Electron microscopy of serum amyloid protein in the presence of calcium; alternative forms of assembly of pentagonal molecules in two-dimensional lattices.

The P component is a protein believed to be of serum origin commonly found in the highly ordered proteinaceous tissue deposits called amyloid. Both the protein recovered from the tissue and its serum counterpart have an identical characteristic appearance in the electron microscope, consisting of pentagonal plates which are often assembled as columns of stacked discs. These images, seen in the absence of calcium, are replaced by three more complex assemblies when low concentrations of calcium are present. One of the structures is crystalline (III), the other two appear to be planar lattices, one having a distinguishable structure with threefold symmetry (1). The structural elements of the other lattice (II) which takes the form of a cylinder are less distinct. It is suggested that the regular arrays which P component forms in the presence of calcium may be the basic framework on which amyloid deposits form.