TALIN DISTRIBUTION AND PHOSPHORYLATION IN THROMBIN-ACTIVATED PLATELETS

We have previously demonstrated that the subcellular distribution of t he adhesion plaque protein, talin, changes dramatically in human plate lets in response to platelet activation (Beckerle et al., J. Cell Biol . 109, 3333-3346, 1989). Talin is uniformly distributed throughout the cytoplasm of resting platelets. However, when platelets are stimulate d to become activated and adhesive, a significant amount of the talin population rapidly redistributes to a peripheral, submembranous locati on. In the present study we have examined talin phosphorylation and pr oteolytic cleavage as possible mechanisms by which talin's subcellular distribution could he regulated in platelets. We have found that thro mbin activation of platelets leads to a fourfold increase in talin pho sphorylation. Proteolytic cleavage of talin, however, is not detected in washed platelets activated with thrombin for as long as 30 minutes. Because talin moves to a submembranous location upon platelet activat ion and has been shown to interact with integrins in vitro, we also in vestigated whether the major platelet integrin, GPIIb-IIIa, is require d for talin redistribution. Using Glanzmann thrombasthenic platelets, which are deficient in GPIIb-IIIa, we found that talin redistribution occurs even in the absence of GPIIb-IIIa. Collectively, our studies su ggest that neither proteolytic cleavage of talin nor interactions betw een talin and GPIIb-IIIa is required for the regulated redistribution of talin in thrombin-activated platelets. Phosphorylation of talin in response to thrombin activation may, however, be one mechanism utilize d by platelets to regulate talin distribution and function in human pl atelets.