Electron microscopy supports a fibrous substructure for lens intermediate filaments

The substructure of intermediate filaments from bovine lens cortical fiber cells was investigated by electron microscopy. Native filaments and synthetic ones regenerated from the total cytoskeletal extract and from the three purified subunits were examined. The morphologies from these various sources were essentially identical, with the exception that filaments reconstituted from one of the purified polypeptides were much shorter, very contorted and showed strings of aggregated protein. The solid cylindrical, unbranching filaments consisted of a helical arrangement of at least two, 5 nm diameter strands. The evidence indicated that each strand was composed of two, 2 nm diameter protofilaments which were also helically constructed (right-handed) with a periodicity of 11.6 nm. Intermediate filament diameter varied widely (8-14.8 nm, average 11.3 nm) and in a direct, linear manner relative to the apparent progression (helical) angle of the strands across the filaments face. These conclusions were obtained from observations on negatively stained intact filaments and reconstituted 4.4 nm fibrils and on positively stained transverse sections of fixed and embedded filaments.