Limited trypsinolysis changes the structural dynamics of myosin subfragment-1

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Abstract
The effects of limited trysinolysis of myosin subfragment 1 (S1) on its structural dynamics were investigated by using the method of transient electric birefringence. Conversion of S1 by trypsin to produce S1(T) did not change the specific Kerr constant [(8.1 .+-. 0.3) .times. 10-7 and (8.0 .+-. 0.3) .times. 10-7 cm2/statvolt2 for S1(T) and S1, respectively] or the degree of alignment in a weak electric field, suggesting that the size of S1 and its permanent electric dipole moment are not modified by trypsin. On the other hand, the relaxation time for the field-free rotation, after achieving a steady-state birefringence signal was reduced from 316 ns for S1 to 269 ns for S1(T), at 3.7.degree. C, suggesting that trypsinolysis increased the flexibility of the connections between S1 segments or introduces additional segmental motions. For both S1 and S1(T), the rate of decay for a steady-state signal was independent of the field strength, between 3.34 and 20.3 statvolt/cm. Shortening the duration of the weak electric field pulses to 0.35 .mu.s, so that steady-state signals were not achieved, decreased the relaxation times for S1 and S1(T) to 240 and 102 ns, respectively, which is consistent with the segmented flexible S1 structure proposed earlier [Highsmith, S., and Eden, D. (1986) Biochemistry 25, 2237]. When the electric field was increased to above 10 statvolt/cm, in order to make the interaction energy for the S1(T) electric dipole moment in the electric field greater than the thermal energy, the relaxation time after a 0.35-.mu.s pulse decreased from 210 to 170 ns as the field was increased from 7 to 20 statvolt/cm. The percent decrease for S1 was about the same. Thus, the elastic distortion of S1 structure that is observed when short strong electric fields are used may not be significantly changed by the action of trypsin on S1.