FIGURE 18-48 Structure and assembly of intermediate filaments. Electron micrographs and drawings of IF protein dimers, tetramers, and mature intermediate filaments from Ascaris, an intestinal parasitic worm. (a) IF proteins form parallel dimers through a highly conserved coiled-coil core domain. The globular heads and tails are quite variable in length and sequence among IF classes. (b) A tetramer is formed by antiparallel, staggered, side-by-side aggregation of two identical dimers. (c) Tetramers aggregate end to end and laterally into a protofibril. In a mature filament, consisting of four protofibrils, the globular domains form beaded clusters on the surface. See N. Geisler et al., 1998, J. Mol. Biol. 282:601; courtesy of Ueli Aebi. (d) Comparison of the structure of vimentin and lamin A. Notice that the lamin protein has a nuclear localization sequence to target it to the nucleus. See H. Hermann et al., 2007, Nat. Rev. Mol. Cell Biol. 8:562. [Micrographs reprinted with permission from Elsevier, from: N. Geisler et al., “Assembly and architecture of invertebrate cytoplasmic intermediate filaments reconcile features of vertebrate cytoplasmic and nuclear lamin-type intermediate filaments,” J. Mol. Biol. 282:601 (1998).]