title: Biochemical and functional consequences of dissociation of the platelet membrane glycoprotein IIb-IIIa complex.
abstract: The platelet membrane glycoproteins, IIb and IIIa, form a Ca2+-dependent heterodimer complex that functions as the fibrinogen receptor in activated platelets to mediate platelet aggregation. Little is known about factors that affect the IIb-IIIa complex within the platelet membrane. It has been observed that platelets incubated with ethylene glycol tetra-acetic acid (EGTA) at 37 degrees C are unable to aggregate or to bind monoclonal antibodies specific for the IIb-IIIa complex. To determine whether this is due to a dissociation of IIb from IIIa, we developed a method for quantitating the complex on nondenaturing, polyacrylamide gradient gels. Platelets were surface-labeled with 125I and then solubilized and electrophoresed in 0.2% Triton and 10 mmol/L CHAPS. Under these conditions and in the presence of 1 mmol/L Ca2+, glycoproteins IIb and IIIa migrated on the gels as a discrete band at Rf = 0.33. Protein that was eluted from this band bound to an immunoaffinity column specific for the IIb-IIIa complex. In contrast, when the IIb-IIIa complex was solubilized and then dissociated with EGTA, the discrete band at Rf = 0.33 was no longer present, and IIb and IIIa were now found in a broad band at Rf = 0.45 to 0.50. To study IIb and IIIa within the surface membrane, the 125I-labeled platelets were first incubated with 0.5 mmol/L EGTA (1 nmol/L free Ca2+) at 22 degrees C and then solubilized in the absence of EGTA. The IIb and IIIa from these platelets migrated at Rf = 0.33, indicating the presence of the intact IIb-IIIa complex. In contrast, when the platelets were incubated at 37 degrees C for one hour with the EGTA, the discrete band at Rf = 0.33 representing the IIb-IIIa complex gradually disappeared. This phenomenon could not be reversed by adding Ca2+ back to the platelets before solubilization and electrophoresis. This loss of the IIb-IIIa complex from intact platelets was accompanied by (a) a progressive and irreversible decrease in adenosine diphosphate (ADP)-induced platelet aggregation and (b) decreased binding of a complex-dependent monoclonal antibody to the platelets. These studies demonstrate that when platelets are exposed to low Ca2+ at 37 degrees C, the IIb-IIIa heterodimer complexes in their surface membranes are irreversibly disrupted. Because intact IIb-IIIa complexes are required for platelet aggregation, the loss of these complexes may account for the failure of these platelets to aggregate in response to ADP.
authors.forename: S J
authors.initials: SJ
authors.lastname: Shattil
authors.forename: L F
authors.initials: LF
authors.lastname: Brass
authors.forename: J S
authors.initials: JS
authors.lastname: Bennett
authors.forename: P
authors.initials: P
authors.lastname: Pandhi
mesh_terms: D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D002118:Calcium / Q000097:blood; D002462:Cell Membrane / Q000502:physiology; D004533:Egtazic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D006023:Glycoproteins / Q000502:physiology*; D006801:Humans; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000502:physiology*; D008970:Molecular Weight; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins; D013696:Temperature
chemical_list: D006023:Glycoproteins; D046911:Macromolecular Substances; D008565:Membrane Proteins; D010980:Platelet Membrane Glycoproteins; D004533:Egtazic Acid; D002118:Calcium