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Abstract
In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.
Highlights
Phosphatidylinositol 3-kinases (PI3Ks)1 are a family of enzymes involved in a multiplicity of cellular functions, including cell proliferation and transformation [1,2,3], lymphocyte activation (4 –7), G protein signaling [8], DNA repair [9], intracellular vesicle trafficking [10, 11], and inhibition of programmed cell death [12, 13]
Tyrosine phosphorylation of the p85 subunit has been shown to occur in many different systems, such as in response to platelet-derived growth factor [3], insulin [35], B cell antigen receptor ligation [4], interleukin-2 [36], and in cells transformed by the Bcr-Abl fusion protein-tyrosine kinase [37,38,39,40]
At least in some Bcr-Abl expressing cells, such as HL-60 cells transfected with Bcr-Abl [37], a fraction of the p85 subunit is included among these substrates for enhanced tyrosine phosphorylation
Summary
Phosphatidylinositol 3-kinases (PI3Ks) are a family of enzymes involved in a multiplicity of cellular functions, including cell proliferation and transformation [1,2,3], lymphocyte activation (4 –7), G protein signaling [8], DNA repair [9], intracellular vesicle trafficking [10, 11], and inhibition of programmed cell death [12, 13]. Tyrosine phosphorylation of the p85 subunit has been shown to occur in many different systems, such as in response to platelet-derived growth factor [3], insulin [35], B cell antigen receptor ligation [4], interleukin-2 [36], and in cells transformed by the Bcr-Abl fusion protein-tyrosine kinase [37,38,39,40]. We have studied the tyrosine phosphorylation of p85 in hematopoietic cells, and report that phosphorylation occurs at least at Tyr-688 in the C-terminal SH2 domain This event does not detectably affect the catalytic activity of PI3K per se, but causes a change in the binding properties of the SH2 domain. This change is likely to modify the function of PI3K in intact cells
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