To a less extent, we also observed colocalization of Tespa1 and LAT1 by 10 min after mast cell activation (Fig. IgE-mediated allergic responses. The dysregulated signaling in KO mast cells was associated with increased activation of Grb2-PLC-1-SLP-76 signaling within the LAT1 (linker for activation of T cells family, member 1) signalosome versus the LAT2 signalosome. Collectively, these findings show that Tespa1 orchestrates mast cell activation by tuning the balance of LAT1 and LAT2 signalosome assembly. Allergic responses are Th2- and IgE-mediated immune responses that evolved to protect against environmental substances and parasites. However, these responses can also lead to allergy when inadvertently activated by noninfectious environmental antigens and cause harmful symptoms such as itching, inflammation, or tissue injury. As to our current understanding, mast cells are at the center of allergic responses (Galli et al., 2005;Bischoff, 2007). Mast cells are BM-derived hematopoietic cells localized under surfaces exposed to the external environment, such as the skin, airways, and intestine. They function as sentinel cells in host defense reactions including immediate hypersensitivity responses and allergic responses (Galli et al., 2005). Activated mast cells trigger allergic responses by releasing preformed granule-associated chemical mediators, producing multiple cytokines and chemokines, and secreting de novo synthesized arachidonic acid metabolites and various proteins (Metcalfe et al., 1997;Bischoff, 2007). Mast cells recognize antigens via MPT0E028 IgE and specific Fc receptors, termed FcRI. Binding of multivalent antigen to FcRI-bound IgE induces receptor aggregation and triggers mast cell activation (Kinet, 1999;Siraganian, 2003). FcRI is expressed on the surface of mast cell as MPT0E028 a tetrameric complex consisting of an IgE-binding subunit, a signal-modulating subunit, and two signal-transduction subunits (Kraft and Kinet, 2007). The signaling cascades elicited by FcRI aggregation in mast cells have been extensively studied (Kalesnikoff and Galli, 2008). Briefly, the conserved immunoreceptor tyrosine-based activation motifs (ITAMs) within the cytoplasmic tails of the and subunits are rapidly phosphorylated upon FcRI stimulation in a Lyn-dependent manner (Garman et al., 1999;Kinet, 1999). Then another tyrosine kinase, Syk, binds to the tyrosine-phosphorylated ITAMs and MPT0E028 initiates the principal axis pathway that includes Grb2, PLC-1, and SLP-76 (Gilfillan and Tkaczyk, 2006;Rivera and Gilfillan, 2006), which ultimately lead to the activation of downstream signaling cascades including mitogen-activated protein kinases, protein MPT0E028 kinase C pathways, and calcium flux. During this signaling process, two similar adaptor proteins, linker for activation of T cells family, member 1 (LAT1) and LAT2, are both phosphorylated, resulting in the formation of two complementary and competitive signalosome complexes known as the LAT1 signalosome and the LAT2 signalosome. Both of these membrane adaptors recruit principal axisrelated molecules including Grb2, SOS, PLC-1, SLP-76, and Vav1. The TIMP2 essential role of LAT1 in mast cell activation is clear because LAT1 deficiency markedly attenuates mast cell responsiveness. However, the function of the LAT2 signalosome in RcRI signaling is still an enigma to many immunologists. In general, LAT2 may down-regulate antigen-mediated signaling in mast cells by either competing with LAT1 for a limited pool of signaling molecules or recruiting of phosphatases and ubiquitin-ligases such as SHP-1 and c-Cbl (Gu et al., 2001;Brdicka et al., 2002;Voln et al., 2004;Rivera, 2005). On the contrary, LAT2 has also been found to compensate for the loss of LAT1 in theLat1/background (Voln et al., 2004;Zhu et al., 2004), either through activation of the principal axis pathway with lower efficiency, or by recruiting another adaptor protein, Gab2, to facilitate Fyn-Gab2-PI3K-Akt signaling termed the complementary axis (Gilfillan and Tkaczyk, 2006;Kalesnikoff and Galli, 2008;Alvarez-Errico et al., 2009). The current model of LAT1 and LAT2 activation raises another unavoidable question of what determines the recruitment of the same pool of signaling molecules to two different adaptor proteins, namely LAT1 and LAT2, with such high similarity in sequence and structure. It is still unclear whether there is a shunt to modulate the allocation of the signal molecules to these different signalosomes. If such a regulator were found, it would be an attractive target for pharmacological intervention for fine-tuning allergic responses and the treatment of mast celldriven disorders such as allergic asthma. Recently, we MPT0E028 identified a novel adaptor protein, Tespa1 (thymocyte-expressed, positive selection-associated 1), which plays a critical role in the positive selection of thymocytes (Wang et al., 2012). KO mice.