Friction measurement between protein-protein by surface forces apparatus

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This study was planned to measure friction between protein and protein by using a surface forces apparatus. To perform the plan, proteins have to be arranged monolayer-like on a substrate. As samples of proteins, actin from rabbit skeletal muscle and recombinant myosin subfragment-1 from Dictyosterium were used. These are selected under a background of the possibility that friction occurs in muscle contraction, which is supported by the author's friction theory. Actin molecules were chemically fixed and arranged on the maleimide groups formed on the silica thin film that was made by an ECR sputtering instrument. The adsorption process and the density were measured precisely by using a quartz crystal microbalance. On the other hand, the developed novel in vitro motility assay showed that sliding speed increases exponentially with increasing the applied force. This result suggests that sliding speed is controlled by the adhesion force between actin and myosin as well as activity of ATP hydrolysis. The author presented, here, a possibility that a balance force with the sliding force is a frictional force that is always produced by rubbing two contact bodies. Such friction is well known as Amontons-Coulomb's law or adhesion theory. If the interface of actin and myosin behaves solid-like in the sliding movement, friction occurs naturally even in solution. In this study, rupture dynamics was shown to play a role of their mediation. The dissociation kinetics of weak noncovalent bonds under external forces was first developed by Bell using a phenomenological model for the dissociation rate. Rupture dynamics of molecular adhesion bonds has been investigated on the basis of Brownian dynamics by independently two groups of Evans and Schulten. In the present work, Bell's expression was saved because it was established in various biological specimens. The load-velocity relation was derived through the knowledge of rupture dynamics or non-equilibrium thermodynamics.