A Catch Bond Suppresses Fluctuations in the Coordinated Actions of Myosin II Motors

Abstract

We study the role of a catch bond, a bond that has an increased lifetime in response to an applied force, in the coordinated actions of myosin II motors. During muscle contraction, it is important for the actomyosin complex to have a steep response in the number of bound myosin motors with respect to an increasing tension, but also small fluctuations in the number of bound motors. This would not be possible at equilibrium, where response and fluctuations are linked through fluctuation-dissipation relationships. But, actomyosin operates out of equilibrium; the individual myosin motors undergo dissipative cycles (driven by ATP hydrolysis) of binding and unbinding. The key feature is that bond release, which has catch-bond behavior, is not the microscopic reverse of bond formation. This detailed-balance breaking, combined with the feedback control of the catch bond, induces a violation of fluctuation-dissipation to achieve a strong myosin II cooperativity that would not be possible at equilibrium but is needed for proper muscle contraction. We also show that a similar mechanism underlies the shear-induced binding of leukocytes and platelets to the blood vessel wall.