Bridging the Gap: Modulatory roles of the Grb2-family adaptor, Gads, in cellular and allergic immune responses

Antigen receptor signaling pathways are organized by adaptor proteins, including three hematopoietic adaptors: LAT, Gads and SLP-76, that are required for antigen responsiveness in T cells and in mast cells. Upon antigen recognition by the TCR or by the FCεRI, the Grb2-family adaptor, Gads, bridges the recruitment of the cytoplasmic adaptor, SLP-76, to the membrane-bound adaptor, LAT. Cooperative binding events contribute to the formation of large LAT-nucleated signaling complexes that are competent to mediate downstream responsiveness; however, the molecular basis for cooperativity was not known. We demonstrated an SH2-intrinsic mechanism of cooperativity, based on Gads SH2 dimerization. Gads dimerization promotes its preferentially paired binding to dual-phosphorylated LAT molecules, and facilitates discrimination between dual and singly phosphorylated LAT molecules. We developed a rapid and sensitive method for measuring cooperativity at LAT. Using this method, we are able to define the regions of Gads and of LAT that stabilize their cooperative interaction. Mutational inactivation of the Gads dimerization interface reduced cooperativity, and abrogated Gads signaling functions in T cells and in mast cells. Dimerization-dependent, cooperative binding of Gads to LAT may increase antigen receptor sensitivity, by reducing signalosome formation at incompletely phosphorylated LAT molecules, thereby prioritizing the formation of stoichiometrically complete signalosomes.