Platelet adhesion to collagen activates a phosphoprotein complex of heat-shock proteins and protein phosphatase 1

Rapid activation of blood platelets is required for effective haemostasis, with shape change, aggregation, secretion of granule contents and cell adhesion occurring in seconds or even milliseconds. Signal-transduction events, evidenced by changes in protein phosphorylation and calcium levels, also take place in this time domain. We have now shown that platelet adhesion to collagen via the alpha 2 beta 1 integrin under arterial shear forces initiated the rapid dephosphorylation of a 67 kDa protein "band" which contained the 70 kDa constitutive heat-shock protein, hsc70. Immunoprecipitation with hsc70 antibodies revealed a large phosphoprotein complex in resting platelets and adhesion caused dissociation of the complex along with dephosphorylation of hsc70. The complex also contained the hsp90 heat-shock protein, protein phosphatase IC, alpha, delta and M subunits, and some 7-8 unidentified phosphoproteins. The data suggest that heat-shock proteins and protein phosphatases are actively involved in integrin-mediated platelet adhesion.