Enzymatic activity of the Src homology 2 domain-containing inositol phosphatase is regulated by a plasma membrane location

The negative regulatory role of the Src homology 2 domain-containing inosit ol B-phosphatase (SHIP) has been invoked in a variety of receptor-mediated signaling pathways. In B lymphocytes, co-clustering of antigen receptor sur face immunoglobulin with Fc gamma RIIb promotes the negative effects of SHI P, but how SHIP activity is regulated is unknown. To explore this issue, we investigated the effect of SHIP phosphorylation, receptor tyrosine engagem ent by its Src homology 2 domain, and membrane recruitment of SHIP on its e nzymatic activity. We examined two SHIP phosphorylation kinase candidates, Lyn and Syk, and observed that the Src protein-tyrosine kinase, Lyn is far superior to Syk in its ability to phosphorylate SHIP both in vitro and in v ivo. However, we found a minimal effect of phosphorylation or receptor tyro sine engagement of SHIP on its enzymatic activity, whereas membrane localiz ation of SHIP significantly reduced cellular phosphatidylinositol 3,4,5-tri phosphate levels. Based on our results, we propose that a membrane localiza tion of SHIP is the crucial event in the induction of its phosphatase effec ts.