X-ray diffraction and electron microscopy of a light meromyosin tactoid

The structure of a tactoid of light meromyosin with a 43-nm periodicity was studied by both X-ray diffraction and electron microscopy. Such tactoids were formed from light meromyosin prepared by a short tryptic digestion (5 min) of myosin.A strong magnetic field (6 kgauss) was employed to obtain oriented specimens of tactoids for X-ray diffraction. The oriented tactoids gave equatorial reflections from a rectangular lattice with a unit cell of 6·5 nm × 3·9 nm (at pH 6·6) in a plane perpendicular to the long axis of the tactoid. This lattice shrank anisotropically when the pH was lowered. The meridional reflections could be indexed as orders of 42·93 ± 0·05 nm.The tactoids were frequently associated with sheet-like structures termed banded sheets. In negative stain these banded sheets showed the same band pattern as the tactoids with 10 nm wide light and 33 nm wide dark bands. However, in thin banded sheets the density of neighbouring dark bands alternated so that the true axial repeat was 86 nm. Optical diffraction showed that the face-on view of the banded sheet had a unit cell of 3·6 nm × 86 nm.From these observations a plausible model for the structure of the light meromyosin tactoid has been deduced. In this model the tactoid is made by a stacking of unit layers. A unit cell (6·5 nm × 3·9 nm × 86 nm) contains four light meromyosin molecules, each 90 nm long and packed co-planar, not all of which are in an identical environment. The molecules make parallel interactions with staggers of 86 and 43 nm and antiparallel interactions with overlaps of 84 and 41 nm.