Human Platelet Glycoprotein Ia

Glycoprotein Ia (GP Ia) is a relatively minor component of human blood platelets thought to be a receptor involved in collagen-induced platelet activation.However, some difficulties exist with the definition of this glycoprotein.The expression of GP Ia on resting (prostacyclin analogue-treated) and thrombin-activated platelets was compared by surface labeling with '251-lactoperoxidase. Intact platelets or platelets solubilized in sodium dodecyl sulfate were labeled with peri~date/[~H]NaBHd.Analysis on two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels showed that GP Ia is very poorly labeled in resting platelets.After activation a new spot (GP Ia*) appears with the same relative molecular mass as GP Ia under reducing conditions.GP Ia and Ia* can be clearly separated by two-dimensional nonreduced/reduced gel electrophoresis.Therefore, two glycoproteins which have been termed GP Ia exist in platelets with similar molecular weight and PI under reducing conditions.One of these (GP Ia*) is only surface-labeled when platelets are activated, indicating that it is only exposed on the surface of activated platelets.Supernatant from activated platelets contains this glycoprotein as well as other granule components.This glycoprotein is missing in platelets from two patients with collagen-response defects.Glycoprotein (GP)' Ia is a relatively minor component of human blood platelets with a molecular mass of 167 kDa under reducing conditions, 153 kDa under nonreducing conditions (l), and an isoelectric point (PI) of 4.5-5.5 (2).The role of GP Ia in the physiological function of platelets is not yet clear, but there is strong evidence that it is involved in the collagen receptor and plays a role in platelet activation by collagen, in the subsequent spreading, and in aggregate formation on injured vessel walls.Nieuwenhuis et al. (3) described a patient whose platelets were totally unresponsive to collagen and were deficient in GP Ia.These platelets were unable to spread on exposed subendothelium (4) and, unlike normal platelets, were not activated by wheat germ agglutinin.Recently, another patient was described by Kehrel et al. ( 5 ) whose platelets also failed to aggregate in the presence of *