Keratin filaments

Together with actin microfilaments and microtubules, keratin filaments make up the cytoskeleton of vertebrate epithelial cells. Keratins belong to a family of intermediate filament proteins that form a-helical coiled-coil dimers that associate laterally and end to end to form 10 nm diameter filaments. Keratin and actin filaments and microtubules form an integrated cytoskeleton that preserves the shape and structural integrity of the ker-atinocyte as well as serves to transmit mechanical loads. Keratins account for about 30% of the total protein in basal cells. 

Cytokeratin Intermediate filament keratins found in epithelial tissue. There are two types of cytokeratins the acidic type 1 cytokeratins and the basic or neutral type 11 cytokeratins. Cytokeratins are thought to play a role in the activation of plasma prekallikrein and plasminogen. See Crewther, W.G., Fraser, R.D., Lennox, F.G., and Lindley, H., The chemistry of keratins, Adv. Protein Chem. 20, 191-346, 1965 Masri, M.S. and Friedman, M., Interactions of keratins with metal ions uptake profiles, mode of binding, and effects on the properties of wool, Arfv. Exp. Med. Biol. 48, 551-587,1974 Fuchs, E. and Green, H., Multiple keratins of cultured human epidermal cells are translated from different mRNA molecules. Cell 17, 573-582, 1979 ... 

Proteins can be broadly classified into fibrous and globular. Many fibrous proteins serve a stmctural role (11). CC-Keratin has been described. Fibroin, the primary protein in silk, has -sheets packed one on top of another. CoUagen, found in connective tissue, has a triple-hehcal stmcture. Other fibrous proteins have a motile function. Skeletal muscle fibers are made up of thick filaments consisting of the protein myosin, and thin filaments consisting of actin, troponin, and tropomyosin. Muscle contraction is achieved when these filaments sHde past each other. Microtubules and flagellin are proteins responsible for the motion of ciUa and bacterial dageUa. 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

North, A.C.T., Steinert, RM., Parry, D.A.D. Coiled-coil stutter and link segments in keratin and other intermediate filament molecules a computer modeling study. Proteins 20 174-184, 1994. 

Chu, Y-W., Runyan, R.B., Oshima, R.G., Hendrix, M.J.C. (1993). Expression of complete keratin filaments in mouse L cells augments cell migration and invasion. Proc. Natl. Acad. Sci. USA 90, 4261-4265. 

Keratins are made of filaments, approximately 10 nm in diameter and hundreds of nanometers in length, via assembly of rod-shaped, coiled-coil proteins. Filament formation is initiated by the creation of a dimer comprising monomeric units 44-54 nm in length. Such dimers may form three types of lateral interactions leading to filament formation from equimolar amounts of acidic and basic dimers. In vitro assembly involves the correct alignment of two, three, or four dimers into a nucleus for further, rapid filament assembly [6]. 

Two major types of muscle fibers are found in humans white (anaerobic) and red (aerobic). The former are particularly used in sprints and the latter in prolonged aerobic exercise. During a sprint, muscle uses creatine phosphate and glycolysis as energy sources in the marathon, oxidation of fatty acids is of major importance during the later phases. Nonmuscle cells perform various types of mechanical work carried out by the structures constituting the cytoskeleton. These strucmres include actin filaments (microfilaments), micrombules (composed primarily of a- mbulin and p-mbulin), and intermediate filaments. The latter include keratins, vimentin-like proteins, neurofilaments, and lamins. 

All three of these are dermatophytes, i.e. filamentous fungi which can utilize keratin for their nutrition. Keratin is the chief protein in skin, hair and nail. Hence, all of these organisms are responsible for superficial mycoses in mammals. It is often stated that dermatophytes are the only fungi to have evolved which rely upon infection for then-own survival. This mistaken belief results from a view which is too human-centred and neglects, for example, the presence of symbiotic fungi in the stomachs of ruminants. 

In the keratins, large parts of the peptide chain show right-handed a-helical coiling. Two chains each form a left-handed superhelix, as is also seen in myosin (see p. 65). The superhelical keratin dimers join to form tetramers, and these aggregate further to form protofilaments, with a diameter of 3 nm. Finally, eight protofilaments then form an intermediate filament, with a diameter of 10 nm (see p.204). 

Similar keratin filaments are found in hair. In a single wool fiber with a diameter of about 20 pm, millions of filaments are bundled together within dead cells. The individual keratin helices are cross-linked and stabilized by numerous disulfide bonds (see p. 72). This fact is exploited in the perming of hair. Initially, the disulfide bonds of hair keratin are disrupted by reduction with thiol compounds (see p. 8). The hair is then styled in the desired shape and heat-dried. In the process, new disulfide bonds are formed by oxidation, which maintain the hairstyle for some time. 

The components of the intermediate filaments belong to five related protein families. They are specific for particular cell types. Typical representatives include the cytokeratins, desmin, vimentin, glial fibrillary acidic protein (GFAP), and neurofilament. These proteins all have a rod-shaped basic structure in the center, which is known as a superhelix ( coiled coil see keratin, p. 70). The dimers are arranged in an antiparallel fashion to form tet-ramers. A staggered head-to-head arrangement produces protofilaments. Eight protofilaments ultimately form an intermediary filament. 

Keratin filaments are visible here in an epithelial cell. Keratin fibers belong to the group of intermediate filaments (see pp. 70, 204 d = nucleus). 

The principal cytoskeletal proteins in non-muscle cells are actin, tubulin, and the components of intermediate filaments. Actin can exist either as monomers ( G-actin ) or polymerized into 70 A diameter double filament ( F-actin ). Polymerized actin usually is localized at the margins of the cells, linked by other proteins to the cell membrane. In contrast, tubulin forms hollow filaments, approximately 250 A in diameter, that are distributed within a cell in association, generally, with cell organelles. Stabilized microtubule structures are found in the flagella and cilia of eucaryotic cells however, in other instances - examples being the mitotic apparatus and the cytoskeletal elements arising in directed cell locomotion - the microtubules are temporal entities. Intermediate filaments, which are composed of keratin-like proteins, are approximately 100 A thick and form stable structural elements that impart rigidity, for example, to nerve axons and epithelial cells. 

C4. Cauhn, C., Salvesen, G. S., and Oshima, R. G., Caspase cleavage of keratin 18 and reorganization of intermediate filaments during epithehal cell apoptosis. J. Cell Biol. 138,1379-1394 (1997). 

SUMMERHAYES, I.C., Cheng, Y.E., SuN, T.T., AND Chen, L.B. (1981). Expression of keratin and vimentin intermediate filaments in rabbit bladder epithelial cells at different stages of benzo[a]pyrene-induced neoplastic progression, J. Cell Bio. 90,63. 

These are cells of transition between the basal and superficial cells. They constitute the thickest and biggest layer. They are polygonal cells, with a convex front side and a concave back side. They are arranged on two or three layers at the center and five to six layers on the edge. Their nucleus is active and stretched out along the big axis of the cell. Their cytoplasm contains a very developed Golgi s apparatus as well as tonofilaments (microtubules and keratin filaments) connected to the desmosomes. Their cytoplasmic membranes are only united desmosomes and gap junctions that enable both the unity of intermediate cells and the union of intermediate and basal cells (Figs. 4.3. 5). 

The stem cells of the basal cells are located at the level of the limbus, which come from centripetal migration. The daughter cells migrate to form the intermediate cells. Their cytoplasm, which is rich in glycogen and mitochondria, shows their high metabolic activity. It also contains a Golgi s apparatus, some microtubules, and some keratin filaments connected to each other by desmosomes and hemidesmo-somes. Most of all this cytoplasm contains some actin... 

---