Correlation between the papain digestibility and the conformation of 10S-myosin from chicken gizzard

In our previous reports, ATP was shown to induce a drastic change in the conformation of gizzard myosin molecules. For example, the sedimentation constant of unphosphorylated myosin (UM) increased from 6S to 10S although an ATP-induced change in the sedimentation constant did not occur with phosphorylated myosin (Suzuki et al. (1978) J. Biochem. 84, 1529). We now report the finding that the ATP-induced formation of 10S-myosin is associated with a drastic change in the papain digestibility of gizzard UM. With 10S-myosin, the cleavage by papain was strongly inhibited at two regions on heavy chains and at one region on light chains; that is, the junction between the 72K dalton and 22K dalton fragments (i.e., a cleavable site in myosin head), the one between the 22K dalton and 130K dalton fragments (i.e., a head-tail junction), and the one between the 3K dalton and 17K dalton fragments of 20K dalton light chains. An even more intimate correlation between the myosin conformation and the papain digestibility of myosin was demonstrated by using thiophosphorylated myosin (thioPM); the cleavages by papain at the 72K–22K dalton junction and the 22K–130K dalton junction were not inhibited when thioPM was digested.