Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Pleckstrin is the major substrate phosphorylated on serine and threonine in response to stimulation of human platelets by thrombin (Abrams, C. S., Zhao, W., Belmonte, E., and Brass, L. F. (1995) J. Biol. Chem. 270, 23317-23321). We now show that pleckstrin in platelets is in a complex with inositol polyphosphate 5-phosphatase I (5-phosphatase I). This enzyme hydrolyzes the 5-phosphate from inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate and thus serves as a calcium signal-terminating enzyme, since the substrates but not the products mobilize intracellular calcium. Pleckstrin co-immunoprecipitates with 5-phosphatase I in homogenates of platelets. Platelet homogenates fractionated by anion exchange chromatography show co-elution of pleckstrin and 5-phosphatase I. Fractions containing phosphorylated pleckstrin have 7-fold greater 5-phosphatase activity than those containing unphosphorylated pleckstrin. Mixing experiments with recombinant 5-phosphatase I and pleckstrin in vitro show that they form a stoichiometric complex. A mutant form of pleckstrin, in which the serine and threonine residues that are phosphorylated by protein kinase C are substituted with glutamic acid (pseudophosphorylated pleckstrin), activates recombinant 5-phosphatase I 2-3-fold while native unphosphorylated pleckstrin does not stimulate the enzyme. Thus pleckstrin functions to terminate calcium signaling in platelets when it is phosphorylated by binding to and activating 5-phosphatase I.
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PMID:Phosphorylation of platelet pleckstrin activates inositol polyphosphate 5-phosphatase I. 899 61

The platelet receptor for the von Willebrand factor (VWF) glycoprotein Ib-IX-V (GPIb-IX-V) complex mediates platelet adhesion at sites of vascular injury. The cytoplasmic tail of the GPIbalpha subunit interacts with the actin-binding protein, filamin, anchoring the receptor in the cytoskeleton. In motile cells, the second messenger phosphatidylinositol 3,4,5 trisphosphate (PtdIns(3,4,5)P3) induces submembraneous actin remodeling. The inositol polyphosphate 5-phosphatase, Src homology 2 domain-containing inositol polyphosphate 5-phosphatase-2 (SHIP-2), hydrolyzes PtdIns(3,4,5)P3 forming phosphatidylinositol 3,4 bisphosphate (PtdIns(3,4)P2) and regulates membrane ruffling via complex formation with filamin. In this study we investigate the intracellular location and association of SHIP-2 with filamin, actin, and the GPIb-IX-V complex in platelets. Immunoprecipitation of SHIP-2 from the Triton-soluble fraction of unstimulated platelets demonstrated association between SHIP-2, filamin, actin, and GPIb-IX-V. SHIP-2 associated with filamin or GPIb-IX-V was active and demonstrated PtdIns(3,4,5)P3 5-phosphatase activity. Following thrombin or VWF-induced platelet activation, detection of the SHIP-2, filamin, and receptor complex decreased in the Triton-soluble fraction, although in control studies the level of SHIP-2, filamin, or GPIb-IX-V immunoprecipitated by their respective antibodies did not change following platelet activation. In activated platelets spreading on a VWF matrix, SHIP-2 localized intensely with actin at the central actin ring and colocalized with actin and filamin at filopodia and lamellipodia. In spread platelets, GPIb-IX-V localized to the center of the platelet and showed little colocalization with filamin at the plasma membrane. These studies demonstrate a functionally active complex between SHIP-2, filamin, actin, and GPIb-IX-V that may orchestrate the localized hydrolysis of PtdIns(3,4,5)P3 and thereby regulate cortical and submembraneous actin.
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PMID:SHIP-2 forms a tetrameric complex with filamin, actin, and GPIb-IX-V: localization of SHIP-2 to the activated platelet actin cytoskeleton. 1267 85