Relation between magnetically-applied force and velocity in beads coated with rabbit myosin, sliding on actin cables in Nitellopsis cells

Sliding movements of molluscan and algal myosin attached to a magnetizable bead under a load controlled by electromagnet

We developed an electromagnetic apparatus to perform a quick change in load in the motility system, using magnetizable beads on which myosin thick filaments from molluscan smooth muscle or green algae, Chara, myosin were attached. The quick change in load to beads (diameter 4.5 microm) was applied in the range of 0-85 pN. The movement of beads was recorded by a video-system and analyzed with special software. When the quick increase in load was applied during the movement of beads under no load, the beads showed a transient movement to the reverse direction before the steady slower movement to the normal direction. When the application of load was stopped, the beads showed a transient fast phase of movement. The change in load-sustaining ability was measured by a double load step. The backward velocity at the second constant test load was smaller when the first preceding step was increased, suggesting that the ability to sustain load was higher with a higher preceding step. These phenomena were observed both in molluscan thick filaments and in Chara myosin, and the time course of the movement of a bead was quite similar to those observed previously in frog single muscle fibers. This suggested that the velocity transients are the intrinsic properties induced by the interaction between actin and myosin, irrespective of the hexagonal lattice structure of filaments, the regular sarcomere structure, and myosin type, namely, that the molecule of myosin itself has the ability to adjust to mechanical circumstances.

Force-velocity relationship of single actin filament interacting with immobilised myosin measured by electromagnetic technique

The effect of applying an external load to actin filaments moving in the in vitro motility assay is studied. Bead-tailed actin filaments were made by polymerising actin onto 2.8 microm diameter Dynabeads conjugated with gelsolin-G actin. These were introduced into a motility cell coated with 100 microg/ml rabbit fast skeletal myosin in the presence of ATP and 0.5% methylcellulose. The motility cell was inserted between the pole-pieces of an electromagnet and the fluorescent beads and filaments were observed. The force-current relationship of the electromagnet was determined from the velocity of free beads in viscous solution and Stokes' equation. The magnet produced up to 6 pN force on the Dynabeads at 1 A. Many bead-tailed actin filaments stuck to the surface, but the beads that did move moved at the same speed as unloaded f-actin in the same cell. Bead-tailed filaments slowed down under an increasing magnetic load, eventually stalled and then slid backward under increasing load before detaching from the surface. Single-filament force-velocity curves were constructed and a stalling force of about 0.6 pN/mm of actin filament estimated.