Effects of Calcium and ATP on the Conformation of F-Actin in Vivo and in Vitro

Abstract

The dichroism spectrum of I-band in a glycerinated single fiber of muscle was very similar to that of an oriented F-actin filament in vitro. The negative dichroism of F-actin in the thin filament in vivo or in vitro was found to change with the addition of Ca ions, suggesting a conformational change of F-actin. In the absence of ATP, a small amount of bound H-meromyosin induced a remarkable decrease of the dichroism of F-actin. Upon the addition of Mg-ATP, the negative dichroism of the F-actin-H-meromyosin complex became larger than that of pure F-actin, suggesting that F-actin interacting with H-meromyosin assumes a different conformation from pure F-actin. A similar increase was also observed in the F-actin-tropomyosin-troponin-H-meromyosin complex. The maximum increase was greater under the condition that super-precipitation was delayed. The possible role of F-actin in muscle contraction is discussed.