The inositol polyphosphate 4-phosphatase forms a complex with phosphatidylinositol 3-kinase in human platelet cytosol

Inositol polyphosphate 4-phosphatase (4-phosphatase) is an enzyme that cata lyses the hydrolysis of the 4-position phosphate front phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P-2]. In human platelets the formation of thi s phosphatidylinositol, by the actions of phosphatidylinositol 3-kinase (PI 3-kinase), correlates with irreversible platelet aggregation. We have show n previously that a phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase forms a complex with the p85 subunit of PI 3-kinase. In this study we inves tigated whether PI 3-kinase also forms a complex with the 4-phosphatase in human platelets. Immunoprecipitates of the p85 subunit of PI 3-kinase from human platelet cytosol contained 3-phosphatase enzyme activity and a 104-kD a polypeptide recognized by specific 1-phosphatase antibodies. Similarly, i mmunoprecipitates made using 4-phosphatase-specific antibodies contained PI 3-kinase enzyme activity and an 85-kDa polypeptide recognized by antibodie s to the p85 adapter subunit of PI 3-kinase. After thrombin activation, the 1-phosphatase translocated to the actin cytoskeleton along with PI3-kinase in an integrin- and aggregation-dependent manner. The majority of the PI 3 -kinase/ 4-phosphatase complex (75%) remained in the cytosolic fraction. We propose that the complex: formed between the two enzymes serves to localiz e the 4-phosphatase to sites of PtdIns(3,4)P-2 production.