Membrane insertion of prothrombin

The nature of the interaction of prothrombin with liposomes was examined by labelling with the apolar reagent 5-[125I] iodonaphthyl-1-azide (INA). When liposomes containing phosphatidylcholine were incubated with prothrombin and INA, the amount of incorporated label was greater in the absence of added calcium. Increasing the phosphatidylserine content of the liposomes, irrespective of CA++ resulted in a significant increase in the labelling of prothrombin by INA. Since INA is confined to the lipid core of the bilayer, these results strongly suggest that prothrombin binds to liposomes, and penetrates into the lipid bilayer.