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J. Cell Biol.,
Volume 139, Number 3, November 3, 1997 675-681
* Department of Clinical Veterinary Sciences, University of Bristol, Langford, Bristol, BS18 7DY, United Kingdom; and 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 (available at http://www.leeds.ac.uk/chb/muscle/
myosinhead.html). The sharply curved conformation
of the atomic model of S1 is seen only rarely in our
data, with straighter heads being more typical.
Biochemistry Department, Eastern Virginia Medical School, Norfolk, Virginia 23507
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