Comparison and functional characterization of C-CAM, glycoprotein IIb/IIIa and integrin beta 1 in rat platelets

Three adhesion-related proteins in rat platelets were compared, namely C-CAM, gpIIb/IIIa, and integrin beta 1. These three proteins behaved as distinct components as judged by both biochemical and immunological analyses. GpIIb/IIIa bound to a GRGDSPC-peptide, but neither beta 1-containing integrins nor C-CAM had any affinity either for this peptide or for a large cell-binding fragment of fibronectin. C-CAM and integrin beta 1 behaved differently when platelets were labeled with 125I, solubilized by detergent, and immunoprecipitated. Significant amounts of labeled C-CAM was precipitated when the platelets were first solubilized with detergent and then 125I-labeled. Almost no labeled C-CAM could be precipitated when intact platelets, that were unactivated or activated by ADP, were labeled. In contrast, labeled integrin beta 1 was immunoprecipitated when unactivated platelets were surface-labeled. However, when platelets that were activated by ADP and calcium ions were labeled almost no labeled integrin beta 1 could be immunoprecipitated. These data indicate 1) that C-CAM in intact platelets is inaccessible to surface-labeling and 2) that beta 1 integrin is less accessible to surface-labeling after platelet activation in the presence of calcium ions.