Intermediate filaments: dynamic processes regulating their assembly, motility, and interactions with other cytoskeletal systems

Intermediate filaments (IF) are major cytoskeletal proteins of animal cells. They are comprised of a large family of . 50 gene products that are capable of polymerizing into ;10 nm diameter IF. The rules for IF polymerization from these protein subunits are complex. Some can form IF from a single type of protein chain (homopolymers), while others can form IF only when they are combined with more than one type of protein chain (heteropolymers). Within the cytoplasm of cells, IF form elaborate and complex networks. These are usually concentrated in the perinuclear region and radiate throughout the cytoplasm, where they are frequently associated with the cell surface (1, 26). The extent of sequence homology, the pattern of cell type specific expression, and the similarity of intron positions of their genes are properties that have been used to classify IF proteins into six different types (1–3). The largest number of IF proteins are categorized as the type I and II keratins, which are expressed mainly in epithelial cells. The keratins are obligatory copolymers that require both a type I and a type II protein to form IF. The expression of specific keratin pairs is precisely regulated during development, and this unique feature has led to their use as differentiation markers for epithelial cells. Vimentin, desmin, glial fibrillary acidic protein (GFAP), and peripherin are the four known type III IF proteins. Each of these type III proteins is capable of assembling into homopolymer IF. Among them, vimentin is unique in that it often forms a scaffold IF network before the expression and assembly of differentiation-specific IF proteins such as desmin, GFAP, and peripherin. Type IV IF proteins consist of the three neurofilament subunit proteins, NF-L, NF-M, and NF-H, as well as a-intenexin. Neurofilaments are obligatory heteropolymers, while a-internexin is self-assembly competent (4). Although the majority of IF proteins are expressed in the cytoplasm, the type V IF proteins (the nuclear lamins) are localized in the nucleus where they polymerize to form the nuclear lamina. The lamina is located at the interface between chromatin and the inner face of the nuclear envelope membrane. Recent observations demonstrate that the nuclear lamina is not only essential for the structural integrity of the nuclear envelope but is also involved in the process of DNA replication during S phase of the cell cycle (5). The type VI IF protein nestin has not been characterized extensively. It was discovered as an early developmental marker of neuroepithelial cells (3), which cannot form IF on its own but is capable of coassembling with vimentin to form IF (28). The IF proteins possess both well-conserved and nonconserved domains. The most conserved region is the central a-helical rod domain consisting of ;310 amino acids, known to form coiled-coils in assembled IF. This central rod is flanked at both ends by NH2and carboxyl-terminal non-a-helical regions that are not as well conserved among the various types of IF proteins. It has often been speculated that these less conserved end domains may be involved in the cell type specific functions of IF, as well as their higher order structure. Within the rod domain, two stretches of sequence are highly conserved that are known to play important roles in IF assembly (1). They are located at the beginning (the IA region) and at the end (the 2B region) of the rod. Although the details of IF structure remain to be determined, the most widely accepted model is that IF are comprised of four protofibrils. Individual protofibrils contain two protofilaments, each of which is constructed from linear arrays of antiparallel and staggered dimers. The basic building block is the dimer, consisting of two parallel and in register protein chains (6). IF are known to provide mechanical integrity to cells. In vitro they exhibit unusual viscoelastic properties making them much more resistant to mechanical stress when compared with microtubules and actin-containing microfilaments (7). Their mechanical role has been demonstrated at the tissue level in vivo by the finding that numerous human blistering diseases are caused by point mutations in epidermal