Isolation and partial characterization of proteins from platelet pseudopods.

Isolated pseudopods of human platelets activated by ADP, thrombin, or low temperature contain, as a major constituent, a 70,000-dalton membrane protein. This protein, which is labeled in intact platelets by [125I]iodonaphthylazide, is either pushed out of the membrane plane during activation or undergoes a conformational change or, as a result of activation, is aggregated in the pseudopod region. The relative amount of actin present in the pseudopod fraction is higher in the case of thrombin activation than that of ADP- or temperature-induced activation. The relative amounts of glycoproteins IIb and III in these pseudopod fractions were found to be similar with each of the preparative methods used and also similar to their relative concentrations in whole platelets.