A Study of Cell Motility Using the Reconstituted Systems

Abstract

The muscle contraction is the process in which the chemical energy of ATP is transformed to the mechanical energy. Since it is very difficult to analyze quantitatively the chemical change accompanying mechanical work in a living muscle, the mechanochemical coupling of energy conversion was studied using a reconstituted motile system, “stream cell”. In the cell circulating flow could be induced by the interaction of heavy meromyosin (HMM) with attached polarized actin, both of which were extracted from skeletal muscle of a rabbit. Improving the way of attaching F-actin, a new biological motor, which was based on microscopic energy conversion, was also developed. The motor, “actomyosin motor”, has six blades, on which F-actin-HMM assembly is formed, and can rotate in a specific directon in the HMM solution. It was shown in these reconstituted systems that the assembly, not necessarily in the fibrous form, produces a vectorial force utilizing the energy of ATP hydrolysis.The physiological significance of these reconstituted systems depends on to what extent chemical and physical properties of the systems resemble those of muscle system under contraction. The reconstituted systems worked only when the physiological condition was the same as that of muscle. In the case of the stream cell, the ATPase activity at higher streaming velocity was higher than that at lower velocity. This corresponds to the Finn's effect in the muscle system. A close analysis of the relation between chemical and mechanical properties during active streaming led to a conclusion that the extent of self-organization of macroscopic streaming, or the resulting flow velocity, controls the molecular dynamics of the elementary cycles, namely, the ATPase activity of individual myosin molecules.