On plant myosin

Abstract

A systematic study has been carried out both on the cellular and the molecular level of the mechanism of protoplasmic streaming in the slime mold, Physarum polycephalum. This study is based on the hypothesis that ATP-protein interaction is an important feature of the mechanism. ATP has been found to have a liquefying effect on the gel structure of the streaming plasmodium. Furthermore, when ATP is allowed to react with protoplasmic material extracted from plasmodia under proper conditions, the viscosity of the solution decreases immediately. This abrupt decrease in viscosity of the protoplasmic solution is followed by a slow recovery. The active protein which is responsible for ATP induced viscosity lowering reaction has been identified by studies of viscosity, electrophoresis and ultracentrifuge. It has been purified to 75% by salt precipitation and differential centrifugation. It is proposed that this protein be termed myxomyosin because of its similarity to actomyosin. Myxomyosin is found to complex with RNA to ca. 9% of its own weight. The RNA does not appear to be essential for the ATP-protein interaction process. RNA does, however, exert a great influence on the physical states of myxomyosin in solution. Myxomyosin preparations possess an ATPase activity. The turnover number of this enzymatic activity is small, namely, 30. A refractory state of myxomyosin has been found. Immediately after myxomyosin reacts with a large excess of ATP, the system enters into a refractory state which persists during the recovery of the viscosity level. In this state, the protein is not sensitive to ATP. Viscosity, sedimentation, flow birefringence and electron microscopy studies reveal that myxomyosin is a rod-like molecule with a weight average molecular weight of 6 millions [plus or minus] 10% and with a range of 4.9 to 10 millions. The molecule possesses a diameter of 50 A [plus or minus] 5 A and a most frequent length of 4000 A with a range of 3000 to 6000 A. The effect of ATP on myxomyosin has been investigated by viscosity, sedimentation, flow birefringence, electron microscopy and electrophoresis studies. There is no indication that myxomyosin breaks down into small units or suffers an extensive change in shape when it reacts with ATP. The present data are best understood on the basis that in the absence of ATP, myxomyosin aggregates in a concentration-dependent manner. The binding of ATP to the myxomyosin moiety reduces the aggregation of the monomers. A proposed mechanism of protoplasmic streaming, suggested by the above observations is presented.