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 catalyses the hydrolysis of the 4-position phosphate from phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P2]. In human platelets the formation of this phosphatidylinositol, by the actions of phosphatidylinositol 3-kinase (PI 3-kinase), correlates with irreversible platelet aggregation. We have shown 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 investigated 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 4-phosphatase enzyme activity and a 104-kDa polypeptide recognized by specific 4-phosphatase antibodies. Similarly, immunoprecipitates made using 4-phosphatase-specific antibodies contained PI 3-kinase enzyme activity and an 85-kDa polypeptide recognized by antibodies to the p85 adapter subunit of PI 3-kinase. After thrombin activation, the 4-phosphatase translocated to the actin cytoskeleton along with PI 3-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 localize the 4-phosphatase to sites of PtdIns(3,4)P2 production.
Munday
A D
AD
Department of Biochemistry and Molecular Biology, Monash University, Clayton Campus, Clayton 3168, Victoria, Australia.
Norris
F A
FA
Caldwell
K K
KK
Brown
S
S
Majerus
P W
PW
Mitchell
C A
CA
D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D003600:Cytosol / Q000201:enzymology; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D019869:Phosphatidylinositol 3-Kinases / Q000097:blood* / Q000737:chemistry / Q000302:isolation & purification; D010744:Phosphoric Monoester Hydrolases / Q000097:blood* / Q000302:isolation & purification; D013917:Thrombin / Q000494:pharmacology
D046911:Macromolecular Substances; D019869:Phosphatidylinositol 3-Kinases; D010744:Phosphoric Monoester Hydrolases; C053153:phosphatidylinositol-3,4-bisphosphate 4-phosphatase; D013917:Thrombin