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Fig. 3. The integrin-actin linkage acts as a molecular clutch. (A,B) The linkage is shown in two positions: integrins not linked to actin (disengaged) (A), and integrins linked to actin (engaged) (B). In A, the actin is not anchored to the substratum, and thus the force produced by actin polymerization (P) is counterbalanced by retrograde flow (R) which is caused by myosin contraction and tension on the membrane in the lamellipodium. In the example, they balance and there is no protrusion. In B, actin is coupled to the substratum by the interaction of actin-binding proteins with integrins. Under these conditions, the force generated by the retrograde flow is partially or fully shunted to the substratum (oblique black arrows). The force produced by actin polymerization then exceeds the force that produces retrograde flow, resulting in a higher protrusion rate. New nascent adhesions assemble as the lamellipodium extends. As the protrusion advances, the boundary between the lamellipodium and the lamellum moves forward. In the lamellum, myosin II activity generates a contractile force that drives retrograde flow. Slippage points that result in differential coupling of adhesion proteins to the actin occur at an as-yet-undetermined level between the 
-actinin (in green) and the other components of the linkage, and/or at the level of interaction of the integrin with the substrate.
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