Enzymatic Activity of the Src Homology 2 Domain-containing Inositol Phosphatase Is Regulated by a Plasma Membrane Location

Hyewon Phee,Anand Jacob,K.Mark Coggeshall

doi:10.1074/jbc.m001093200

Abstract

The negative regulatory role of the Src homology 2 domain-containing inositol 5-phosphatase (SHIP) has been invoked in a variety of receptor-mediated signaling pathways. In B lymphocytes, co-clustering of antigen receptor surface immunoglobulin with FcgammaRIIb promotes the negative effects of SHIP, but how SHIP activity is regulated is unknown. To explore this issue, we investigated the effect of SHIP phosphorylation, receptor tyrosine engagement by its Src homology 2 domain, and membrane recruitment of SHIP on its enzymatic 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 vivo. However, we found a minimal effect of phosphorylation or receptor tyrosine engagement of SHIP on its enzymatic activity, whereas membrane localization of SHIP significantly reduced cellular phosphatidylinositol 3,4, 5-triphosphate levels. Based on our results, we propose that a membrane localization of SHIP is the crucial event in the induction of its phosphatase effects.

Highlights

Clustering of the B cell surface immunoglobulin1 antigen receptor by binding of foreign antigen initiates a set of biochemical events termed positive signaling which activate B lymphocytes to proliferate and secrete soluble antigen specific Ig
We investigated the effect of Src homology domain-containing inositol 5-phosphatase (SHIP) phosphorylation, receptor tyrosine engagement by its Src homology 2 domain, and membrane recruitment of SHIP on its enzymatic activity
We propose that SHIP negatively affects PtdIns[3,4,5]P3-dependent enzymes by altering its subcellular location from cytosol to the plasma membrane, which is accomplished by recruitment to phosphorylated cytoplasmic tyrosines of receptors

Summary

Introduction

Clustering of the B cell surface immunoglobulin (sIg)1 antigen receptor by binding of foreign antigen initiates a set of biochemical events termed positive signaling which activate B lymphocytes to proliferate and secrete soluble antigen specific Ig (reviewed in Refs. 1–3). We investigated the effect of SHIP phosphorylation, receptor tyrosine engagement by its Src homology 2 domain, and membrane recruitment of SHIP on its enzymatic activity.

Results

Conclusion