Self-association of bound fibrinogen on platelet surfaces.

Binding of fibrinogen to receptors on the surfaces of activated platelets triggers movement of the ligand-bound receptors. In this study this process was followed by using native and colloidal gold-labeled fibrinogen. Both labeled and unlabeled proteins on platelet surfaces could be visualized by low-voltage, high-resolution scanning electron microscopy. Fibrinogen and gold-conjugated fibrinogen were observed to bind to platelet surfaces and to trigger identical patterns of receptor-ligand complex redistribution. In addition to previously described long- and short-range translocation patterns, fibrinogen, either unlabeled or conjugated to gold particles, formed small, specific, nonfibrillar aggregates after binding to platelet surface receptors. Similar triggering and movement resulted from binding of gold-conjugated antibody to the fibrinogen receptor, but no subsequent self-association of the antibody-gold was observed.