Two distinct tyrosine-based motifs enable the inhibitory receptor FčRIIB to cooperatively recruit the inositol phosphatases SHIP1/2 and the adapters Grb2/Grap

Abstract

FcγRIIB are low-affinity receptors for IgG that contain an immunoreceptor tyrosine-based inhibition motif (ITIM) and inhibit immunoreceptor tyrosine-based activation motif (ITAM)-dependent cell activation. When coaggregated with ITAM-bearing receptors, FcγRIIB become tyrosyl-phosphorylated and recruit the Src homology 2 (SH2) domain-containing inositol 5′-phosphatases SHIP1 and SHIP2, which mediate inhibition. The FcγRIIB ITIM was proposed to be necessary and sufficient for recruiting SHIP1/2. We show here that a second tyrosine-containing motif in the intracytoplasmic domain of FcγRIIB is required for SHIP1/2 to be coprecipitated with the receptor. This motif functions as a docking site for the SH2 domain-containing adapters Grb2 and Grap. These adapters interact via their C-terminal SH3 domain with SHIP1/2 to form a stable receptor-phosphatase- adapter trimolecular complex. Both Grb2 and Grap are required for an optimal coprecipitation of SHIP with FcγRIIB, but one adapter is sufficient for the phosphatase to coprecipitate in a detectable manner with the receptors. In addition to facilitating the recruitment of SHIPs, the second tyrosine-based motif may confer upon FcγRIIB the properties of scaffold proteins capable of altering the composition and stability of the signaling complexes generated following receptor engagement.