This paper describes the use of digital image processing to combine the images of 2400 myosin heads from electron microscopy of purified myosin molecules. The type of flexibility inferred in the heads has previously been proposed as a way of storing elastic energy of attached heads in muscle. This would allow gradual dissipation of the chemical energy derived from ATP. The Abstract from the paper is shown.
The motor domain, which has actin binding and ATPase activity is at the top. The two light chains comprising the regulatory domain (bottom) vary in position. The whole head is 16 nm long.
Electron microscopy of negatively stained myosin has previously revealed three discrete regions within the heads of the molecule. However, despite a probable resolution of ~2 nm, it is difficult to discern directly consistent details within these regions. This is due to variability in both head conformation and in staining. In this study, we applied single particle image processing and classified heads into homogeneous groups. The improved signal-to-noise ratio after averaging these groups reveals substantially improved detail. The image averages were compared to a model simulating negative staining of the atomic structure of subfragment-1 (S1); this shows that the three head regions correspond to the motor domain and the essential and regulatory light chains. The image averages were very similar to particular views of the S1 model. They also revealed considerable flexibility between the motor and regulatory domains, despite the molecules having been prepared in the absence of nucleotide. This flexibility probably results from rotation of the regulatory domain about the motor domain, where the relative movement of the regulatory light chain is up to 12 nm, and is most clearly illustrated in animated sequences. The sharply curved conformation of the atomic model of S1 is seen only rarely in our data, with straighter heads being more typical.