ABSTRACT
The volume-regulating protein, ICln, interacts with the conserved KxGFFKR alpha-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 alpha(IIb)beta(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 alpha(IIb)beta(3) during platelet activation. His-tagged recombinant ICln bound equally to purified alpha(IIb)beta(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 alpha(IIb)beta(3) to occur. Thus, although ICln has the inherent capacity to bind to alpha(IIb)beta(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.
