PLATELETS EXPRESS A MEMBRANE PROTEIN COMPLEX IMMUNOLOGICALLY RELATED TO THE FIBROBLAST FIBRONECTIN RECEPTOR

The heterodimer complex GpIIb-IIIa on human platelets can specifically bind fibronectin (FN) only when platelets are activated by thrombin. However unstimulated platelets can adhere and spread on a FN substratum. This suggests the existence of a second binding site for FN on the platelet surface that does not require activation for its expression. We have previously identified and characterized a membrane glycoprotein complex (Gp 150/135) that functions as fibronectin receptor (FN-R) in mouse fibroblast adhesion. To investigate whether this molecule was also present in platelets we have produced an affinity purified polyclonal antibodies monospecific for the lower subunit of the fibroblast FN-R. These antibodies specifically stained human and rat platelet surface as determined by fluorescence flow cytometric analysis and reacted with a component of 138 Kd m w in Western blot of platelet membranes. Experiments of differential extraction revealed that the 138 Kd component is an integral membrane protein. Moreover the antibodies precipitated the 138 Kd component together with a 160 Kd protein suggesting that the two molecules are associated in a supramolecular complex. A comparative analysis indicated that this protein complex is clearly distinct from the GpIIb-IIIa. In addition platelets from a thrombastenic patient reacted normally with 138 Kd but not with GpIIb-IIIa antibodies by Western blot analysis. These data indicate that normal human platelets express both GpIIb-IIIa and FN-R on their membrane and that these receptors are composed of structurally and antigenically distinct proteins.