Electron microscopy of muscle phosphorylase a

Electron microscopy of negatively stained preparations of phosphorylase a (tetrameric form) from solutions with and without added protamine revealed three characteristic types of particle images. In preparations with added protamine several types of crystalline formation were observed: three types of plane monolayers, tubes and small three-dimensional crystals. The tubes have been studied by optical diffraction and filtering. In the main class of tubes the particles form $\text{26}\text{9}$ three-start helices (it may well be that narrow tubes with different parameters exist). Now and again, one can observe tubes twice as large in diameter, and tubes with two-layer walls. Analysis of the images of particles in solution and in crystalline formations showed that their structure can be characterized in terms of one model consisting of four elongated bent subunits arranged with the point-group symmetry 222 at the vertices of a tetrahedron. The structure of phosphorylase b particles, previously studied (Kiselev et al., 1971), can also be characterized satisfactorily by the same model. The difference in the structure of these two forms expresses itself in a different character of mutual aggregation of the molecules in plane layers, and in the parameters of the helical packing of the molecules in tubes.