Intermediate filaments (IFs), microfilaments (MFs), and microtubules (MTs) are the key structural proteins that form the cytoskeleton. Intermediate filaments acquired their name for being a size that is ‘intermediate’ between actin microfilaments (MF; 7–8 nm) and microtubules (MT; 25 nm). Several other features distinguish intermediate filaments from MFs and MTs. These include intermediate filament structural diversity, tissue- and cell-selective ex
The first intermediate filament protein was described in the late 1960s based on experiments involving muscle. Since that first discovery, researchers have identified 65 different intermediate filaments, classifying them into six groups (I–VI) according to sequence homology (Table X). Types I–IV IFs are found in the cytoplasm, the type V IF proteins are found in the nucleus, and those classified as type VI are found exclusively in the lens. Type I and type II IFs are made up of keratin IFs and are found in epithelial cells. Type III IFs include vimentin, desmin, glial acidic fibrillary protein (GFAP) and peripherin and are found in mesenchymal cells, muscle cells, astrocytes, or peripheral neurons, respectively. Type IV IFs include α-internexin, found in neurons of the central nervous system, nestin, found in neuroepithelial cells, syncoilin and synemin, which are both observed in muscle cells. The type V nuclear IFs are known as Lamins and are found in nuclear lamina of various cells. Finally, the type VI IFs of the lens include Phakinin and Filensin.
Structure and Assembly
Despite being encoded by approximately 70 conserved genes and having diverse tissue distribution, IFs in many different cell types are morphologically similar, forming filaments with a diameter of 10-12 nm. All intermediate filament proteins are composed of a tripartide domain structure. This structure consists of a structurally conserved central “rod” domain, composed of heptad repeats that are required for the coiled-coil interactions involved in the assembly of IFs; the rod domain is flanked on both ends by non-helical N- (“head”) and C-terminal (“tail”) domains. These domains vary in their lengths, sequences, and properties, and consequently distinguish IF proteins from one another. Monomers assemble into coiled-coiled dimers, which are oriented in parallel and interact to form a very stable coiled-coil rod. Some IFs, such as keratins, form obligate heteropolymers (i.e., a type I and a type II), whereas vimentin IFs form homopolymers. Dimers then associate in an anti-parallel, half staggered manner to form a larger unit which in turn grows in length to form apolar tetramers, also known as protofilaments. Lateral associations of the tetramers bundle into octamers and so on, until the stable 10 nm intermediate filament is assembled. Due to their incredible stability (i.e., insolubility and self-assembly), intermediate filament proteins have only been partially crystallized.
Organization and Dynamics
In most vertebrate cells, cytoplasmic IFs span the cytoplasm, binding to the nucleus while interacting with adhesion sites, such as desmosomes or integrins at the cell periphery. IFs also interact with multiple proteins throughout the cytoplasm, including actin, microtubules, and plectin. The resulting network integrates and organizes the cytoplasm, providing mechanical integrity crucial for proper tissue function. There is a measure of variability as to the distribution of the cytoplasmic IFs, which is tissue- and cell-type dependent. For example, keratin IFs localize to different areas of the cytoplasm depending upon whether one is looking at keratinocytes or polarized epithelial cells.
Intermediate filaments were once thought to be static structures within the cell. Now it is clear that although IF polypeptides have long half-lives and are biochemically stable, they routinely rearrange via disassembly and reassembly in response to various environmental stresses in order to maintain cell integrity and stability. This occurs in spite of the fact that they do not require metabolism of nucleotides ATP or GTP as do actin or microtubules. However, there is conflicting evidence as to whether filament subunit exchange occurs along the ends, core, or along the length of the filament.
For many years, there were two well-defined functions of intermediate filaments: shape maintenance and mechanical integrity. But as is often the case in biology, new knowledge comes with new techniques. New advances came in the late 1980s, when Dr. Robert Goldman microinjected IF protein into cells, and simultaneously, Dr. Elaine Fuchs or transfected cells with keratin cDNA, meanwhile demonstrating that IF proteins were rapidly incorporated into existing, endogenous IF networks (of the same type). One year later, other researchers came to the same conclusion that IFs are dynamic. Thus, the idea of IFs are static had to be reassessed, and more studies determined that they were indeed highly motile and involved in numerous other functions, including cell cycle, apoptosis, cell adhesion, vesicle transport, environmental stress and injury.
As a major cytoskeletal component, intermediate filaments are important contributors to the mechanical integrity, contractility, and rigidity of tissues and cells, particularly during stress. In part, this is because of their unique mechanical properties compared to microfilaments and microtubules. Intermediate filaments are extremely flexible with the unique ability to withstand large deformations before rupturing, further also possessing unique strain hardening properties compared to the other major cytoskeletal components. Finally, of the three major cytoskeletal components, only intermediate filaments are capable of transferring mechanical load from cell surface to the nucleus due to interactions at both the nucleus and plasma membrane. Collectively, studies demonstrate that the properties of vimentin make it well-suited to be intimately involved in maintaining cellular integrity and function.