PROTEOLYSIS OF PLATELET CORTACTIN BY CALPAIN

Cortactin, a substrate of pp60(c-src) and a potent filamentous actin b inding and cross-linking protein, is abundant in circulating platelets . After stimulation of platelet aggregation with collagen, cortactin u ndergoes a dramatic increase in tyrosine phosphorylation followed by a rapid degradation. The cleavage of platelet cortactin was detected in lysates prepared using either Triton-containing buffer or SDS-sample buffer. However, the degradation of cortactin was not observed in plat elets derived from a Glanzmann's patient, who lacked functional integr in alpha(IIb)beta(3) (GPIIb-IIIa). In addition, the proteolysis of cor tactin was abolished by treating platelets before but not after collag en stimulation with EGTA or calpeptin. Furthermore, recombinant cortac tin was digested by mu-calpain in vitro in a dose-dependent manner, in dicating that cortactin is a substrate for calpain. We also observed t hat the calpain-mediated digestion in vitro is dependent on the presen ce of a sequence containing a proline-rich region and multiple tyrosin e residues that are phosphorylated by pp(60c-src). Tyrosine phosphoryl ation by pp60(c-src) up-regulates the activity of calpain toward corta ctin. Our data suggest that the calpain-mediated proteolysis of tyrosi ne-phosphorylated cortactin may provide a mechanism to remodel irrever sibly the cytoskeleton in response to platelet agonists.