Microscopic Measurement of Periodic Cross-Bridge Formation in Skeletal Myofibril

Abstract

Electron microscopy and X-ray diffraction studies have been usually used for the examination of the interactions between actin and myosin, i.e., cross-bridge formation, in muscle fibers. These studies suggested the existence of “target zones [regions]” in actin (thin) filament, which are composed of three to four actin monomers that myosin heads in myosin thick filament can form cross-bridges. Direct evidence for the target zones, however, has been missing in myofibrils. Here we studied the interaction between a single actin filament and the thick filaments of rabbit skeletal myofibril under optical microscope. Single bead-tailed actin filament was manipulated by optical tweezers to make rigor cross-bridges on the outer surface of myofibril, and rupturing events were directly detected. We found the periodic cross-bridge formation, and frequently observed gaps, which are probably due to the incommensurate helical pitches between the thin and the thick filaments.