The force required to break the bond between skeletal muscle actin and alpha-actinin (unbinding force) was measured at the level of individual molecules with an optical trapping technique. An actin filament, to the barbed-end of which was attached a gelsolin-coated polystyrene bead, was bound to alpha-actinin molecules adsorbed to a nitrocellulose-coated glass surface (approximately equal to 1 alpha-actinin molecule per 1 micron actin filament). The filament-bound bead was held by the optical trap and the force was applied to break the bond by pulling the bead. The unbinding force ranged from 1.4 to 44 pN. The average magnitude of the force was approximately equal to 18 pN. As the probability of the bond breakage has been suggested to be governed by the magnitude of the external force, the relationship was studied between the magnitude of the unbinding force and the time required to break the bond (unbinding time). The unbinding time ranged from approximately equal to 0.1 to approximately equal to 20 seconds, and tended to become shorter as the unbinding force became larger. The unbinding time seemed to be classifiable into two major groups: one group having a time value of 1 sec or less and the other having a time value ranging from several to 20 seconds. This suggests the existence of at least two classes of the actin-actinin bonds.
Actinin Actins