: Our aim was to improve the selectivity of anti-Syk therapies by impeding the interaction of Syk with its cellular partners, instead of targeting its catalytic site. METHODS: We have previously studied the inhibitory effects of the anti-Syk intracellular antibody G4G11 on Fc epsilonRI-induced release of allergic mediators. A compound collection was screened by using an antibody displacement assay to identify functional mimics of G4G11 that act as potential inhibitors of the allergic response. The effects of the selected druglike compounds on mast cell activation were evaluated in vitro and in vivo. RESULTS: We discovered compound 13, a small molecule that inhibits Fc epsilonRI-induced mast cell degranulation in vitro and anaphylactic shock in vivo. Importantly, compound 13 was efficient when administered orally to mice. Structural analysis, docking, and site-directed mutagenesis allowed us to identify the binding cavity of this compound, located at the interface between the 2 Src homology 2 domains and the interdomain A of Syk. CONCLUSION: We have isolated a new class of druglike compounds that modulate the interaction of Syk with some of its macromolecular substrates implicated in the degranulation pathway in mast cells.