Compartmentalization regulates the interaction between the platelet integrin α IIb β 3 and ICln

Summary The volume‐regulating protein, ICln, interacts with the conserved KxGFFKR α‐integrin signature motif. ICln is an abundant protein (4455 ± 650 molecules/platelet) found exclusively in the soluble cytosolic fraction of unactivated platelets. In contrast, its binding partner, the platelet integrin α IIb β 3 , is present in detergent‐insoluble fractions associated with membrane and cytoskeleton subcellular localizations. This study investigated factors that regulate the interaction of ICln with α IIb β 3 during platelet activation. His‐tagged recombinant ICln bound equally to purified α IIb β 3 and to integrin from resting or activated platelets. Binding was not affected by direct integrin activation with Mn ++ or by inhibitors of integrin occupancy (abciximab, RGD). However, the capacity for interaction between integrin and recombinant ICln was slowly downregulated following prolonged platelet activation for >300 s. In parallel, ICln redistributed to membrane and cytoskeletal platelet subcellular fractions. The time‐course of this redistribution preceded the downregulation of integrin binding capacity and suggests that only a short window of opportunity exists for ICln interaction with α IIb β 3 to occur. Thus, although ICln has the inherent capacity to bind to α IIb β 3 regardless of its activation state, it can only do so following platelet activation. Activation‐dependent subcellular redistribution of ICln represents a novel, temporally‐regulated mechanism for control of integrin function in platelets.