Separation of Myosin Subfragment 1 into Two Fractions, One Having the Burst Site and the Other Having the Non-burst Site

Abstract

During Mn(II)-ATP hydrolysis by myosin, the predominant intermediate formed at the burst site of the enzyme below 10° is the myosin-ADP complex formed by adding ADP to myosin, while above 10° it is the myosin*-ADP-P₁ complex generated by ATP hydrolysis (Yazawa, Morita, & Yagi (1973) J. Biochem. 74, 1107; Hozumi & Tawada (1975) Biochim. Biophys. Acta 376, 1; Tawada & Yoshida (1975) J. Biochem. 78, 293). It is suggested that the second (non-burst) site of myosin predominantly forms the myosin-ATP complex (Hozumi & Tawada, ibid.). From these findings, it is expected that (i) myosin subfragment 1 (S1) having the burst site is bound to actin in Mn(II)-ATP solution containing ADP below 10°, because it forms the S1-ADP complex even in the presence of ATP; (ii) the other S1, i.e., that having the non-burst site, is dissociated from actin, because it forms the S1-ATP complex. These two expectations were confirmed by viscosity measurements of acto-S1 solutions, giving a basis for the separation of S1 into two fractions: one having the burst site and the other having the non-burst site. S1 having the non-burst site could be extracted from partially papain [EC 3.4.22.2]-digested myofibrils of rabbit skeletal muscle with a solution containing MnCl2, ATP, and ADP at 0°. S1 having the burst site was extracted from myofibrils already used for the extraction of S1 having the non-burst site, with a solution containing MgCl₂ and ATP at 20°. The former S1 fraction had Mg-ATPase [EC 3.6.1.3] activity, but scarcely showed any initial burst of P₁ liberation. The latter S1 showed a P₁ burst of more than 0.5 (M/M). The steady state ATPase activity of the former S1 was slightly higher than that of the latter. The burst size of normal S1, i.e., that extracted from papain-digested myofibrils with Mg-PP₁ or Mg-ATP, was 0.5 (M/M). The ultraviolet absorption spectrum of the non-burst type S1 was not changed by ADP but was changed by ATP, though the difference spectrum was distinct from that of normal S1 and the difference molar extinction coefficient at 289 nm was only 20% of that of normal S1. No significant difference was seen in the compositions of these two S1's and normal S1, as determined by SDS gel electrophoresis.