| Engagement of immune receptors leads to the recruitment of adaptor proteins and activation of downstream pathways. Ras proteins are essential components of immunoreceptor signaling, where they interact with multiple molecules and regulate development and activation of immune cells. There are two major pathways involved in Ras activation in the immune system: Grb2/Sos and RasGRPs (Ras Guanyl nucleotide-Releasing Protein).;RasGRPs are Ras activators that contain a DAG-binding C1 domain. They are recruited to the membrane by DAG and mediate Ras activation. Among the four RasGRP family members, RasGRP1 plays an important role in TCR-induced Ras-Erk activation. Here, we report a new finding that RasGRP1 plays an essential role in Fc&egr;RI-mediated PI3K activation and cytoskeleton rearrangement in mast cells. Genetic inactivation of RasGRP1 in mast cells leads to markedly impaired degranulation, cytokine production, and anaphylactic allergic reactions. While Fc&egr;RI-mediated Erk activation is surprisingly normal, PI3K activation is significantly reduced. Consequently, activation of Akt, PDK1, and PKCdelta is defective. We further show that N-Ras mediates PI3K activation downstream of RasGRP1. As the direct mechanism of impaired degranulation, intracellular granule translocation and RhoA activation is markedly defective in RasGRP1 -/- BMMCs. These results identify RasGRP1 as an essential regulator of N-Ras and PI3K in Fc&egr;RI-evoked allergic responses.;Grb2/Sos is another important pathway leading to Ras activation in immune cells. While LAT recruits Grb2/Sos to the membrane in T cells, it is not clear what molecules recruit Grb2/Sos to the plasma membrane in B cells. Here, we describe the identification and gene targeting of a novel LAT-like adaptor protein that is highly expressed in B cells and myeloid cells-GAPT (Grb2-binding Adaptor Protein, Transmembrane). GAPT is not phosphorylated upon BCR ligation, but constitutively associates with Grb2 through its proline-rich region. Targeted disruption of GAPT in mice does not affect immune cell development and humoral immune response. However, the proliferation of GAPT deficient B cells is enhanced upon BCR stimulation, indicating that GAPT plays a limited role in B cell activation and proliferation. |
