ON ROLE OF ACTIN IN CONTRACTILE COMPLEX OF INSECT FLIGHT MUSCLE

The effect of the globular (G-) and fibrous (F-) forms of actin on insect myosin ATPase has been studied. Whereas G-actin is at least as effective as F-actin in overcoming the inhibitory effect of overoptimal substrate concentrations, only the fibrous form inhibits myosin ATPase at very low levels of Mg++ or at high concentrations of Ca++. Further, rapid superprecipitation under a variety of conditions can only be observed with the F-actin-myosin complex. The possibility of cyclic transformations between the two forms of actin occurring within individual subunits in isolated F-actin or the intact actin filaments was tested by studying the incorporation of nucleotides. A comparison between the time course and extent of the nucleotide exchange observed with natural and repolymerized actin, actomyosin and myofibrils indicates that this exchange reflects a structural lability of the preparation. Nucleotide binding can also be detected with glycerinated muscle fibers. This binding is unaffected by the level of Ca++ and is not related to the incubation time or the enzymic and mechanical activity of the fibers. The finding that the nucleotide binding to muscle fibers depends on the establishment of the rigor state upon removal of ATP during the washing-out procedure, suggests that the nucleotide is bound to the myosin cross-bridges attaching permanently to the actin under these conditions. © 1969.