title: Isolation of InsP4 and InsP6 binding proteins from human platelets: InsP4 promotes Ca2+ efflux from inside-out plasma membrane vesicles containing 104 kDa GAP1IP4BP protein.
abstract: A low-density membrane fraction from human platelets contained the plasma membrane marker glycoprotein Ib (GpIb) and selective binding sites for InsP4 and InsP6. It was separated from the bulk of InsP3-receptor-containing membranes, but was heterogeneous, probably also containing surface-connected canalicular system and some lighter elements of the internal dense tubule system. After loading with calcium oxalate and re-centrifugation on Percoll gradients, this mixed fraction was subfractionated into light membranes containing all of the GpIb, high-affinity InsP4 binding sites (KD = 18 nM) and phosphate-stimulated Ca2+ transport activity. InsP4 (EC50 0.6 microM), but not InsP3 or InsP6, released up to 35% of the accumulated Ca2+ from these vesicles, which were shown to be inside-out plasma membrane vesicles by a biotinylation labelling technique and selective removal of right-side-out plasma membrane vesicles with streptavidin-agarose. Most of the InsP4, and all of the InsP6, binding was present in the much denser calcium oxalate-loaded subfractions, which were free of GpIb. InsP6 binding activity was chromatographically purified as a 116 kDa protein (KD for InsP6 = 5.9 nM), with an amino acid content and two internal peptide sequences identical to those of 116 kDa vinculin. A 104 kDa InsP4 binding protein (KD for InsP4 = 12 nM), probably identical to GAP1IP4BP described by Cullen, Hsuan, Truong, Letcher, Jackson, Dawson and Irvine [(1995) Nature (London) 376, 527-530], was also isolated. This InsP4 receptor may mediate Ca2+ influx in platelets that occurs subsequent to receptor-stimulated production of InsP3 and unloading of internal Ca2+ stores.
authors.affiliation: Department of Pharmacology, University of Connecticut School of Medicine, Farmington 06030, USA.
authors.forename: F
authors.initials: F
authors.lastname: O'Rourke
authors.forename: E
authors.initials: E
authors.lastname: Matthews
authors.forename: M B
authors.initials: MB
authors.lastname: Feinstein
mesh_terms: D001665:Binding Sites; D001693:Biological Transport, Active; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000097:blood / Q000302:isolation & purification* / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D020690:GTPase-Activating Proteins; D006801:Humans; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D008024:Ligands; D010833:Phytic Acid / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D015290:Second Messenger Systems; D015398:Signal Transduction
chemical_list: D002352:Carrier Proteins; D020690:GTPase-Activating Proteins; D008024:Ligands; D011506:Proteins; D018160:Receptors, Cytoplasmic and Nuclear; C061107:inositol-1,3,4,5-tetrakisphosphate receptor; D010833:Phytic Acid; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium