Abstract
Two kinds of subfragment-1 of myosin, S-1(T) and S-1(CT), were prepared by two-step tryptic [EC 3. 4. 21. 4] digestion of myosin that had been modified with about 1 mol of p-chloromercuri-benzoate (CMB) per mol of myosin, and one-step chymotryptic [EC 3. 4. 21. 1] digestion of the myosin, respectively. The amount of bound CMB was about 0.82-0.90 mol per 2 mol of S-1.
Both kinds of S-1 modified with CMB equally inhibited superprecipitation of myosin B from rabbit skeletal muscle. About 2 mol of CMB-S-1 (1 mol of CMB-S-IA) inhibited the function of 1 mol of actin monomer on the superprecipitation of actomyosin reconstituted from myosin and fibrous actin(FA) with relaxing protein (RP). CMB-S-1 also effectively inhibited superprecipitation of myosin B from the plasmodia of the slime mold Physarum polycephalum.
The ATPase [EC 3. 6. 1. 3] activity of CMB-S-1(T) was similar to that of CMB-S-l(CT) in the absence of FA, but was not enhanced as effectively by FA as the latter. In the presence of 0.3mg/ml of FA with RP, the activity of CMB-S-1(T) was only one-fifth of that of CMB-S-1 (CT).
CMB-S-1(T) did not affect the activities of ATPase from animal cells outside actomyosin systems, such as the Ca2+-dependent ATPase [EC 3. 6. 1. 3] of the SR prepared from rabbit skeletal muscle and the Na+, K+-dependent ATPase [EC 3. 6. 1. 3] from porcine kidney. It also scarcely affected Ca2+-uptake by the SR at concentrations lower than 0.2mg/ml.
However, CMB-S-1(T) strongly inhibited the polymerization and depolymerization of tubulin prepared from bovine brain. At 0.15 mol per mol of tubulin heterodimer, CMB-S-1(T)
