Second, the on-rate constant for capping protein binding to free barbed ends predicts that actin filaments could grow to the length of filaments observed in vivo during one lifetime. Third, capping protein beta-subunit isoforms did not differ in their actin binding properties, even in tests with different actin isoforms. A major hypothesis for why capping protein beta-subunit isoforms exist is thereby excluded. Fourth, the proposed capping protein regulators, Hsc70 and S100, had no effect on capping protein binding to actin in vitro . Model for actin assembly in stimulated cells.
Actin filaments grow at their free barbed ends and push the plasma membrane outward.
Elongated filaments then break, due either to mechanical forces or to the action of a severing protein. Finally, new free barbed ends situated far from the membrane where capping protein is active become capped. The net effects are protrusion of the plasma membrane and the creation of new filaments the same length as the existing filaments, of new pointed ends, and of new barbed ends.