Identification of glycoprotein IIb as the lipoprotein (a)-binding protein on platelets - Lipoprotein(a) binding is independent of an arginyl-glycyl-aspartate tripeptide located in apolipoprotein(a)

Abstract

Lipoprotein(a) [Lp(a)] plays an important role in atherosclerosis. The amino acid sequence of apolipoprotein(a) [apo(a)] reveals an arginyl-glycyl-aspartate (RGD) tripeptide that is the consensus sequence for binding of adhesive plasma proteins of the fibrinolytic system, such as fibrinogen and von Willebrand factor, to the platelet membrane glycoprotein IIb-IIIa (GPIIb-IIIa) complex. Therefore, we undertook the present study to further investigate the role of Lp(a) in hemostasis. Binding of 125I-Lp(a) to a single platelet membrane-associated protein (137±6 kD) comigrating with platelet GPIIb (140 kD) was found to be specific, saturable, and Ca2+ independent. Binding of 125I-Lp(a) to resting human blood platelets was saturable, insensitive to temperature, and independent of the apo(a) isoform (B, S1 through S3). Scatchard analysis revealed a Kd of 7.2±1.8×10-9 mol/L, with 729±313 Lp(a) molecules bound per platelet. Monoclonal anti-GPIIb IgG diminished Lp(a) binding by approximately 80%, monoclonal anti-GPIIb-IIIa IgG by 60%, and anti-GPIIIa IgG by just 15%. 125I-Lp(a) binding was competitively inhibited to the same extent by either unlabeled Lp(a) or fibrinogen. Low- and high-density lipoproteins were much weaker competitors. A polyclonal antibody raised against the RGDGQSYRGT sequence of apo(a) was used to verify the presence of an ROD sequence in the different Lp(a) preparations investigated. However, two lines of evidence indicated that the RGD sequence is not the binding domain mediating Lp(a) binding to platelets. First, incubation of platelets with isolated RGD tripeptide did not influence Lp(a) binding. Second, coincubation of 125I-Lp(a) with anti-CRGDGQSY-RGT IgG or anti-CRGDGQSYRGT IgG F(ab′)2 fragments had no effect on the binding of 125I-Lp(a) to intact, resting platelets. These data strongly suggest that platelet GPIIb is the major Lp(a) binding protein, but binding of Lp(a) is apparently not mediated by the RGD peptide motif to GPIIb-IIIa.