Sudden increase in speed of an actin filament moving on myosin cross-bridges of "mismatched" polarity observed when its leading end begins to interact with cross-bridges of "matched" polarity.

Under in vitro movement assay conditions, actin filaments move about 10 times faster toward, than away from, the center of large bipolar thick filaments of molluscan smooth muscle. Using thick filaments isolated from the anterior byssus retractor muscle of Mytilus edulis, the two speed modes of movement were studied in detail. Some thick filaments crossed over each other on the surface of the assay chamber, allowing actin filaments that moved into the crossover region to transfer to other thick filaments. When an actin filament that had been moving in the low speed mode crossed over to another thick filament and the speed changed to fast, the entire actin filament started to move in the high speed mode at the moment of transfer of its leading end, leaving the trailing part still in contact with the original thick filament. This indicates that myosin cross-bridges interacting in the slow mode do not impose a significant load on the cross-bridges interacting in the fast mode. Assuming the theoretical model of Tawada and Sekimoto [Biophys. J. 59, 343-356 (1991)], we suggest that the magnitude of force developed, as well as the speed of unloaded movement, differs greatly, depending on the orientation of the myosin cross-bridges.