| Research Abstract |
The contractile system of striated muscle, i.e., skeletal and cardiac muscle, spontaneously oscillates under the conditions intermediate between relaxation and contraction. We call this phenomenon SPOC (Spontaneous Oscillatory Contraction), in which each sarcomere oscillates in a thaw-tooth wave form composed of rapid yielding and slow shortening phase. Also, the length oscillation of sarcomere length propagates to the adjacent sarcomeres as a SPOC wave. (1) We found that the oscillation of sarcomere length is synchronized with the external mechanical impulse. When the repetitive stretching and shortening of rectangular wave form were externally imposed on a myofibril with an amplitude of about 10 nm every half-sarcomere, all the sarcomeres began to yield in phase following not only quick stretching but also quick shortening of myofibril. This result demonstrates that myosin motors, which function as a stochastic molecular machine on a single molecular level, are synchronized with the external mechanical impulse. This may be a typical example showing inter-molecular synchronization of proteins (enzymes) in an organized system (MS in prep.). (2) Kinesin is a processive molecular motor that carries organelles towards a plus end of a microtubule (MT) in a cell. We examined the unbinding force of kinesin from MT by using optical tweezers that trap the plastic bead on which single kinesin molecule was attached The unbinding force was measured by pulling a bead along the MT at a constant velocity. We found that, in the presence of 1 mM ADP, the unbinding force distribution showed only one peak at about 3pN. Hence, we could show that the single-headed binding in the presence of ADP was weak according to the model analysis. Combining the results obtained last year, we conclude that kinesin takes either single-headed or double-headed binding depending on the nucleotide state, which is consistent with the hand-over-hand mechanism (MS in prep.).
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