Fc gamma receptor IIb modulates the molecular Grb2 interaction network in activated B cells.

B cells require signals transduced by the B cell antigen receptor (BCR) to provide humoral adaptive immunity. These signals are modulated by co-receptors like the Fcγ receptor IIb (FcγRIIb) that prevents activation of B cells after co-ligation with the BCR. Positive and negative effectors need to be precisely organized into signaling complexes, which requires adapter proteins like the growth factor receptor-bound protein 2 (Grb2). Here, we address the question how Grb2-mediated signal integration is affected by FcγRIIb. Our data reveal that concomitant engagement of BCR and FcγRIIb leads to markedly increased Grb2-mediated formation of ternary protein complexes comprising downstream of kinase-3 (Dok-3), Grb2, and the SH2 domain-containing inositol phosphatase (SHIP). Consistently, we found Grb2 to be required for full FcγRIIb-mediated negative regulation. To investigate how FcγRIIb influences the entire Grb2 interactions, we utilized quantitative mass spectrometry to make a differential interactome analysis. This approach revealed a shift of Grb2 interactions towards negative regulators like Dok-3, SHIP and SHP-2 and reduced binding to other proteins like CD19. Hence, we provide evidence that Grb2-mediated signal integration is a dynamic process that is important for the crosstalk between the BCR and its co-receptor FcγRIIb.