Abstract
The structure of a light meromyosin (LMM) tactoid with a 43-nm periodicity was studied by X-ray diffraction and electron microscopy.
A strong magnetic field was applied to obtain oriented specimens of tactoids for X-ray diffraction. The tactoid was found to contain a planer rectangular lattice with a unit cell of 6.5×3.9nm in a plane perpendicular to its long axis which is parallel to the length of LLM molecule. Electron microscopy showed that the tactoid was made of sheet-like units which had a planer lattice of 3.6×86nm.
From these observations a model for the molecular packing in LMM tactoid was deduced. In this model the tactoid is made of a stacking of Unit layers. A Unit Cell (6.5×3.9×86nm) contains tour LMM molecules, each of which is 90nm long and packed co-planar. The molecules make parallel interactions with staggers of 86 and 43nm and anti-parallel interactions with overlaps of 84 and 41nm. The two parallel interactions may play an important role in the formation of the thick filament backbone if it has a rope-like struncture.
