Image Analysis of the Complex of Actin-Tropomyosin and Myosin Subfragment 1
A three-dimensional image of the “rigor” complex of actin-tropomyosin-S1 was reconstituted from both low dose (10 electrons/Å2) and high dose (>500 electrons/Å2) electron microscopic images of specimens embedded in unbroken and unbacked stain sheets of uranyl acetate over the holes of perforated carbon films.
Myosin S1 shows multi-domain submolecular structure as has been earlier observed in actin-S1 (Wakabayshi & Toyoshima, 1981) and actin-heavy mepomyosin (Katayama & Wakabayashi, 1981). The morphological unit of the actin-tropomyosin-S1 was found to be composed of at least three domains (domains A, B and D) and three regions (C, E and H).
A myosin S1 molecule has a complex shape, which cannot be represented by a simple rod with one major axis. The shape of S1 should be approximated by at least two rods.
The domain D is identified as the main part of S1. The angle between the major axis of this domain and the axis of actin helix was about 72°, which is almost right angle.
The angle between the axis of actin helix and major axis of the region E, which is less bulky than the domain D and makes no contact with actin, is much smaller than the value for the domain D.
The resolution of reconstituted images from both high and low dose micrographs was improved so that the radial resolution became about 15 Å and the axial one became about 25 Å. Due to the improvement of resolution in both the radial and axial direction, all major domains A, B and D split into two domains, i.e. into A1 and A2, B1 and B2, and D1 and D2 respectively.
Though unambiguous assignment of actin is not yet achieved by us, it can be confirmed that a S1 molecule interacts morphologically with actin at two sites (Wakabayashi & Toyoshima, 1981).
KeywordsThin Filament Actin Monomer Type Assignment Myosin Molecule Helix Axis
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