Myosin subfragment 1 has tertiary structural domains

Abstract

Transient electrical birefringence measurements were made on skeletal muscle myosin subfragment 1 (S1) at 3.7 degrees C in 10 mM tris(hydroxymethyl)aminomethane-acetate and 0.10 mM MgCl2, pH 7.0. The specific birefringence for 4.5 microM S1 was determined from steady-state measurements to be (8.1 +/- 0.3) X 10(-7) (cm/statvolt)2. For electric fields in the range of 2.47-24.7 statvolts/cm, the alignment was due to a large permanent dipole moment for S1, estimated to be 8500 +/- 2000 D. The duration and the strength of the transient electric field was varied, and the temporal response of the decay of the birefringence signal was analyzed. The rate of rotational motion after the field was removed increased with increasing field strength for short (0.35-microseconds) pulses and decreased with increasing pulse lengths for all field strengths. The rate of decay from a steady-state birefringence signal was independent of field strength. A model of S1 structure is proposed, which is consistent with these data and most other data on S1 structure. In this model, S1 is composed of two tertiary structural domains that are connected by a flexible linkage with a substantial restoring force. The electric dipole moments on the two domains are arranged head to tail. The segmental movement of the domains is restricted to certain directions. The average conformation of the molecule is elongated, but it can be made more compact by the torque exerted by an electric field. The structural changes depend on the strength and duration of the pulse.(ABSTRACT TRUNCATED AT 250 WORDS)