Interaction of myosin B with adenosine triphosphate, a flow-birefringence study

Abstract

The interaction of myosin B, or natural actomyosin, with adenosine triphosphate (ATP) was investigated employing chiefly the flow birefringence technique. Unless the myosin B sample was clarified, anomalous behaviour of the extinction angle (χ) in the low velocity gradient (G) range was frequently observed on the addition of ATP. On addition of ATP, the rotary diffusion constant of myosin B extrapolated to G = 0 increased slightly from 0.7 to 1.2 sec −1. The molecular size in the myosin B solution was polydisperse. The range of length distribution was from 1 to 3 μ, and in the presence of ATP from 0.7 to 2.4 μ. No disaggregation of myosin B was observed upon dilution to 0.005%. The birefringence greatly decreased on the addition of ATP throughout the G range tested. In the high G range between 500 and 1,000 sec −1, χ increased from 8° to 14° on addition of ATP. The myosin B solution was centrifuged in the presence of ATP at 100,000 g, and the supernatant showed the same flow birefringence characteristics as myosin A solution. The rotary diffusion constant of myosin A is 1,800 sec −1. The minimum effective concentration of ATP on flow birefringence was 5.10 −5 M. Pyrophosphate was also effective in the presence of Mg ++. The influence of KCl at a high concentration was similar to that of ATP. A much more drastic effect was caused by KOH and KI. In discussing the effects of ATP, the most adequate explanation of the change in the flow birefringence properties of the clarified solution with ATP, seems to be the dissociation of myosin B into actin and myosin.