The Effect of Cleavage at Site 1 of Gizzard HMM in the Interaction with Skeletal Muscle Actin

Two different classes of gizzard heavy meromyosins (HMMs) were prepared from phosphorylated myosin by chymotryptic digestion in the presence and absence of ATP and were compared with respect to their actin-activated Mg2+-ATPase reactions. One class of HMM, named HMM(+), had a cleavage at site 1 in the N terminal portion of the heavy chain and the other class of HMM, named HMM(—), had no cleavage at this site. Maximum turnover rate (Vmax of the skeletal acto gizzard HMM Mg2+-ATPase reaction was obviously different between HMM(+) and HMM(—). The Vmax value of HMM(+) was 2.5-fold larger than that of HMM(—). On the other hand, the apparent association constants (K2) of skeletal muscle actin for both HMMs which were deduced from double reciprocal plots (v−1 versus [actin]−1) seemed to be identical. The difference in Vmax value was attributed to the cleavage at site 1 since a following chymotryptic cleavage of 1-IMM(—) at Site I caused a 2.5-fold increase in the Vmax value. That site 1 in the N terminal portion of the gizzard myosin heavy chain was the key locus for the actin-myosin interaction was shown in addition to our previous finding of the effects of cleavage at site I on the ATPase activity and nucleotide binding ability of gizzard HM M (Okamoto, Y. & Sekine, T. (1981) J. Biochem. 90, 833–842; 843–849).