Thrombin and the thrombin receptor activation peptide SFLLRN stimulate the rapid cleavage of talin by calpain. (A) Unstirred suspensions of washed human platelets were incubated in 5 units/mL of thrombi or 50 μM of SFLLRN for 5 minutes in the absence or presence of ALLN, after which they were lysed with 50 mM of tris(hydroxymethyl)aminomethane (Tris) buffer containing 1% NP-40, 4 mM of EDTA, 1 mM of ALLM, and protease and phosphatase inhibitor cocktails. Dissolved proteins were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotted with the anti–talin-1 head domain monoclonal antibody TA205. (B) Unstirred suspensions of washed human platelets were incubated with 5 units/mL of thrombin for the indicated times, after which they were lysed with 50 mM of Tris buffer containing 1% NP-40, 4 mM of EDTA, 1 mM of ALLM, and protease and phosphatase inhibitor cocktails. Dissolved proteins were separated by SDS-PAGE and immunoblotted with the anti–talin-1 head domain monoclonal antibody TA205.