J Neural Transm (1997) 104:1037-1047 _Journal of_ Neural Transmission 9 Springer-Verlag t997 Printed in Austria Platelet adhesion to collagen activates a phosphoprotein complex of heat-shock proteins and protein phosphatase 1 A. R. L. Gear, C. G. Simon, and R. Polanowska-Grabowska Department of Biochemistry, University of Virginia, Charlottesville, VA, U. S. A. Accepted October 14, 1997 Summary. Rapid activation of blood platelets is required for effective hae- mostasis, 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 c~ 2131 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 anti- bodies revealed a large phosphoprotein complex in resting platelets and adhe- sion caused dissociation of the complex along with dephosphorylation of hsc70. The complex also contained the hsp90 heat-shock protein, protein phosphataselC, c~, 6 and M subunits, and some 7-8 unidentified phosphopro- teins. The data suggest that heat-shock proteins and protein phosphatases are actively involved in integrin-mediated platelet adhesion. Keywords: Platelets, collagen, heat-shock protein, protein phosphatase 1 Introduction Blood platelets are essential for effective haemostasis and understanding their mechanisms of activation is important for learning how altered function con- tributes to the development of atherosclerosis, as well as to situations where enhanced bleeding occurs (Andrews et al., 1997). In addition, the ability of platelets to function under conditions of high shear stress as seen in the arteri- al circulation, is critical. In case of injury to a tissue, platelet aggregation, ad- hesion, and secretion of granule contents must all take place rapidly and with high efficiency to ensure effective development of platelet thrombi and the formation of a fibrin clot. Indeed, at blood flow rates seen in the arterial circu- lation of about 10 cm/sec, platelet activation must occur within about 50 ms (Born and Richardson, 1980). In the last 20 years considerable experimental evidence has emerged showing second and even subsecond activation times. Platelet "shape change" and aggregation induced by agonists such as ADP and