Flexibility of myosin in pyrophosphate and NaCl solutions. An electric birefringence study

The orientational relaxation time of myosin has been reported as 38 microseconds when measured in pyrophosphate media at elevated pH (Hvidt et al. 1984) and as 17 microseconds when measured in 0.3 M KCl at pH 7.3 (Bernengo and Cardinaud 1982). This discrepancy, which is reexamined in the present report, suggests that in KCl solution the rod portion of the myosin molecule is bent with an average angle close to 110 degrees, whereas in pyrophosphate at elevated pH it assumes a nearly straight and rigid conformation. Electric birefringence shows that the amount of dimeric and polymeric species in pyrophosphate media at pH's 8.0 and 8.5 is certainly greater than usually thought. In these media, relaxation times can be measured correctly at pH 9.0. A comparative analysis of the influence of protein concentration, field strength, medium composition and concentration, pH and temperature showed that a high relaxation time is associated with the presence of pyrophosphate and that the myosin tail is significantly stiffened in the presence of this anion.