Model of prothrombinase assembly on myosin. Factor Xa and factor Va can colocalize on purified myosin and activate prothrombin. This activity requires factor Va and myosin cannot promote factor Xa independent activity. The Gla domain of factor Xa is not absolutely required for interaction, which provides evidence that factor Xa binding to myosin is different than binding to phospholipids. See Figure 7 in the article by Deguchi et al that begins on page 1870.