The Inositol 5′-Phosphatase SHIP Binds to Immunoreceptor Signaling Motifs and Responds to High Affinity IgE Receptor Aggregation

Immunoreceptors such as the high affinity IgE receptor, FcϵRI, and T-cell receptor-associated proteins share a common motif, the immunoreceptor tyrosine-based activation motif (ITAM). We used the yeast tribrid system to identify downstream effectors of the phosphorylated FcϵRI ITAM-containing subunits β and γ. One novel cDNA was isolated that encodes a protein that is phosphorylated on tyrosine, contains a Src-homology 2 ( SH2 ) domain, i nositolpoly p hosphate 5-phosphatase activity, three N XX Y motifs, several proline-rich regions, and is called SHIP. Mutation of the conserved tyrosine or leucine residues within the FcϵRI β or γ ITAMs eliminates SHIP binding and indicates that the SHIP-ITAM interaction is specific. SHIP also binds to ITAMs from the CD3 complex and T cell receptor ζ chain in vitro . SHIP protein possesses both phosphatidylinositol-3,4,5-trisphosphate 5′-phosphatase and inositol-1,3,4,5-tetrakisphosphate 5′-phosphatase activity. Phosphorylation of SHIP by a protein-tyrosine kinase, Lck, results in a reduction in enzyme activity. FcϵRI activation induces the association of several tyrosine phosphoproteins with SHIP. SHIP is constitutively tyrosine-phosphorylated and associated with Shc and Grb2. These data suggest that SHIP may serve as a multifunctional linker protein in receptor activation.